Volcanic Dam Hypothesis and Possible Sherwin Glaciation Age for the
McGee Till, Long Valley Caldera, Sierra Nevada, California
David R Stockton
The boulder deposits upon the summit plateau of McGee Mt have long been recognized
as glacial in origin, but the exact nature of their emplacement has lacked a satisfactory
explanation. The nearest bedrock exposures of the Round Valley Peak granodiorite from
which these boulders are derived, lie to the south, four kilometers up canyon, with the
most likely cirques of origin situated some eight kilometers distant from McGee Mt.
McGee Creek Canyon intervenes between the granodiorite boulder deposits and their
source. The highest till deposits on McGee Mt sit some 800 meters above McGee Creek,
which exits the range-front through a deep canyon along the southeast base of McGee
Mt. Late Pleistocene, Tahoe and Tioga age glaciers followed the present course of
McGee Creek onto the Long Valley floor, building massive moraines which extend
beyond the canyon mouth. Tahoe/Tioga lateral moraines along the base of McGee Mt lie
hundreds of meters below the McGee Till deposits.
These topographic relationships make it improbable that the McGee Till could have been
emplaced upon the present landscape. In his 1931 treatise on eastern Sierra Nevada
glaciation, Eliot Blackwelder hypothesized that the McGee Till was deposited upon an
earlier topography of subdued relief, before incision of the present McGee Creek Canyon.
This explanation gives the till great antiquity, and as a result, the McGee Mt deposits
represent the oldest recognized Sierra Nevada glaciation (Fullerton 1986, D. Clark et al
2003, Gillespie and Zehfuss 2004). Dalrymple (1963) potassium-argon dated the tephra
covered basalt, which underlies portions of the McGee Till, at 2.6 ma, constraining the
maximum age for the deposits. But subsequent attempts by Sharp (1969), and Birkeland
and Janda (1971), to date the McGee Till using weathering characteristics showed no
significant differences between the McGee Till and nearby type Sherwin glacial deposits,
which have a currently accepted mean age of 820 ka (D. Clark et al 2003, Gillespie and
Zehfuss 2004, Kaufman et al 2004). What differentiates the McGee Till from the
Sherwin is its unique elevated and isolated location upon McGee Mt (Bateman and
Wharhaftig 1966, M.M. Clark 1967, Sharp 1969).
The McGee Mt block shows a smoothed and subdued relief as if having been overridden
by ice. Compared to the surrounding jagged towering peaks, the relatively flat McGee
Mt plateau is an anomaly. Noticeably rounded is Aggie-McGee Ridge which extends
southwest of McGee Mt to Mt Aggie. To the west, across Aggie-McGee Ridge from
McGee Creek Canyon, lies Aggie-Morrison Canyon which drains into Convict
Canyon/Tobacco Flat. The upper shaded portion of this hanging valley is narrow and
squeezed between Mt Aggie and Mt Morrison. But west of McGee Mt the AggieMorrison Canyon widens considerably, and the Tahoe/Tioga moraine complex here
appears underfit, only occupying the left-hand portion of the lower canyon. Evidently
this short drainage did not produce enough ice to fill the width of the lower canyon.
These circumstances correlate with a McGee Creek Canyon glacier spilling across
McGee Mt and Aggie-McGee Ridge, into the lower portion of Aggie-Morrison Canyon,
significantly eroding and widening the canyon in the process.
McGee Mountain is located in the central eastern Sierra Nevada, California,
overlooking Lake Crowley, between the towns of Bishop and Mammoth Lakes.
Figure 1
Continuity of Highest Deposits Defines a Possible Ice Surface
Glacial deposits along canyon sides can be correlated if they show continuity with a
projected ice surface (M.M. Clark 1967). Three high boulder deposits along McGee
Creek Canyon correlate with and define a possible McGee Creek Canyon glacier
highstand. These three boulder fields are; granodiorites on an old upland surface between
Esha Canyon and McGee Creek Canyon, granodiorites on the McGee Mt southeast rim,
and quartzites on Mt Aggie’s north shoulder. All three are within 180 m elevation.
The Esha-McGee upland deposits, recognized and mapped by Rinehart and Ross (1964),
are the furthest up canyon and at 3440 m are the highest in elevation. They are on the
eastern canyon wall and so would be glacier right lateral. By all appearances they are a
right lateral moraine deposited in an embayment where the ridge protrudes into the
canyon. These boulders are perched above a steep chute, but uphill behind them is a
gentle erosional surface. Had the ice once been higher than this location, granitic debris
would be expected to extend further up slope. The elevation of these boulders can be
interpreted as a maximum elevation for glacier ice at this location.
Quartzite boulders where the south end of Aggie-McGee ridge abuts the north shoulder
of Mt Aggie lie across and slightly down canyon from the Esha-Mcgee upland. The
Aggie North Shoulder deposit is at 3320 m, 120 m below the elevation of the EshaMcGee deposits. These scattered quartzite boulders rest upon the relatively horizontal
bedrock surface of Aggie-McGee Ridge which is smoothed as if having been overrun by
ice. This surface then was in contact with the bottom of the overriding ice mass. The
upper ice surface would have been many meters higher in elevation. There is no apparent
upper ice surface trimline along the adjacent north slopes of Mt Aggie, though a faint
lower scour-line projects across from the surface of Aggie-McGee Ridge. These Mt
Aggie north shoulder deposits are glacier left lateral.
The McGee Mt Southeast Rim granodiorite boulder deposit sits in a protected location
above a bedrock outcrop at 3260 m. This small boulder field is an eastern outlier of the
South Boulder Ridge deposits. The surrounding slopes are too steep to retain surficial
deposits and another level protected site some twenty meters higher along the ridge
towards the McGee Mt summit shows no evidence of glacial deposits. The Southeast
Rim deposits are free of the metasedimentaries characteristic of McGee Creek Canyon
glacier-left ice. If the entire ice flow was across the McGee-Aggie complex then this site
would be right-lateral. If the glacier bifurcated, with some ice exiting the range through
the present McGee Creek Canyon outlet, then the Southeast Rim boulders would be midglacier.
Esha-McGee Upland
Looking north down onto the Esha-McGee Upland. The right hand portion of this old erosional surface is hidden behind
a ridge. Visible in the photo, slightly below center is the granodiorite boulder field. Mid-background is McGee Mt, SE
Rim Deposit shows as a light patch on the right. Horizontal white band is granodiorite covered South Boulder Ridge.
View of the Esha-McGee Boulder Deposit looking to the north towards McGee Mt. This small granodiorite boulder field
is perched at the top of a large chute which is undercutting the deposit. Boulders sit upon a colluvial surface covering
metasedimentary bedrock. Visible in the background is the McGee-Aggie complex. On McGee-Aggie skyline from right
to left; the SE Rim Deposit, South Boulder Ridge, SW Corner Hill, Aggie-McGee Ridge, Mt Aggie is a shadow
underneath Mt Morrison. This photo shows the three high deposits which define the ice surface; Esha-McGee Upland,
SE Rim, and Aggie North Shoulder. In between is McGee Creek Canyon
Closer view of Esha-McGee Upland boulders looking towards the southeast. Had these originated from a source on Mt
Morgan (N), then it would be expected that granodiorite boulders and pebbles would be scattered across the slopes above.
This boulder field appears to be remnants of a McGee Creek Canyon glacier right-lateral moraine. Boulders from this site
are actively working their way down the chute to the right.
Boulders at the Esha-McGee Upland site show typical characteristics of the Round Valley Peak granodiorite found in the
cirques up canyon; nonpegmatic, equigranular, with dark elliptical mafic inclusions.
The photo above left shows the Esha-McGee Upland. Esha Canyon is to the left below Mt Morgan (N).
Right hand photo shows Mt Aggie and Aggie-McGee Ridge extending to right. Both photos were taken
from McGee Mt summit, looking south, up McGee Creek Canyon.
Left; Looking across the Southeast Rim boulder deposit towards the Esha-McGee Upland.
granodiorite boulders originated in the cirques visible up canyon.
These
Right; Looking across what is possibly an old eroded whaleback towards the Aggie North Shoulder. This
linear sandstone form is aligned with what would have been the direction of ice flow. Light patch with
trees is granodiorite on South Boulder Ridge. Note the horizontal scour line across the north face of Mt
Aggie, level with the surface of Aggie-McGee Ridge.
Figure 2
Bedrock within the McGee Creek Canyon drainage.
Lithic Correlation of Deposits with Glaciation
Upper McGee Creek Canyon is underlain by both granitic and metasedimentary bedrock.
The canyon drains from north to south. Cirques in the eastern headwaters which would
feed into the right side of a McGee Creek Canyon glacier are situated on Round Valley
Peak Granodiorite. Bedrock in the western headwaters of McGee Creek Canyon, which
would feed into the left side of the glacier, is composed of both Round Valley Peak
Granodiorite and Paleozoic metasedimentaries of the Morrison pendant. The two cirques
lying between Mt Aggie and Mt Baldwin (Aggie-Baldwin Cirques N and S) have
headwalls of Round Valley Peak granodiorite, but are underlain by the quartzite rich
lower silicious member of the Convict Lake formation as mapped by Rinehart and Ross
(1964). These two cirques would be the last to feed into the left side of a glacier before
the ice flowed around Mt Aggie (which is composed of stratified brown hornfels) and
spilled across Aggie-McGee Ridge and the McGee Mt plateau.
From the distribution of rock types underlying the McGee Creek Canyon cirques, the
expected lithological composition of mid-canyon glacial till is that glacier-right deposits
will be derived from Round Valley Peak granodiorite. Debris deposited by glacier-left
ice will contain a mixture of granodiorite and metasedimentary lithics. Far left lateral
deposits should contain a significant quartzite component as well as prominently layered
hornfels with some granitics.
Except for a quartzite boulder deposit on the McGee Mt North Rim, all the McGee Till
deposits correspond to the expected pattern. On the east side of McGee Creek Canyon,
the Esha-McGee Upland boulders are far right lateral and composed of granodiorite. The
easternmost deposit on the McGee Mt block is the Southeast Rim boulder field which
appears to be exclusively granodiorite. Working westward along the South Boulder
Ridge, one finds an increasing metasedimentary component amongst the boulders
(Putnam 1962). At the Southwest Corner Hill (the juncture of South Boulder Ridge,
West Boulder Ridge, and Aggie-McGee Ridge) the number of quartzite boulders and
other metasedimentaries is quite noticeable (then slowly decreases downstream along the
West Boulder Ridge). Continuing across the glacial path along Aggie McGee Ridge are
found some enormous granodiorite boulders amongst a scattering of metasedimentaries.
Climbing up past the low saddle in Aggie-McGee Ridge there is a sharp transition from
granodiorite to metasedimentary boulders. Rinehart and Ross (1964) mapped this as the
southern termination of the McGee Mt deposits. Continuing south along Aggie-McGee
Ridge the number of boulders decreases, but there is still a scattering of
metasedimentaries, mostly quartzite, but completely absent of boulders of the typical
Round Valley Peak granodiorite, all the way up to the shoulder of Mt Aggie. Putnam
(1962) mapped these metasedimentary surficial deposits as McGee Till.
These deposits correlate with the expected pattern for a McGee Creek Canyon glacier
spilling across the McGee-Mt Aggie complex into Aggie-McGee Canyon. Because
Rinehart and Rosss (1964) mapped the Aggie-Baldwin Cirques as heading in a narrow
band of Round Valley Peak granodiorite, it seems that this rock type should be found in
the Aggie North Shoulder deposits. Along the southern Aggie-McGee Ridge are
numerous granitic pebbles which could be from this narrow extension of the pluton. At
Aggie Col the granodiorite as mapped by Rinehart and Ross, crosses the col as a dike and
does not show the typical appearance of RVP granodiorite. Being a very narrow terminal
extension of the pluton, it may have a slightly different composition and undoubtedly
experienced a different cooling history from the central plutonic mass. Whether the
granodiorite exposed in headwalls has a similar appearance is uncertain. Curry (1968)
stated that in the Convict Lake region, it was to be expected that morainal lithologic
mixtures should change between glacial advances as cirques eroded headward into
differing rock types. In an earlier time the cirque headwalls would mostly be within the
quartzite bearing lower Convict Lake formation, and granodiorite feeding into the glacier
would be from the exterior portion of the pluton. Future work here needs to focus on
comparing the composition of the McGee Till with differing lithologies in the AggieBaldwin cirques, particularly textural and compositional differences within the pluton.
Why the granitic debris on south Aggie-McGee Ridge is only found as pebbles, is an
interesting problem. These granitic pebbles are not shown on the glacial lithology maps
in this manuscript.
North Rim Quartzites
The boulder deposit on the McGee Mt northern bench is an anomaly. Rinehart and Ross
(1964) identified these quartzite erratics as originating from the lower silicious member
of the Convict Lake formation. If derived from a McGee Creek Canyon glacier, these
quartzite boulders would have originated within the Aggie-Baldwin Cirques (Curry
1968). In this case they should be glacier-left to any granodiorite boulders. But the
North Rim Quartzites lie glacier-right to significant granodiorite boulder fields along
Cinder Ridge, West Boulder Ridge, Aggie-McGee Ridge, and the western portion of
South Boulder Ridge. If these quartzite boulders came from the west as suggested by
Rinehart and Ross (1964) then it requires emplacement by a separate ice flow from that
which carried the granodiorite from the headwalls of upper McGee Creek Canyon.
Aggie-Morrison Canyon contains this quartzite but it is difficult to imagine how this
small drainage could generate enough ice to flow up onto the McGee Mt North Bench, or
why. Curry (1968) interpreted the quartzites as resting upon the basalt but possibly lying
stratigraphically under or concurrent with the tephra deposits, and suggested a separate
glacial advance for emplacement of this quartzite boulder field.
The presence of the quartzite boulders sitting on the North Bench contradicts
Blackwelder’s hypothesis for emplacement of the McGee Till. If the McGee Mt deposits
were derived from the upper McGee Creek Canyon drainage, and emplaced upon an
earlier landscape in which the drainage flowed across the McGee Mt plateau, then there
should be no significant granodiorite deposits to glacier-left of the quartzites.
Quartzite boulders scattered across basalt on North Bench. Quartzite outcrops above Aggie-Baldwin Cirque N.
EVOLUTION OF THE DRAINAGE
Currently the widely accepted interpretation, is that the early central Sierra Nevada was a
gentle upland surface of subdued relief (Christensen 1966, Gillespie 1991, Huber 1981,
see references in Clark, M.K., et al 2005). Extensional rifting in the Pliocene initiated the
breakup of the Sierra-Owens-White/Inyo plateau, with associated down dropping of the
Owens Valley block (Bachman 1978, Bateman and Wahrhaftig 1966, Phillips and
McIntosh 2000) and formation of an eastern Sierra Nevada range-front escarpment.
Within the Long Valley region this rifting was accompanied by extrusion of extensive
mafic volcanic flows, which along San Joaquin Ridge beheaded the upper San Joaquin
River, the headwaters of which became the upper Owens River drainage (Bailey et al
1976, Bailey 1989, Huber 1981).
These mafic extrusions were followed by rhyolitic volcanic activity resulting in
voluminous pyroclastic flows and construction of the Glass Mountain complex along the
Benton Range, bordering northeastern Long Valley (Bailey et al 1976, Bailey 1989, Metz
and Mahood 1985), possibly in response to development of the Sierra escarpment (Bailey
et al 1976). Silicic based volcanic activity continued south of Glass Mountain,
culminating in the massive eruption 760 ka (Bogaard 1995, Sarna-Wojcicki et al 2000) of
the Bishop Tuff and collapse of the Long Valley caldera floor (Bailey et al 1976, Bailey
1989, Metz and Mahood 1985). Possibly half the development of vertical range-front
relief of the eastern Sierra Nevada surrounding the Long Valley region appears to have
occurred in relation to, and since, the Bishop Tuff eruption (Gillespie 1991).
Before the Bishop Tuff eruption, the Sierra were extensively glaciated during the well
documented Sherwin glacial stage, considered to have a mean age of 820 ka (D. Clark et
al 2003, Gillespie and Zehfuss 2004, Kaufman et al 2004) based on estimates of
weathering before burial underneath the Bishop Tuff; of 40 ky (Sharp 1968, Sharp and
Glazner 1997), of 50 ky (Birkeland et al 1980), and a cosmogenic exposure before burial
of 53-67 ky (Nishiizumi et al 1989). These dates are in conflict with a paleo-magnetic
study of the Sherwin Till in which Easterbrook (1978, Gillespie 1982) evidently found
the deposits to have a normal magnetic polarization, placing the Sherwin deposits within
the Brunhes Normal Chron which began 15 ky before the Bishop Ash event (SarnaWojcicki et al 2000). The Sherwin Glaciation was preceded 100 ky earlier by deposition
of the “Old Red Till” of Lower Rock Creek (Sharp 1968, weathering estimate by
Birkland et al 1980). Presence of diamictons (including the McGee Till) in protected
highland sites on mountain passes and summit flats have been taken as evidence of earlier
glaciations (Gillespie 1982, Curry 1984, Phillips and McIntosh 2000, Brocklehurst et al
2002, Gillespie and Zehfuss 2004).
The 2.7 ma basalt layer (Dalrymple 1963, recalculated by Huber 1981) that caps the
northwestern portion of McGee Mt, is abruptly terminated along the northern escarpment
overlooking Long Valley, but does follow a decreasing elevation down the slope of the
northern portion of West Boulder Ridge on the McGee Mt northwest shoulder. This
suggests that the lava flowed into and across the site of the present McGee plateau
drainage outlet, across West Boulder Ridge, and into the Aggie-Morrison Canyon
drainage. Since the McGee Till rests upon this volcanic layer, this indicates that the
Aggie-Morrison Canyon predates emplacement of the McGee boulder deposits.
Both Blackwelder’s (1931) and Bailey’s (1989) explanations for emplacement of the till
require that it take place upon an upland surface before incision of McGee Creek Canyon.
For ice to travel from the McGee Creek Canyon headwaters (Blackwelder) or from Mt
Morgan (N) (Bailey), across the moraine bearing portions of the McGee Mt plateau
requires the ice to exit across the McGee Mt northern rim directly into Long Valley, to
the northwest onto Tobacco Flat, and to the west into Aggie-Morrison Canyon which
drains into Convict Canyon and Tobacco Flat. The topographic form of the McGee MtMt Aggie complex, as well as distribution and lithological composition of McGee Till
deposits, indicates that this occurred. For glacial ice to flow across Aggie-McGee Ridge
and the southwestern McGee Mt plateau, it follows that this was the course of the
preglacial McGee Creek drainage. If this was the case, then the present McGee Creek
Canyon outlet would not have existed during emplacement of the till, and the drainage
course would have been to the west of the present McGee Mt summit, emptying into
Aggie-Morrison Canyon/Tobacco Flat and then into the Convict Canyon drainage.
Elevated subdued surfaces of Mt Morgan (N), Esha-McGee Upland, and McGee Mt, can
be imagined as once having been continuous before incision of Esha and McGee Creek
Canyons. Upper McGee Creek Canyon is aligned and drains from south to north,
heading directly towards the McGee Mt plateau before abruptly making a right turn
above Buzztail Springs to exit the range-front through a deep canyon incision. This
“Buzztail Bend” could represent an elbow of capture, where the former northerly stream
course into the Convict system was hijacked by a drainage opening to the east because of
range-front faulting. McGee Creek exits the range-front where the northern Round
Valley (Wheeler Crest) fault terminates along the Hilton Creek fault (Rinehart and Ross
1964, Bailey 1989).
Blackwelder’s and Bailey’s explanation for emplacement of the McGee Till require that
deposition occurred before any significant incision of the current McGee Creek Canyon
outlet. The present three low spots along the Aggie-McGee/South Boulder Ridgeline are
very similar in elevation, with the saddle in Aggie-McGee Ridge being the lowest. If this
saddle represents the original drainage course for the McGee Creek basin which is much
larger than the Aggie-Morrison drainage, then the upper Aggie-Morrison Canyon would
have been a tributary of the early McGee Creek, and stream capture by abscension at
Buzztail Bend should have beheaded Aggie-Morrison canyon, capturing its upper
drainage.
Alternately, if the original McGee Creek path was across the McGee Mt plateau, then
why did such significant amounts of ice overrun Aggie-McGee Ridge to flow down into
Aggie-Morrison canyon? If the McGee plateau was the original drainage course for the
relatively large McGee Creek drainage, then there is no reason for the closely parallel but
very much smaller drainage of Aggie-McGee Canyon to be incised below the level of the
paleo-McGee Creek drainage, but distribution of the 2.7 ma basalt indicates that this was
the case before deposition of the till.
If the pre-McGee Till drainage was across McGee Mt, then this should also have been
the post glacial drainage. The lowest entry onto the McGee plateau is across South
Boulder Ridge between SW Corner Hill and Middle Hill. This saddle is an area of the
thickest till deposits. The west branch of the central plateau drainage that would have
been the expected course of a paleo-McGee Creek crossing South Boulder Ridge, heads
against the till deposits here where it is undercutting a headwall of boulders. If this were
the site of the postglacial drainage, then one would expect there to be here an erosional
channel, or a mass of stratified outwash debris, not a stack of granodiorite boulders with
little sand or silt matrix and no hint of stratification. There is no evidence of a postglacial stream across South Boulder Ridge.
Though one could argue that a recessional moraine across South Boulder Ridge blocked
the pre-glacial outlet, diverting the drainage to the east, the evidence strongly suggests
that the McGee Till was emplaced upon a drainage topography very similar to the current
landscape, after range-front faulting had established the present canyon outlet. If the
Buzztail Bend did result from abscension, this stream capture likely occurred in the
Pliocene before Aggie-Morrison Canyon was significantly developed, and before
emplacement of the McGee Till. The present McGee Creek Canyon outlet predates the
McGee Mt boulder deposits. This contradicts both Blackwelder’s and Bailey’s models
for deposition of the McGee Till. [One could argue that emplacement occurred when the
present drainage pattern was established but McGee Creek Canyon was very shallow
(Sharp 1965, Christensen 1966)]
ICE POOLING AND EXPLOITATION OF MCGEE PLATEAU BY LEFT
LATERAL PORTION OF MCGEE CREEK CANYON GLACIER
The distribution pattern of deposits on the McGee Mt plateau shows a central section of
granodiorite boulders with outlying deposits of metasedimentary rocks to the northeast
and southwest. Up canyon from McGee Mtn, much of the west-side walls of McGee
Creek Canyon are composed of metasedimentary rock while the east-side bedrock is
Round Valley Granodiorite. For a McGee Canyon glacier, left-lateral moraine debris
would be predominantly metasedimentary while right-lateral material would be
granodiorite. The erratic metasedimentaries on the north shoulder of Mt. Aggie would
correspond to left-lateral debris from the McGee Creek Canyon glacier flowing across the
McGee plateau (fig 3). The metasedimentary quartzite deposits northwest of the Tip Top
Prospect though are an anomaly since they would be glacier-right to the extensive
granodiorite deposits.
If the North Bench quartzite deposits are the result of the McGee Creek Canyon glacier,
their presence argues against that glacier following a preglacial stream course into the
Convict drainage. The only McGee Creek Canyon quartzite deposits mapped by
Rinehart and Ross (1964) are in the Aggie-Baldwin Cirques which would leave glacier
left-lateral deposits.
The North Bench quartzite deposits can be explained as remnants of an initial glacial
advance across the McGee Mt plateau. If the McGee Creek Canyon glacier were unable
to move its entire ice volume through the present day canyon outlet, then ice would back
up (fig. 4), raising the surface level until ice overflowed across the McGee-Aggie
complex. As the glacier exploited this new outlet across the McGee plateau, it would do
so with left-lateral ice carrying a burden of quartzite from the Aggie-Baldwin Cirques
(fig, 5).
Once ice flow began moving across the broad McGee Mt plateau and the adjoining
Aggie-McGee Ridge, flow would concentrate in those areas that were more easily
scoured and lowered by ice. If the southwest portion of the McGee plateau and the
adjoining ridge were lowered by erosion, then quartzite laden ice would be left to
stagnate on the outlying North Bench as ice flow increased across the lowered southwest
plateau and Aggie-McGee Ridge saddle. The initial exploratory ice flow across McGee
Mt would be left-lateral, but as this new outlet was scoured by glacial ice, then the flow
would increase and ice would be pulled from the central, granodiorite laden portion of the
glacier. This secondary ice stream accounts for the central granodiorite portion of the
McGee Till (fig 6).
This watershed breaching by glacial ice explains the landform and surficial deposits of
the McGee Mt complex, but the explanation in itself does not provide a cause for a
McGee Creek Canyon ice pool. Evidence indicates that McGee Creek Canyon drained
through its current outlet before deposition of the McGee Till. Ice pooling and watershed
breaching requires that glacial ice was unable to find adequate passage through the
McGee Creek Canyon outlet, causing the ice flow to detour across the McGee Mt
plateau.
Figure 3
Expected lithological content of a McGee Creek Canyon Glacier. Granitics of Aggie-Baldwin cirque
headwalls not included in these maps. See text for discussion.
Figure 4.
Obstruction of canyon outlet results in development of an ice pool which fills to the watershed divide.
Topography at this time would be somewhat different, showing less sculpture.
Figure 5.
Initial exploitation of McGee Mt plateau would be with quartzite laden left-lateral ice. Initially the
easiest passage for ice would be across South Boulder Ridge and the McGee Mt central plateau,
exiting over a low northern escarpment. At this time the McGee Mt central plateau outlet was
probably filled with basalt.
Figure 6.
Erosion of the narrow divide across Aggie-McGeee Ridge, as well as scouring and removal of the
basalt filling the central plateau outlet would focus an increasing ice flow in these regions, pulling in
granodiorite laden glacier-center ice. Quartzite laden ice on the North Bench would be left to
stagnate. These events could have extended over several glacial advances.
Figure 7
After 800 ky or more of erosion, the McGee Till is limited to ridgelines and protected flats. Note the
Esha-McGee Upland deposit at lower right, and a scattering of boulders north of Devonian Hill.
This map does not show the granitic pebbles scattered across the southern Aggie-McGee Ridge,
probably derived from the Aggie-Baldwin Cirques’ headwalls.
LONG VALLEY CALDERA COLLAPSE
Other than location, the McGee Till lacks any obvious characteristics to separate it from
the nearby Sherwin age glacial drift, which is considered to be much younger. Credible
explanations for the emplacement of the McGee Till have struggled with the dichotomy
of requiring a great length of time in which the McGee Mt range-front experienced nearly
one kilometer of vertical offset, and McGee Creek Canyon was incised 800 m into the
bedrock of a former landscape, … but through all this the surficial McGee Till deposits
somehow managed to escape obliteration.
The catastrophic Bishop Tuff eruption at 760 ka allows the possibility of fitting these
processes into a much shorter time frame, reducing the exposure of the till to weathering
and erosion. Previous explanations for the emplacement of the McGee Till do not take
into account that McGee Mt is directly adjacent to the southeastern Long Valley Caldera
boundary. During the Bishop Tuff eruption, it is likely that within a one week period,
McGee Creek experienced a base level reduction on the order of two kilometers (Bailey
et al 1976, Bailey 1989, Hildreth and Mahood 1986, Wilson and Hildreth 1997)
During the Bishop Tuff eruption, the eastern caldera floor was dropped 3 km (Bailey et al
1976, Bailey 1989). Although the southern ring fracture bounding the subsiding caldera
plate is 5 km from the McGee Creek Canyon outlet (Bailey 1989), slumping and rock
avalanching likely extended out into the Hilton Creek range-front embayment along the
Hilton Creek fault. Scalloped embayments along caldera margins can often be attributed
to collapsed sections of wall rock (Lipman 1984, 1997). The presence of the Hilton
Creek fault and the northern extension of the Round Valley (Wheeler Crest) fault along
the range-front would separate the southeastern caldera wall rock from the Sierra block,
promoting instability leading to the caving of this portion of the caldera rim. Gravity
measurements show a pronounced arm of gravity low extending into the Hilton Creek
embayment (Pakiser et al 1964, Jachens 1985, Carle 1988). Caldera infalling at the
mouth of McGee Creek Canyon would radically lower the base level of McGee Creek,
rapidly accelerating canyon incision.
Under these scenarios the upper age constraint upon the McGee Till is that it be
precaldera, allowing a possible Sherwin Glacial age designation. The Long Valley event
could be used to fit Blackwelder’s and Bailey’s hypotheses into a shorter time frame,
arguing that the precaldera McGee drainage originally flowed across the McGee Mt
plateau, before incision of the present canyon outlet which was created by the caldera
collapse. But this explanation is contradicted by the presence of Aggie-McGee Ridge.
Recent incision of a new McGee Creek Canyon outlet would have resulted in capture of
the Aggie-Morrison Canyon drainage. The over widened lower Aggie-Morrison Canyon,
distribution of basalts, and lithological distribution of till deposits on the sculpted
McGee-Aggie complex, argue that the McGee Till was deposited upon a landscape with a
drainage pattern essentially the same as today.
Development of a growing ice pool within McGee Creek Canyon, which spilled across
the McGee-Aggie complex into the Convict/Tobacco Flat drainage, provides an
explanation that matches the topography, and the bedrock and surficial rock types of the
McGee Creek Canyon area. Impoundment of an ice pool requires that the McGee Creek
Canyon outlet be unable to accommodate glacial ice flow. The present day outlet is a
deep U-shaped trough that easily allowed passage of the Tioga and Tahoe age glaciers
through the range front onto the Long Valley floor. The canyon here is not overly narrow
and constricted, and it is hard to imagine how in this landscape, ice flow could be
obstructed, even by massive landslides. But it is likely that the current McGee Creek
Canyon outlet is very different than that which existed during deposition of the McGee
Till.
The Hartley Springs fault is a northern analog to the Hilton Creek fault in that it shows
significant range-front offsetting but then seems to dissipate after entering the caldera.
Along the Hartley Springs fault, Tertiary San Joaquin Mountain complex andesites are
displaced by 450 m while the Bishop tuff is displaced by about 300 m [Some of the
range-front faulting here has been accommodated by another unnamed fault further to the
west towards Deadman Pass (Huber 1981)]. Christensen estimated that approximately
80% of the McGee Mt range-front escarpment along Hilton Creek fault was formed after
the McGee Glaciation (Christensen 1966, 1968, Huber 1981). Gillespie (1991)
hypothesized that tectonic activity along the central Sierra range-front increased in the
time period surrounding the Bishop Tuff eruption. It is possible that range-front tectonic
movements were suppressed during formation of the magma chamber, and then
accelerated immediately after the caldera collapse since the emptied magma reservoir
would not accommodate extensional stress, by dike intrusion into Hilton Creek fault (see
Bacon 1982, Gillespie 1991, discussion in Bursik and Sieh 1989).
M.M. Clark and Gillespie (1981, 1993) found that the Hilton Creek fault showed a
northward increasing amount of offset which peaked at the intersection with the McGee
Creek Canyon left-lateral moraine. They found the Tahoe age (60-130 ka) moraine to be
vertically displaced by 130 m. Dip along the fault is 55 degrees (M.M. Clark et al 1984).
M. Berry (1997) obtained a similar offset for the Tahoe age moraine and calculated
vertical slip rates at McGee Creek of 0.9-1.2mm/yr for the last 15-20 ky, 1.0-1.3 mm/yr
for the last 20-25 ky, and 0.9-2.0 mm/yr for the last 65-140 ky. Assuming uniform rates
of displacement (which Clark and Gillespie cautioned against), a slip rate of 1.0mm/yr
since the 760 ka Bishop tuff eruption would produce a vertical offset of 760 m. The
summit of McGee Mt rises 1,000 m above where McGee Creek crosses the Hilton Creek
fault.
Collapse of the caldera floor would have radically lowered the base level for McGee
Creek, resulting in significantly increased canyon incision. The McGee Till was most
likely deposited on a landscape of much more subdued relief with a much shallower
McGee Creek Canyon. Currently the McGee Mt SE rim deposits are situated some 800
m above the McGee Creek Canyon floor. During precaldera times this vertical reach
would have been much smaller, requiring a shallower ice pool to overspill across the
McGee Mt plateau than in the present landscape.
The McGee Creek Canyon empties into the Hilton Creek range-front embayment which
is underlain by glacial drift, and alluvium. The Hilton Creek fault separates the Sierra
block from the Hilton Creek embayment which is bounded on the south by exposures of
granitic plutons which make up the range-front (Rinehart and Ross 1964). A branch of
the northern termination of the Round Valley fault ends in the Hilton Creek embayment
(Rinehart and Ross 1964, Bailey 1989). Pakiser et al (1964) found a significant Bouger
gravity low extending from Long Valley up into the Hilton Creek embayment. Caldera
wall collapse could account for this gravity low within the embayment. The presence of
the Hilton Creek and northern Round Valley faults could have contributed to the
instability of this wall rock.
The McGee Creek, Long Valley floor terminal moraine complex is lacking in material
compared to Convict moraines, though both drainages are comparable in size (Sharp and
Glazner 1997). Caldera wall collapse could have taken more of the McGee Creek
Canyon outlet, removing all pre-collapse deposits and leaving a hole to be filled. In
contrast to the Hilton Creek embayment, the Convict range-front does not show a gravity
low (Pakiser et al 1964, Jachens and Roberts 1985, Carle 1988). This can be taken as
evidence of caldera wall collapse extending into the Hilton Creek embayment. Sharp and
Glazner explained this difference in moraine mass as being the result of more ice
overflowing from the upper San Joaquin ice field across the Sierra crest into Convict
Canyon, than into McGee Creek Canyon, with the resulting larger Convict glacier
generating a larger terminal moraine complex.
The Hilton Creek fault system predates the caldera collapse (Bailey et al 1976). Any
extension of the range-front past the Hilton Creek fault would probably have been lower
in elevation than the precaldera McGee Mt or Nevahbe Ridge which currently form the
range-front. It is unlikely that any such extension would have contributed to a greater
possibility of canyon obstruction by landslide. Inflation of the Long Valley magma
chamber could have raised the elevation of an extended McGee Creek Canyon, but it is
difficult to imagine this alone causing ice to back up enough to overspill into another
drainage. It also seems unlikely that a McGee Creek Canyon glacier could build up such
an extensive terminal moraine that the unconsolidated drift prevented further forward
motion of glacier ice.
If the McGee Till were deposited by the first glacial advance to utilize the present McGee
Creek Canyon outlet, then the unglaciated canyon could have been very much narrower,
and more easily obstructed by landslides. It seems that the most likely source for a
massive landslide would be collapse of the canyon walls bounding the current lower
reaches of McGee Creek Canyon, possibly induced by oversteepening from rapid canyon
incision resulting from uplift along the Hilton Creek fault. But the deepest and narrowest
canyon passage lies directly between two high till deposits; the Esha-McGee Upland
granodiorites, and the McGee Mt SE Rim granodiorites. These deposits define the
glacial ice surface. Presence of the till deposits on highland flats precludes the possibility
that these surfaces have been lowered, since surficial erosion would have removed the
deposits. So the precaldera canyon walls could have been little higher than these old
erosional upland surfaces. Landslide deposits lie lower than their source. Even the most
massive canyon wall collapse would not have blocked the outlet to an elevation as high
as the uppermost canyon walls.
Although the catastrophic nature of the Long Valley volcanic event would hasten the
post-McGee Till canyon incision required of Blackwelder’s and Bailey’s hypotheses,
topography and lithological composition of surficial deposits indicate that canyon
incision preceded deposition of the till. Evidence on the ground supports the presence of
an overflowing ice pool within the canyon. None of the possibilities discussed so far
provide a satisfactory mechanism for the origin of a McGee Creek Canyon ice pool.
LAVA DAM HYPOTHESIS FOR THE MCGEE TILL
The Bishop Tuff event was initiated by a towering Plinian style eruption from a single
vent source, later accompanied by long fountains of ash issuing from spreading ring
fracture vents as the caldera floor fractured and subsided into the emptying magma
chamber (Hildreth and Mahood 1986, Wilson and Hildreth 1997). A great quantity of
country rock was entrained within the resulting ashfall and pyroclastic flows which
billowed across the surrounding landscape. Much of these entrained rock fragments were
ripped loose from bedrock surrounding the volcanic vents. By careful examination of the
lithic inclusions found within the early ashfall deposits and their comparison with caldera
wallrock, Hildreth and Mahood (1986) were able to place the location of the eruption
initiating Plinian phase vent. The most critical rock types used to restrict the location of
the vent were entrained fragments of lower Convict Formation quartzite, and Wheeler
Crest quartz monzonite, as mapped by Rinehart and Ross (1964). The possible area for
this Plinian phase vent location was mapped as being along and to the west of the
branching Hilton Creek fault complex which extends from the northern base of McGee
Mt out into the Long Valley caldera.
The Bishop Tuff eruption was an event of enormous magnitude, expelling some 600
cubic kilometers of volcanic rock and ash, most of which ponded within the subsiding
caldera (Bailey et al 1976). Measurable ashfall deposits blanketed western North
America as far east as Nebraska (Izzett 1970). Pyroclastic flows buried the northern
Owens Valley under a hundred meter-plus thick blanket of volcanic ash, creating the
Volcanic Tablelands which lie between Long Valley and the present town of Bishop.
Ash flows extended to the north towards Mono Lake and west through the Sierra Nevada,
traveling down the San Joaquin River drainage (Bailey et al 1976, Bailey 1989). Yet the
volcano which initiated this event is gone, all remnants of which were either blasted
across the countryside or consumed in the ensuing caldera collapse when the roof of the
emptied magma chamber foundered and fell back into the earth. Assuming that there was
a precaldera “Long Valley Volcano” at the site of the Plinian vent, how large could it
have been?
Large calderas are likely to have been the site of a volcanic field with a diffuse vent
pattern above the large, shallow magma chamber (Bacon 1985, Lipman 1984). Rhyolitic
volcanism was initiated on the present northeastern Long Valley caldera boundary with
the Glass Mountain volcanic complex, possibly in response to development of the Sierra
escarpment (Bailey 1976). A minimum of 4.8 cubic km of high silica rhyolite was
erupted between 2.13 and 1.20 Ma, and a minimum of 10.5 cubic km of lavas erupted in
a second episode between 1.10 and 0.79 Ma (Metz and Mahood 1985). Ash fall
attributed to the Glass Mountain eruptions has been found as far away as Beaver County
Utah and Ventura County California (Izzet 1981, Izzet et al 1988). Bacon (1985) stated
that the distribution of tuff and ash flows suggested the Glass Mountain field of rhyolitic
vents was at least 11 x 12 km. The southwestern portion of Glass Mountain, estimated at
10-20 cubic km (Hildreth 2004), was lost into the caldera collapse (Bailey et al 1976,
Metz and Mahood 1985. The precaldera volcano could have been taller.
At an elevation of 3390 m, Glass Mountain stands some 1200 m above the caldera floor,
taller than McGee Mt at 3318 m. Pakiser (1964) and Bailey et al (1976) found little
evidence of recent faulting along the southern base of Glass Mountain, so this elevation
can be assumed to pre-date the caldera collapse, unlike the McGee Mt escarpment which
has evidently undergone considerable offset since the Bishop Tuff eruption. If an
equivalent volcano were placed upon the intersection of the caldera ring fracture with the
Hilton Creek fault, it would match or exceed the height of McGee Mt. But at the time of
the caldera forming event, the McGee Mt escarpment was probably only half its present
height, or less. Such a large stratovolcano would have dwarfed the precaldera McGee
Mt, and it’s not unreasonable that its footprint could encompass the precaldera McGee
Creek outlet.
The extensive Glass Mountain complex is the result of over a million years of eruptions
from multiple vents (Metz and Mahood 1985, Metz and Bailey 1993). Wilson and
Hildreth (1997) found that part of the initial ignimbrite flow (Ig1Ea) of the Bishop Tuff
eruption followed the current path of the Owens River. This ignimbrite did not entrain
the rhyolitic lithics characteristic of flows which were emplaced to the east and northeast,
and which crossed the Glass Mountain volcanoclastic fan. Pakiser (1964) suggested that
gravity measurements showed a wide trough which was the paleo-Owens River drainage
under where the river presently now flows. Sherwin age glacial outwash from Rock
Creek rests upon a 3.2 Ma basalt flow within the Owens Gorge (Putnam 1960, Dalrymple
1963, Sharp 1968). This outwash is buried under Bishop Tuff. The long series of
eruptions which would have been needed to build a massive stratovolcano upon the
Plinian vent site should have left evidence, which doesn’t seem to be present.
Construction of such a volcano should have emplaced a significant tephra apron upon (if
not also underneath) the Sherwin outwash within the Owens Gorge. (One could argue
here that such deposits were emplaced but subsequently eroded by the Owens River and
Rock Creek.)
Metz and Mahood (1985) found that many of the Glass Mountain lavas were more
evolved than the Bishop Tuff, and interpreted this to indicate that the caldera forming
event was not the result of magma differentiation developing to a critical state. They also
suggested that a 60 ky gap between the last Glass Mountain eruption and the Bishop Tuff
event showed that pressure release in the magma chamber from a Glass Mountain vent
did not initiate the Bishop Tuff eruption [Sarna-Wojcicki et al’s (2000) dating the Bishop
Ash at 760 ka reduces this time gap to 30 ky]. The Hilton Creek fault predates the Long
Valley caldera (Bailey et al 1976, Hildreth 2004). Gravity studies show that the Hilton
Creek fault extends across the Long Valley caldera floor, and that west of the fault the
floor is down-dropped two kilometers, and three kilometers to the east (Pakiser 1964,
Bailey et al 1976). It is uncertain how much of this offset occurred previous to or during
the subsidence of the magma chamber roof (Bailey et al 1976). Metz and Mahood (1985)
suggested that the Bishop Tuff eruption was tectonically triggered by rupture along the
Hilton Creek-Hartley Springs fault zone. This scenario does not require a long history of
volcanic activity at the initial vent site.
Although the probable location of the Plinian vent was along the Hilton Creek fault to the
north or northwest of McGee Mountain, it is likely that other volcanic vents were situated
along the fault where magma was able to work its way to the surface. Large calderas are
usually the former sites of a volcanic field (Lipman 1984), with a diffuse, radial or
arcuate pattern of vents (Bacon 1985). The Sierra Nevada microplate is moving in a
northwest direction (Wernicke and Snow 1998) and the orientation of the Hilton Creek
fault provides a releasing geometry (Unruh et al 2003), creating an extensional
environment at the mouth of McGee Creek Canyon. Although the McGee Creek Canyon
outlet lies five kilometers south of the caldera ring fracture as mapped by Bailey (1989),
volcanic activity could have extended this far along the Hilton Creek fault. The Hilton
Creek fault, and possibly the presence of a northern termination of the Round Valley fault
would structurally weaken the crust here allowing passage of extrusive magmas into the
southern portion of the Long Valley pull-apart basin (see discussion in Bursik and Sieh
1989).
The precaldera range-front near McGee Creek Canyon had a much lower vertical relief
than the present day escarpment. A pre-Hilton Creek embayment volcano would need to
reach a height no more than half of the present day range-front to provide an obstruction
sufficiently tall enough to divert ice flow across the McGee Mt plateau. Before creation
of the Hilton Creek embayment, this area could have shown bedrock exposures of
moderate relief, providing a raised foundation for a volcanic cone to build upon. If the
precaldera McGee Creek Canyon extended across or along the Hilton Creek fault before
opening into the paleo-Long Valley, then volcanic activity on the fault would occur in a
narrow confined space, preventing the spreading of lavas and tephra, and facilitating the
buildup of an obstruction within the canyon.
It was previously discussed how caldera wall collapse could have created the Hilton
Creek embayment as indicated by fault geometry and gravity measurements. Formation
of this embayment by mass wasting would have removed any volcanic obstruction and
associated moraine material in the mouth of McGee Creek Canyon. There is no hard
evidence for a volcanic buildup within the Hilton Creek embayment. Whatever was once
there, now lies buried with the caldera collapse breccia underneath a kilometer or more of
volcanic ash, glacial drift, and alluvium. All that remains is the debris and sculpted
landscape left behind by a misplaced glacier. Much of the evidence cited in this
argument could be called circumstantial, but a volcanic dam hypothesis, though very
speculative, is the only explanation proffered so far that fits the observed facts.
A POSSIBLE HISTORY OF EMPLACEMENT FOR THE MCGEE TILL
The early McGee Creek drained an old elevated Sierra Nevada landscape of subdued
relief. The drainage outflow likely crossed the McGee Mt block. The land between Mt
Morgan (N) and McGee Mt would have been one continuous surface.
During Pliocene breakup of the Sierra-Owens-Inyo-White Mt block, mafic magmas
exploited the weakness of the resulting fault lines. The main drainage outlet for the
paleo-Long Valley was filled with flood basalts. Rifting, that would result in formation
of the McGee Mt northeast range-front escarpment above Tobacco Flats, allowed basaltic
andesite to flow across the present McGee Mt north bench, down onto the McGee Mt
central plateau, and into lower Aggie-Morrison Canyon. This andesite-basalt flow was
followed by a tephra eruption from several vents that covered the northwest portion of
McGee Mt with cinders.
Faulting enhanced a low range-front escarpment above Tobacco Flats, separating McGee
Mt from much of the volcanic flows on the northwest. Along the northeast side of the
McGee Mt block, offset on the Hilton Creek fault encouraged headward erosion into the
developing range-front, resulting in the capture of McGee Creek and beheading the
original gentle drainage which crossed the McGee Mt block. Rejuvenated by the baselevel changes resulting from continued fault offset, McGee Creek rapidly entrenched
itself through its new outlet channel, exiting through the hills along the base of the
growing range-front.
To the north across the precaldera Long Valley, silica rich magma formed a magma
chamber along the southern Benton Range. The resulting rhyolitic eruptions birthed a
chain of volcanoes. Glass Mountain dominated this early Long Valley volcanic field.
Pyroclastic flows billowed out many kilometers from this stratovolcano and winds
carried the ashfall far to the east.
The growing magma chamber pushed further to the south. Silica rich melt intruded the
northern Hilton Creek fault complex underneath the valley floor, extending the volcanic
field to the Sierra Nevada range-front. A vent developed north of McGee Mt.
Associated vents extruded rhyolitic lavas and ash south along the Hilton Creek fault,
extending the footprint of this volcano complex into the mouth of McGee Creek Canyon.
At this time the range-front escarpment would have grown to somewhat less than half its
current height, though with McGee Mt this would be hard to judge because of the
volcanoes nestled along its sides.
Early landscape before initiation of volcanic activity in southern paleo-Long Valley
Rhyolitic volcanic activity along southern ring fracture and Hilton Creek fault.
Although glaciation could have started earlier in the Sierra Nevada, by 900 ka, glacial ice
had left till deposits below the mouth of Rock Creek Canyon. This Old Red Till was
followed some 100 ky later by the Sherwin advance which dumped extensive deposits of
drift. Outwash from the Rock Creek, Sherwin age glacier spread granitic boulders out
onto the earlier basalts along the paleo-Owens River valley floor.
During one of these climatic episodes, the glacial ice flow in McGee Creek Canyon was
obstructed by volcanic activity within the canyon mouth on Hilton Creek fault. Unable to
exit the canyon the ice backed up creating a great ice-pool which filled McGee Creek
Canyon to the brim, leaving granodiorite boulders high upon the Esha-McGee Upland on
the southeast side of the canyon. When the ice-pool surface reached the elevation of the
old Pliocene paleo-channel it began to spill left-lateral ice across the broad Mt AggieMcGee plateau. At this time the two cirques underneath Mt Baldwin which lie
immediately south of Mt Aggie headwalled in massive quartzite deposits of the lower
Convict Lake formation. These Aggie-Baldwin Cirques fed the quartzite rich left-lateral
ice into the ice-pool which exploited the pathway across the McGee Mt plateau. Ice flow
was across the entire plateau and emptied over the northern escarpment to mingle with
the Convict glacier ice accumulating drift against the base of a volcano.
Obstruction of McGee Creek Canyon glacier resulted in diversion of ice flow across the McGee-Aggie
complex, spilling over the northern McGee Mt escarpment.
The overspilling glacier scoured out the basalt which had filled the McGee plateau
drainage and also downcut the saddle which lay between Mt Aggie and McGee Mt,
finding this narrower path across Aggie-McGee Ridge easier to erode. An increasing
portion of the overflow was now exiting through the lower Aggie-Morrison Canyon
which was scoured and widened by the ice. At some point this lowering of the ice
pathways across Aggie-McGee Ridge and the southwestern McGee Mt plateau channeled
the ice flow to the south of Cinder Ridge, leaving ice to stagnate and abandon its
quartzite burden on the McGee Mt North Bench. As this watershed breaching continued
and outlets were scoured, the volume of ice flow increased which pulled granodiorite
laden ice from the ice-pool into the glacial stream across western McGee Mt and northern
Aggie McGee Ridge to exit down onto the Convict Canyon glacier. It is entirely possible
that this sequence of events could have extended through several glacial epochs, which
could correlate the North Bench Quartzites with Sharp’s Old Red Till or earlier, and the
McGee Mt granodiorite deposits with the Sherwin advance.
Downcutting of Aggie-McGee Ridge, South Boulder Ridge, and the central plateau outlet focuses and
increases the ice flow. McGee Mt North Bench is abandoned by glacier.
With the waning of the Sherwin glacial period, summers were warmer, and ablation
increased on the McGee Creek Canyon ice-pool which developed a thick blanket of
debris. With lowering of the ice-pool surface, the last ice to flow onto the McGee Mt
plateau was heavily burdened with rock material. Thick moraine deposits were banked
onto this glacial spillway by the retreating ice. After melting of the ice, these thick till
deposits would have been dusted with ashfall from the adjacent and increasingly active
volcanos.
760,000 years ago a major earthquake wrenched the Long Valley region. A sudden shift
along the Hilton Creek fault ruptured the roof above the magma chamber. This vent
began erupting voluminous quantities of light colored pumice, then let loose with an
enormous Plinian outburst which sent a towering column of ash to the lower stratosphere.
Fragments of the basement rock surrounding this vent were ripped loose and ejected by
the violent passage of expanding magma. McGee Mt was blanketed with ashfall. While
the Plinian phase of the eruption continued, associated ground hugging ignimbrite flows
raced down the paleo-Owens River drainage. Further ignimbrites billowed across these
deposits, burying the eastern and southeastern landscape. Situated within a basin, the
volcanic vent sometimes choked and gargled on the products of its eruption, increasing
the frequency of these lateral pyroclastic flows. After several days of eruption the
pressure within the underlying magma chamber subsided. The roof of the chamber which
had previously been uplifted along a surrounding ring-fracture system, now began to
subside along these very same faults. Shifting of the crust along this ring-fracture system
allowed more magma to escape. Like popping the cork on a champagne bottle, the
integral failure of the magma chamber roof caused a sudden release in confining pressure,
allowing gases to expand in a cascading cycle of eruptive violence. Great fissures,
kilometers in length, opened to the east and north of the Plinian vent. Huge fountains of
exploding magma poured forth as long vertical curtains of ash which spread out as
ignimbrite flows.
As the eastern caldera floor foundered and fell back three kilometers into the emptying
magma chamber, weaker sections of wall rock, which surrounded the 12 km by 22 km
collapse plate, failed and caved into the caldron as collapse breccias. The volcanic dam
at the mouth of McGee Creek Canyon was part of this caldera wall failure. Separated
from the Sierra basement rock by the Hilton Creek fault and possibly the northern
terminus of the Round Valley fault, and weakened by pressure from intrusive magmas,
the caldera wall rock at the mouth of McGee Creek Canyon gave way. The resulting
mass wasting event emptied the mouth of McGee Creek Canyon, including the volcanic
dam, into the caldera, creating the Hilton Creek range-front embayment.
The dropping of the caldera floor and accelerated dip slip faulting along the range-front
rejuvenated McGee Creek which increased incision of the canyon bottom. Later glacial
episodes scoured and widened the lower canyon. Impoundment of Lake Long Valley
hastened the infilling of the caldera with alluvium since sediments weren’t exiting the
valley through agency of the Owens River.
Caldera collapse removed the volcanic dam and radically lowered the base level of McGee Creek.
Mid-Pleistocene glaciation with Lake Long Valley. The lake waters enhanced deposition of sediments and
infilling of the caldera basin.
On McGee Mt, the drainage outlet to the central plateau was clearing itself of glacial
debris. This central drainage system slowly eroded headward through the till deposits as
climatic cycles waxed and waned. Eventually most of the overburden of glacial till upon
the McGee Mt plateau was removed through the northwest drainage outlet. More till was
removed as the steep sided margins of the mountain block slowly eroded inward,
undercutting the edges of the upland surface above. A large avalanche gulley on the
northern escarpment began to undercut the mountain’s basalt north bench and its
quartzite boulder field. Growth of a small cirque on the north side of Mt Aggie, undercut
and narrowed the Aggie-McGee Ridge. During colder climate cycles accumulating snow
scoured nivation hollows on north facing slopes on the McGee Mt plateau. Earthquakes
shook loose any boulders that weren’t resting upon a level surface. Over thousands of
years the weaker pelitic and calcareous metasedimentary boulders within the till
weathered away, leaving the more resistant granitics and silicic metasedimentaries.
Eventually all that remained was a weathered till, capping ridgelines and flat benches
receiving little runoff from upper slopes.
Description of the Bishop Ash eruption from; Bailey et al 1976, Bailey 1989, Bailey et al
1989, Hildreth and Mahood 1986, Wilson and Hildreth 1997, Hildreth 2004.
POSSIBLE EXPLANATIONS FOR THE MCGEE TILL
NON-GLACIAL ORIGIN
McGee Mt boulder deposit consists of corestones, resulting from spheroidal
weathering of a granitic pluton. (JP Schaffer 1997)
PRO
 The McGee Till is mostly granodiorite boulders.
CON
o No exposures of obvious Round Valley Peak granodiorite bedrock on McGee Mt.
o McGee Till boulder arrangement shows no alignment along jointing planes.
o Lack of contact metamorphism in bedrock adjacent to granodiorite boulders.
o Contradicted by granodiorite erratics resting upon strata near a sandstone hilltop
along the South Boulder Ridge.
o Contradicted by large upright granodiorite boulder resting upon basalt with no
granodiorite source upslope. This is the easternmost boulder of the Cinder Ridge
deposit.
o No explanation for metasedimentary boulders resting upon granodiorite along
West Boulder Ridge.
o No explaining power for separate deposit of quartzite erratics on North Rim.
o No explanation for switch from granodiorite to metasedimentary boulders heading
south on Aggie-McGee Ridge.
o Near the NW volcanic vent, the very clean straight-line boundary between
reddish basalt/cinder and granodiorite deposits supports glacial origin- ice cutting
across volcanics, depositing granitics on tephra, not tephra overlapping granitic
boulder field.
o No explaining power for Esha-McGee upland granodiorite boulders.
o This hypothesis requires the presence of an exposed pluton, several kilometers
across, which went unrecognized by every miner and geologist who has worked
on McGee Mt for the last eighty years. (see Wise 1996, Greene and Stevens 2002)
McGee boulder deposit is the remnants of a mudflow. (considered and provisionally
rejected by Eliot Blackwelder 1931)
PRO
 Allows emplacement of “tilloid” before region uplifted above ELA.
CON
o Sources of granodiorite are several miles away with little intervening slope.
o Little explaining power for separate deposit of quartzite boulders on North Rim.
o Little explaining power for quartzite boulders on Mt Aggie N shoulder.
o Requires a very long period of time, in which the canyon was cut 800 meters
through a new outlet but the “tilloid” managed to survive.
GLACIAL ORIGIN
McGee boulder deposit is a glacial “tilloid” deposited by ice which flowed from
current head of drainage, and across the present McGee Mountain plateau before
McGee Creek Canyon was incised. (Eliot Blackwelder 1931, with reservations)
PRO
 Consistent with location of RVP granodiorite based cirques along Sierra crest.
 Corresponds to directional trend of drainages in area.
CON
o Requires a very long period of time, in which the canyon was cut 800 meters
through a new outlet but the till deposit managed to survive.
o Sharp (1969) and Birkeland and Janda (1971) found no significant evidence to
conclude that the till is older than Sherwin age.
o Evidence indicates that the current McGee Creek Canyon outlet predates the till.
o Little explaining power for separate deposit of quartzite boulders on North Rim.
McGee Mt granodiorite boulder deposits were derived from the head of the
drainage to the south, and carried and deposited by ice on the McGee Plateau
before incision of McGee Creek Canyon, while the North Rim quartzites (derived
from lower silicious hornfels member of the Convict Lake Formation) were carried
by ice from the west. (Rinehart and Ross 1964)
PRO
 Consistent with location of RVP granodiorite based cirques at head of drainage.
 Consistent with location of quartzite exposures in Aggie-Morrison Canyon.
 This explanation addresses the North Rim Quartzites.
CON
o Requires a very long period of time, in which the canyon was cut 800 meters
through a new outlet but the till deposit somehow managed to survive.
o Sharp (1969) and Birkeland and Janda (1971) found no significant evidence to
conclude that the till is older than Sherwin age.
o Evidence indicates that the current McGee Creek Canyon outlet predates the till.
o Uses two separate events to explain emplacement of McGee Till.
o Difficult to explain the emplacement of the North Rim quartzites, how did ice
from Aggie-Morrison Cyn get up onto the North Rim of McGee Mt?
McGee boulder deposits are a left lateral “alb” moraine, resulting from the McGee
Creek Canyon glacier banking up onto the McGee Mt Plateau as a result of the ice’s
momentum as the glacier followed the canyon’s turn to the right. (Lovejoy 1965)
PRO
 Allows for emplacement of till in more recent time, upon current landscape.
CON
o This explanation contradicts observed glacier behavior.
o No explaining power for right-lateral Esha-McGee Upland deposit (Christensen
1966).
o To obtain the elevation required to overflow McGee Mt, the ice would be too
deep to sustain itself. Internal shear would cause glacial ice to flow out the
canyon mouth into the valley (Christensen 1966).
o Quartzite boulders above Buzztail Bend, on Mt Aggie N shoulder are high to the
left and out of glacier’s path if momentum was carrying it straight down canyon..
McGee boulder deposit was derived from ice which flowed from Mount Morgan N,
across the present McGee Mountain. (Roy Bailey 1989, in map legend)
PRO
 Consistent with location of RVP granodiorite as a boulder source.
 Reasonable projection of a former landscape surface from McGee Mt to EshaMcGee Upland to Mt Morgan N.
 Possible right lateral source of quartzite in upper Esha Canyon (Convict Lake
Formation, from Greene and Stevens 2002) or in the material lost to MCC
incision?
 Accounts for Esha-McGee Upland granodiorite boulder deposit.
CON
o Requires a very long period of time, in which the canyon was cut 800 meters
through a new outlet but the till deposit managed to survive.
o Sharp (1969) and Birkeland and Janda (1971) found no significant evidence to
conclude that the till is older than Sherwin age.
o Emplacement would be before establishment of current McGee Creek and Esha
drainages. Evidence indicates that the current McGee Creek Canyon outlet
predates the till.
o Direction of transport is perpendicular to regional drainage trends.
o Difficult to explain the clean upslope termination of granodiorite boulders on the
Mt Morgan N side of the Esha-McGee Upland surface.
o Current landscape provides little clue as to the location of a cirque for this ice
flow.
o (some of these arguments also apply to the mudflow hypothesis using Mt Morgan
(N) as a material source).
INVOLVEMENT OF LONG VALLEY CALDERA COLLAPSE
“Because the Sherwin till is found at the base of fault scarps like those bordering
McGee Mountain, Blackwelder (1931) and all subsequent workers have
concluded that the Sherwin and McGee are separate glaciations, and that several
thousand feet of displacement occurred on the range-front fault in the interval
between them. Although this is probably so, one cannot really prove that the
Sherwin and McGee are separate glaciations.” (Bateman and Wahrhaftig 1966)
“The vertical separation of 3000 feet between McGee till and type Sherwin till
nearby suggests but does not prove they represent different glaciations.” (M.M.
Clark 1967)
“The McGee till is clearly much older than the deposits of Tahoe and younger age
in the canyons below, but its chronological relationship to the Sherwin is as yet
undemonstrated on any absolute basis. The semiquantitative data obtained are of
minimal value in differentiating the McGee from the type Sherwin at Rock
Creek.”…“The unusual topographic setting of the McGee is the principal reason
for regarding it as older than Sherwin.” (Sharp 1969)
“The postulate that the type Sherwin Till and the type McGee Till (Bateman and
Wahrhaftig 1966) record two separate glaciations is not supported by compelling
stratigraphic or physiographic evidence. They cannot be adequately differentiated
by soil profile development or by the semiquantitative methods of Sharp (1969).”
(Birkeland and Janda 1971)
The McGee Till is considered unique because of its isolated elevated position on
McGee Mt. Previous explanations have required lengthy periods of time for uplift of
the mountain block and incision of McGee Creek Canyon, giving the McGee till great
antiquity. Prehistoric uplift along the range-front is indistinguishable from
downdropping of the valley floor. The catastrophic Long Valley Caldera event at 760
ka allows the possibility of fitting these processes into a much shorter time frame. At
the time of the Bishop tuff eruption the range-front escarpment at the McGee Creek
Canyon outlet could have been half its current height, or less. It is possible that rangefront tectonic movements were accelerated in the time after the caldera collapse
(Gillespie 1991).
During the Bishop Tuff eruption, the eastern caldera floor was dropped 3 km. Although
the southern ring fracture bounding the subsiding caldera collapse plate is 5 km from
the MCC outlet, slumping and rock avalanching likely extended out into the Hilton
Creek range-front embayment along the Hilton Creek fault. Caldera infalling at the
mouth of MCC would radically lower the base level of McGee Creek, rapidly
accelerating canyon incision. Under these scenarios the upper age constraint upon the
McGee Till is that it be precaldera, allowing a possible Sherwin Glaciation age
designation.
McGee boulder deposits were transported by ice of a glacier heading in cirques on a
formerly much larger and taller McGee Mountain which was lost into the caldera
collapse.
PRO
 Possible source for quartzite as Convict Lake Formation outcrops on eastern
escarpment of McGee Mt. (Greene and Stevens’ map, not Rinehart and Ross)
 Shortened time that till was exposed to weathering and erosion, only required to
be precaldera.
CON
o No remnant of a RVP granodiorite pluton has been mapped on McGee Mountain.
o Would require the precaldera range-front to lie significantly east of the Hilton
Creek fault, not just foothills but an upward extension in elevation of the McGee
Mt block.
o Would require SW facing cirques close to the ELA.
o Contradicted by extensive granodiorite boulder deposits extending out onto
Aggie-McGee Ridge where such ice couldn’t flow along a catwalk.
o No explaining power for granodiorite boulders on Esha-McGee Upland.
o No explaining power for quartzite boulders on Mt Aggie north shoulder.
o Direction of ice flow is perpendicular to whale-back.
McGee drainage originally flowed across the McGee Mountain plateau before
incision of the present canyon outlet which was created by the caldera collapse.
PRO
 Allows for a more recent emplacement, shortening time deposits were exposed to
weathering and erosion, only required to be precaldera.
 Explains left lateral metasedimentaries on Aggie-McGee Ridge.
 Canyon bend at Buzztail Springs could be an elbow of capture.
 Northernmost extension of Round Valley fault could intersect the HCF near the
mouth of MCC, adding to crustal weakness at that location.
 MCC Long Valley floor terminal moraine complex is lacking mass relative to
Convict moraines, though both drainages are comparable in size (Sharp and
Glazner 1997). If caldera wall collapse opened the present MCC outlet, there
would be no deposits of Sherwin age or earlier. (Sharp explained this as the result
of more ice overflowing the Sierra crest into Convict)
CON
o Little explaining power for North Rim quartzites which lie glacier right to central
granodiorite deposits.
o Contradicted by presence of Aggie-McGee Ridge. Recent incision of a new MCC
outlet should have resulted in capture of Aggie-Morrison drainage. Since the low
point of spillover is on Aggie-McGee Ridge, this requires arguing that precaldera
drainage centered on McGee Mt plateau closely paralleling Aggie-Morrison Cyn,
and even though ice overran Aggie-McGee and West Boulder Ridges down
cutting Aggie-McGee Ridge to the level of or below the entry to the McGee
Plateau drainage, the McGee Creek and Aggie-Morrison drainages remained
separate.
McGee Creek Canyon outlet could not accommodate the ice flow, forming an ice
pool, resulting in ice spilling across the McGee Mountain plateau. Caldera collapse
later opened the McGee Creek Canyon outlet.
PRO











Ice pooling allows an explanation for North Rim quartzite erratics.
Explains left lateral metamorphics on Aggie-McGee Ridge.
Ice pooling accounts for right lateral Esha-McGee Upland deposits.
If the precaldera elevation difference between summit and canyon bottom was
greatly reduced, then blockage of MCC and overflow across McGee Mt would
not be nearly as difficult as in the current landscape.
Lower McGee Creek Canyon is deep and relatively narrow. A narrow passage is
more easily obstructed, and is evidence of more recent incision.
Precollapse canyon could have extended along or beyond Hilton Creek fault.
Inflation of the magma chamber could have raised the precaldera MCC outlet
elevation.
Precaldera tectonic movements could have induced massive landslides into the
canyon narrows.
Northernmost extension of Round Valley fault could intersect the HCF near the
mouth of MCC, adding to crustal weakness at that location.
MCC valley floor terminal moraine complex is lacking mass relative to Convict
moraines, though both drainages are comparable in size (Sharp and Glazner).
Caldera wall collapse could have taken more of the MCC outlet, removing all
precollapse deposits and leaving a hole to be filled. In contrast to the Hilton Creek
embayment, the Convict range-front does not show a gravity low. (Sharp
explained this as the result of more ice overflowing the Sierra crest into Convict)
Shortened time that till was exposed to weathering and erosion, only required to
be precaldera.
CON
o Requires a pre-established canyon outlet of a large drainage basin to be unable to
accommodate ice passage, or the glacier to obstruct its own outlet with morainal
material, to such a degree that ice was backed up and overflowed into another
drainage.
Massive landslides blocked the canyon outlet, causing the formation of an ice-pool
which overflowed across McGee Mt. Caldera collapse removed the blockage.
PRO
 Explains formation of ice-pool
CON
o The Esha-McGee Upland, and McGee Mt SE Rim Deposits which sit across
canyon from each other define the probable ice surface. The topography doesn’t
allow for a landslide source higher than these locations. A landslide cannot build
an obstruction which is above the source of the slide. (roller coaster physics)
Basalt flows originating on McGee Mt or within the canyon 2.7 Ma, obstructed the
canyon outlet forcing the glacier to detour across McGee Mt. Caldera collapse
removed the basalts.
PRO
 A 2.7 Ma basalt flow with probable vent source sits on the NE section of McGee
Mt.
 The incipient range-front would have been very subdued at this time.
CON
o It seems unlikely that such an obstruction could survive 1.5 My or so of stream
erosion, and so any rerouting of glacial deposits would have to occurred at a much
earlier time. This would contradict the soil weathering studies of Sharp (1969)
and Birkeland and Janda (1971) who found little to differentiate between the
McGee and Sherwin deposits (one could argue here that because of its elevation,
the McGee deposits weather more slowly).
o If a basaltic obstruction did reroute an early glacier, then the previous arguments
against a late stream capture would apply (unless McGee Creek incised a deep
narrow passage through the basalt mass which could not accommodate a glacier).
o Rhyolitic extrusives typically pile up, but mafic lavas are fluid and would be
likely to flow out the canyon mouth.
As a result of rhyolitic volcanic blockage, McGee Creek Canyon outlet could not
accommodate the ice flow, resulting in formation of an ice-pool and ice spilling
across the McGee Mountain plateau. Caldera collapse later opened the McGee
Creek Canyon outlet
PRO












Ice pooling allows an explanation for North Rim quartzite erratics.
Explains left lateral metamorphics on southern Aggie-McGee Ridge.
Ice pooling accounts for right lateral Esha-McGee Upland deposits.
If the precaldera elevation difference between summit and canyon bottom was
greatly reduced, then blockage of MCC and overflow across McGee Mt would
not be nearly as difficult as in the current landscape.
Lower McGee Creek Canyon is deep and relatively narrow. A narrow passage is
more easily obstructed, and would confine volcanic products allowing the
construction of a higher volcanic dam.
The vent for the Plinian phase of the Bishop Tuff eruption was adjacent to McGee
Mt, on or near Hilton Creek fault (Hildreth and Mahood 1986). The footprint of
the volcano could have extended to the mouth of a precaldera McGee Creek
Canyon.
Felsic volcanics extending beyond LVC ring fracture, on HCF would provide
explanation for blockage of MCC. Significant Long Valley precollapse volcanics
needn’t be limited to exact Plinian vent location.
Magma intrusion on HCF could contribute to destabilization and collapse of
caldera wallrock within the Hilton Creek embayment during caldera subsidence.
Northernmost extension of Round Valley fault could intersect the HCF near the
mouth of MCC, adding to crustal weakness at that location.
MCC valley floor moraine complex is lacking mass relative to Convict moraines,
though both drainages are comparable in size (Sharp and Glazner 1997). Caldera
wall collapse could have taken more of the MCC outlet, removing all precollapse
deposits and leaving a hole to be filled. In contrast to the Hilton Creek
embayment, the Convict range-front does not show a gravity low. (Sharp
explained this as the result of more ice overflowing the Sierra crest into Convict)
Precollapse MCC could have extended beyond or along the Hilton Creek fault out
toward the ring fracture.
Shortened time that till exposed to weathering and erosion, only required to be
precaldera.
CON
o Plinian vent location is mapped as north to northwest of McGee Mt, not to the
southeast at the mouth of the MCC.
o Bailey mapped the caldera boundary 5km north of MCC outlet, not extending into
the Hilton Creek embayment southeast of McGee Mt. Volcanic blockage requires
extracaldera volcanic activity extending along Hilton Creek fault outside of the
mapped ring fracture system.
o There is currently no evidence to be found of precollapse volcanics within the
Hilton Creek embayment.
Potential for Cosmogenic Dating of Quartzites
Dating of old boulders is limited by granular disintegration and spalling of the surface,
removing the mineral grains which contain the history recorded in cosmogenic nuclides.
Quartzite lacks the clay forming minerals of granitics which encourage weathering. The
Convict Lake formation quartzite within the McGee and Convict drainages may hold the
potential for a recoverable record of glaciation extending back close to a million years.
Comparison of quartzite and granodiorite boulders within the same moraine could allow a
cross correlation which helps refine the glacial chronology.
This manuscript has not been subject to peer review.
Acknowledgements
Early writings on the McGee Till were read and critiqued by Curtis Woodcock.
I feel very fortunate to have attended the 2003 INQUA Sierra Nevada Glaciation
fieldtrip. I learned a lot from the leaders and those in attendance. Especially helpful
were discussions about the McGee Till with Alan Gillespie, Malcolm Clark, and Robert
Curry. I am grateful for the encouragement and support offered by Bud Burke and Allan
James. Most helpful has been discussions with Malcolm Clark regarding the McGee Till
problem. I hope that by singling him out, the egregious errors that I’ve made within this
manuscript don’t reflect upon him. My mistakes are my own.
None of this would have been possible without my mother Jean Marie Bryant Stockton,
who taught me to look for evidence of past glaciers while in the high country.
BIBLIOGRAPHY
Bachman, S.B., 1978, Pliocene-Pleistocene breakup of the Sierra Nevada-White –Inyo
Mountains block and the formation of Owens Valley: Geology, volume 6, p. 461-463.
Bacon, C.R. 1982, Time-predictable bimodal volcanism in the Coso Range, California;
Geology, vol. 10, p. 65-69.
Bacon, C.R. 1985, Implications of Silicic Vent Patterns for the Presence of Large
Crustal Magma Chambers; Journal of Geophysical Research, vol. 90 (B13), p. 11,24311,252.
Bailey, Roy A., G. Brent Dalrymple, and Marvin A. Lanphere 1976, Volcanism,
Structure, and Geochronology of Long Valley Caldera, Mono County, California;
Journal of Geophysical Research, volume 81(5), p. 725-744.
Bailey, Roy A. 1989a, Geologic Map of the Long Valley Caldera, Mono-Inyo Craters
Volcanic Chain, and Vicinity, Eastern California; U.S. Geological Survey,
Miscellaneous Investigations Series, Map 1-1933.
Bailey, Roy A., C. Dan Miller, Kerry Sieh 1989b, Quaternary Volcanism of Long
Valley Caldera and Mono-Inyo Craters, Eastern California; Field Trip Guidebook T313.
American Geophysical Union.
Bateman, Paul C. and Clyde Wahrhaftig 1966, Geology of the Sierra Nevada; in, Edgar
H. Bailey editor, Geology of Northern California, Bulletin 190, California Division of
Mines and Geology, pages 107-172.
Berry, Margaret E. 1997, Geomorphic analysis of late quaternary faulting on Hilton
Creek, round Valley, and Coyote Warp faults, east-central Sierra Nevada, California,
USA; Geomorphology, 20(1-2), pages 177-195.
Birkeland, Peter W., and Richard J. Janda 1971, Clay minerology of soils developed
from Quaternary deposits of the eastern Sierra Nevada, California; Geological Society of
America Bulleitn, volume 82, pages 2495-2514.
Birkeland, P.W., R.M. Burke, and A.L. Walker 1980, Soils and subsurface rockweathering features of Sherwin and pre-Sherwin glacial deposits, eastern Sierra Nevada,
California; Geological Society of America Bulletin, vol. 91 (1), p. 238-244.
Blackwelder, Eliot 1931, Pleistocene Glaciation in the Sierra Nevada and Basin Ranges;
Geological Society of America Bulletin, volume 42, pages 865-922.
Bogaard, P. van den, and C. Schmick 1995, 40Ar/39Ar laser probe ages of Bishop tuff
quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United
States; Geology, volume 23, pages 759-762.
Brocklehurst, S.H., D.E. Granger, and K.X. Whipple, 2002, Implications of old,
glaciated surfaces at high elevations in the Sierra Nevada and Basin Ranges; EOS,
Transactions American Geophysical Union, 83 (47), Fall Meeting Supplement, Abstract
H22B-0884.
Bursik, M. and K. Sieh, 1989, Range Front Faulting and Volcanism in the Mono Basin,
Eastern California; Journal of Geophysical Research, vol. 94 (B11), p. 15,587-15,609.
Carle, S.F. 1988, Three dimensional gravity modeling of the geologic structure of Long
Valley Caldera; Journal of Geophysical Research, 93, (11), p. 13237-13250.
Christensen, M. N., 1966, Late Cenozoic crustal movements in the Sierra Nevada of
California: Geological Society of America Bulletin, vol. 77, p. 163-182.
Clark, Douglas H., Malcolm M. Clark, Alan R. Gillespie, and Bud Burke 2003,
Mountain glaciations of the Sierra Nevada: in Easterbrook, D.J. editor, Quaternary
Geology of the United States, INQUA 2003 Field Guide Volume, Desert Research
Institute, Reno, NV, p. 287-311.
Clark, M.K., G. Maheo, J. Saleeby, and K.A. Farley 2005, The non-equilibrium
landscape of the southern Sierra Nevada, California, GSA Today, vol. 15 (9), pp. 4-10.
Clark, M.M. 1967, Pleistocene glaciation of the drainage of the West Walker River,
Sierra Nevada, California. Ph.D. dissertation, Stanford University.
Clark, Malcolm M, and A.R. Gillespie 1981, Record of late Quaternary faulting along
the Hilton Creek Fault in the Sierra Nevada, Califonia [abs], Seismological Society of
America, Earthquake Notes, 52(1), page 46.
Clark, Malcolm M, and A.R. Gillespie 1993, Variations in late Quaternary behavior
along and among range-front faults of the Sierra Nevada, California (abstr.), Geological
Society of America Abstracts with Programs: 25, (5), 21
Clark, M.M., K.K. Harms, J.J. Lienkaemper, D.S. Harwood, K.R. Lajoie, J.C. Matti, J.A.
Perkins, M.J. Rymer, A.M. Sarna-Wojcicki, R.V. Sharp, J.D. Sims, J.C. Tinsley III, J.I.
Ziony, 1984, Preliminary slip-rate table and map of late-Quaternary faults of
California., U.S. Geological Survey Open-file Report, 84-106.
Curry, Robert R. 1968, Quaternary Climatic and Glacial History of the Sierra Nevada,
California, Ph.D. thesis, U.C. Berkeley.
Curry, Robert R., 1984, Mountain Summit Glacial Tills and Their Tectonic
Implications, Eastern Sierra Nevada, California; Geological Society of America Abstracts
with Programs, Reno, 16(6), p 481
Dalrymple, G. Brent 1963, Potassium-Argon Dates of Some Cenozoic Volcanic Rocks
of the Sierra Nevada, California; Geological Society of America Bulletin, volume 74,
pages 379-390.
Easterbrook, D.J. 1978, Paleomagnetism of Pleistocene tills and age of BrunhesMatuyama boundaries; Geological society of America Abstracts with Programs, volume
10, p. 394.
Fullerton, David S. 1986, Chronology and Correlation of Glacial Deposits in the Sierra
Nevada, California, in Quaternary Glaciations in the Northern Hemisphere, edited by V.
Sibrava, D.Q. Bowen, and G.M. Richmond, Quaternary Science Reviews, vol 5, pp. 161169.
Gillespie, Alan R., 1982, Quaternary Glaciation and Tectonism in the Southeastern
Sierra Nevada, Inyo County, California. Ph.D. thesis, California Institute of Technology
Gillespie, Alan R. 1991a, Quaternary Subsidence of Owens Valley, California; in,
Natural History of Eastern California and High Altitude Research, White Mountain
Research Station Symposium Volume 3, edited by Clarence A. Hall Jr., Victoria DoyleJones, and Babara Widawski. Regents of the University of California
Gillespie, Alan R., and Paul H. Zehfuss, 2004, Glaciations of the Sierra Nevada,
California, USA: in, Quaternary Glaciations-Extent and Chronology, Part II, editors,
J.Ehlers and P.L. Gibbard, Elsevier
Greene, David C., and Calvin H. Stevens, 2002, Geologic Map of Paleozoic Rocks in the
Mount Morrison Pendant, Eastern Sierra Nevada, California: California division of Mines
and Geology.
Hildreth, Wes, and Gail A. Mahood 1986, Ring-fracture eruption of the Bishop Tuff;
Geological Society of America Bulletin, volume 97, pages 396-403.
Hildreth, Wes 2004, Volcanological perspectives on Long Valley, Mammoth Mountain,
and Mono Craters: several contiguous but discrete systems, Journal of Volcanology and
Geothermal Research, vol. 136, pp 169-198.
Huber, N. King 1981, Amount and Timing of Late Cenozoic Uplift and Tilt of the
Central Sierra Nevada, California – Evidence from the Upper San Joaquin River Basin,
US Geological Survey Professional Paper 1197.
Izett, G.A. , Ray E. Wilcox, H.A. Powers, and G.A. Desborough 1970, The Bishop Ash
Bed, a Pleistocene Marker Bed in the Western United States; Quaternary Research, 1, p.
121-132.
Izett, Glen A. 1981, Volcanic Ash Beds: Recorders of Upper Cenozoic Silicic
Pyroclastic Volcanism in the Western United States, Journal of Geophysical Reearch,
volume 86 (B11), pp. 10200-1222.
Izett, G.A., J.D. Obradovich, and H.H. Mehnert 1988, The Bishop Ash Bed (Middle
Pleistocene) and Some Older (Pliocene and Pleistocene) Chemically and Mineralogically
Similar Ash Beds in California, Nevada, and Utah, US Geological Survey Bulletin 1675
Jachens, R.C., and C.W. Roberts, 1985, Temporal and Areal Gravity Investigations at
Long Valley Caldera, California; Journal of Geophysical Research, vol. 90, (B13), p.
11,210-11,218.
Kaufman, D.S., S.C. Porter, and A.R. Gillespie, 2004, Quaternary Alpine Glaciation in
Alaska, the Pacific Northwest, Sierra Nevada, and Hawaii, pp77-103: in A.R. Gillespie,
S.C. Porter, and B.F. Atwater editors, The Quaternary Period in the United States;
Elsevier
Lipman, Peter W. 1984, The Roots of Ash Flow Caldera in Western North America:
Windows Into the Tops of Granitic Batholiths; Journal of Geophysical Research, volume
89, (B10), p. 8801-8841.
Lipman, Peter W. 1997, Subsidence of ash-flow caldera: relation to caldera size and
magma-chamber geometry. Bulletin of Volcanology, 59, p. 198-218.
Lovejoy, E. M., 1965, Evidence of Plio-Pleistocene faulting at McGee Mountain, Mono
County, California – critical study (abstract): Geological Society of America,
Cordilleran Section, 61st Annual Meeting, Fresno, California, 1915, Program, p. 34.
Metz, J.M. and G.A. Mahood, 1985, Precursors to the Bishop Tuff Eruption: Glass
Mountain, Long Valley, California; Journal of Geophysical Research, volume 90 (B13),
p. 11,121-11,126.
Metz, Jenny M. and Roy A. Bailey, 1993, Geologic Map of Glass Mountain, Mono
County, California, US Geological Survey, Miscellaneous Investigations Series, Map I1995, scal 1:24,000.
Nishiizumi, K., Winterer, E.L., Kohl,C.P., Klein, J., Fink, D., Middleton, R., Lal, D., and
Arnold, J.R., 1989, In situ Be-10 and Al-26 in granitic rocks with glacially polished
surfaces; Journal of Geophysical Research, 94, p. 17907-17915.
Pakiser, L.C., 1964, A Gravity study of Long Valley; in, Rinehart, C. Dean, and Donald
C. Ross ; Geology and Mineral Deposits of the Mount Morrison Quadrangle, Sierra
Nevada, California; Geological Survey Professional Paper 385.
Putnam, William C. 1960, Origin of Rock Creek and Owens River Gorges Mono
County, California; University of California Publications in Geological Sciences, volume
34 (5), pages 221-280.
Putnam, William C. 1962, Late Cenozioc Geology of McGee Mountain Mono County,
California; University of California Publications in Geological Sciences, volume 40 (3),
pages 181-218.
Rinehart, C. Dean, and Donald C. Ross 1964, Geology and Mineral Deposits of the
Mount Morrison Quadrangle, Sierra Nevada, California; Geological Survey Professional
Paper 385.
Sarna-Wojcicki, A.M., M.S. Pringle, and J. Wijbrans, 2000, New 40Ar/39Ar age of the
Bishop Tuff from multiple sites and sediment rate calibration for the Matuyama-Brunhes
Boundary, Journal of Geophysical Research, volume 105 (B9), pp. 21,431-21,443.
Schaffer, Jeffrey P. 1997, The Geomorphic Evolution of the Yosemite Valley and
Sierra Nevada Landscapes, Solving the Riddles in the Rocks; Wilderness Press, Berkeley,
California.
Sharp, Robert P. 1965, Northern Great Basin and California, Guidebook for Field
Conference I, INQUA VII Congress: editors; C. Wahrhaftig, R.B. Morrison, P.W.
Birkeland
Sharp, Robert P. 1968, Sherwin Till-Bishop Tuff Geological Relationships, Sierra
Nevada, California; Geological Society of America Bulletin, volume 79, pages 351-364.
Sharp, Robert P. 1969, Semiquantitative Differentiation of Glacial Moraines Near
Convict Lake, Sierra Nevada, California; Journal of Geology, volume 77, p. 68-91.
Sharp, Robert P and Allen F. Glazner 1997, Geology Underfoot in Death Valley and
Owens Valley; Mountain Press Publishing, Missoula Montana.
Unruh, Jeffrey, James Humphrey, and Andrew Barron 2003, Transtensional model for
the Sierra Nevada frontal fault system, eastern California; Geology, volume 31 (4), pages
327-330.
Wernicke, Brian, and J. Kent Snow 1998, Cenozoic Tectonism in the Central Basin and
Range: Motion of the Sierran-Great Valley Block, International Geology Review, volume
40, pp403-410.
Wilson, Colin J. N., and Wes Hildreth 1997, The Bishop Tuff: New Insights from
Eruptive Stratigraphy, The Journal of Geology, volume 105, pages 407-439.
Wise, J.M., 1996, Structure and stratigraphy of the Convict Lake block, Mount
Morrison pendant, eastern Sierra Nevada, California [M.S. thesis]: Reno, University of
Nevada.