R. Franklin
Written Preliminary Exam
Presented to O. Davis
Introduction: Glacial activity is broadly synchronous across the continent of western North America. After the
ice of the last Pleistocene glaciation retreated approximately 18,000 – 10,000 years ago, the continent entered the
Holocene (interglacial), a period of variable climate and associated glacial activity. The Holocene covers the past
10,000 years and is marked by very warm post-glacial temperatures at 8 – 6 k years ago, termed the Holocene
climatic optimum or altithermal, with associated rises in treeline and disappearance of glaciers in many regions
and then a gradual decrease of temperature over the next 5 – 6 thousand years. The past 5000 – 3500 years are
termed the neoglacial, a term in part describing the readvance (though highly variable) in this time period of many
mountain glaciers (Denton and Karlen 1973).
The neoglacial period in western North America has been differentiated into three periods of glacial activity, the
early neoglacial, mid neoglacial and late neoglacial, commonly termed the Little Ice Age (LIA). I have broken the
geography of western North America into five climatically and geographically distinct mountainous regions in
order to address the patterns that vary on a regional scale and also to compare large broadly-synchronous patterns
that emerge. These regions are 1) Northern Rocky Mountains (Montana and British Columbia), 2) Southern
Rocky Mountains (Wyoming and Colorado Front Range), 3) the Coast Mountains (Pacific Coast ranges), 4)
Cascades (Oregon and Washington), and 5) the Sierra Nevada. I will first describe the advance sequences in each
region and then the glacial-advance patterns that are synchronous across the whole of western North America.
Glacial retreat immediately follows the lower age brackets for these advances, as the nature of a moraine (the
geomorphological unit used to bracket a glacial advance) implies subsequent glacial retreat.
In the Northern Rocky Mountains a glacial advance termed the Peyto Advance has been taken in part from the
dates of detrital logs and in situ stumps in glacial forefields at more than ten different glacier systems. This
advance, correlated with the Tiedemann Advance in western British Columbia, initiated at approximately 3100
BP and began retreat at 2500 BP (Luckman et al 1993). A readvance at c. 1800 – 1700 BP constituted a weak
“mid-neoglacial” advance. The earliest of the late neoglacial advances or “Cavell Advance” for British Columbia
began in AD 1150 – 1330 and then again from AD 1750 – 1850. This advance is the most extensive neoglacial
advance in the Canadian Rockies (ibid).
In the southern Rocky Mountains there are many well dated deposits and moraines in Wyoming and Colorado’s
Front Range. An advance predating the neoglacial and extending until 3000 – 3500 BP is tentatively termed
“Triple Lakes”. After that came the widespread Audubon Advance, which has been correlated across the Front
Range of Colorado, the La Sal Mountains of Utah, the Grand Tetons and the Wind River Range of Wyoming to
span from 2500 – 950 BP. The most recent, LIA, advance in the southern Rocky Mountains is termed Gannett
Peak (Wyoming) or Arapahoe Peak (Colorado) and occurred from 350 – 100 BP (Gillespie et al 2004).
The Coast Mountains had two separate periods of glacial advance and retreat, a mid-neoglacial advance 2500 –
1300 BP and LIA advances at AD 1700 and 1800 – 1900. The Cascade Range has well dated deposits that put an
early neoglacial advance between 3300 BP and 2200 BP based on different tephra dates. This advance is termed
the Burroughs Mountain Stade. The Cascade glaciers did not readvance past that position until their late
neoglacial LIA advance which culminated at multiple moraines retreated from AD 1550 – 1850 (Sigafoos &
Hendricks 1972).
In the Sierra Nevada of California an early neoglacial advance was recently identified as having initiated post3400 BP (based on rock-glacier lichens and low-OM lake sediments) and extended presumably to the period of
increased temperatures c. 2000 BP (Konrad & Clark 1998). In the intervening period between the early
neoglacial and the late neoglacial (Matthes for the Sierra Nevada) it is unknown if there was a second neoglacial
advance, as the Matthes advance overrode all material from the last 10,000 years. The Matthes advance in the
Sierra Nevada came in three stages: AD 1300 – 1400, 1550 – 1700 and 1800 – 1850 (Scuderi 1987).
The general trends for North America can be seen in figure 1 where three distinct glacial events take place over
the neoglacial period. The early neoglacial in the Rocky Mountains (N and S), Cascades and the Sierra Nevada at
3500 – 2500 BP, the mid-neoglacial in the Southern Rocky Mountains and Coast Mountains and the ubiquitous
and most extensive of the neoglacial, the late Neoglacial which initiates as early as AD 1200 in many locations
but is broadly synchronous from AD 1700 -1900. Since the end of the LIA retreat has continued until present.
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R. Franklin
Written Preliminary Exam
Presented to O. Davis
2. Methods: The primary methods of measuring glacial retreat occurring in the above-mentioned
neoglacial period were, for the early and mid-neoglacial advances/retreats, soil and rock weathering,
lake sediment varves, tephrachronology, lichenometry, radiocarbon dating of in situ stumps, exposed
previously buried logs and detrital material found in glacier forefields and for the late neoglacial,
dendrochronology, historical documentation, radiocarbon dating, lichenometry and tephrochronology
(see figure 2).
Rates of glacier retreat after an advance are measured by determining the furthest extent of a glacier
down-slope or down-valley (i.e. at time t) and comparing that measurement, over time, to another
measurement made of the position of the glacier terminus at time t+1. Usually this measurement is
made in meters or kilometers per year. This technique is more straightforward when measuring glacial
recession without interceding advances as the most recent glacial advance will “wipe the slate clean”
and destroy (in most cases) evidence of prior glacial activity be it advances or retreats. Dates are usually
taken from end moraines which indicate a glacier’s furthest extent to compare to other moraines or
referenced locations.
When a glacier recedes from its end moraine, it leaves a surface that can be colonized by vegetation
which can be dated to year of formation (in the case of tree species and some forb and shrub species).
Difficulties arise when the amount of time from the exposure (“ecesis time”) of the surface to the
establishment of vegetation (vascular or non-vascular plants in the case of lichens) is unknown. Studies
addressing this question (McCarthy and Luckman 1993) have found times varying from seven to 100
years between the exposure of a surface and establishment dates of tree species.
Using
herb/shrubchronology to derive time since retreat is ideal for areas that have suitable species as the
ecesis times for these species usually are far less than that of trees, dateable species such as Dryas spp
establish within 5 or so years of glacial retreat and can live upwards of 200 years (Schweingruber and
Poschlod 2005, Franklin unpublished data).
In some cases, such as at the Saskatchewan and Yoho Glaciers and Peyto and Robson Glaciers in
Alberta, Canada, wood is revealed by glacial termini retreating from a previous advance or is flushed out
of meltwater at a glacier terminus. This is a fortuitous event as the radiocarbon (usually these types of
detrital wood or tree stumps are too old to be dated by dendrochronological methods) dates derived from
once-standing trees mown down by an advancing glacier give a closely approximate date of the previous
advance into a standing forest. Thus, prior glacial advances can be dated even though subsequent
glacier advances obliterate any morainal evidence of the activity. However dates for mid and early
neoglacial advances usually carry an error associated with them of +/- 100 years (Luckman et al 1993).
Varve layers in lake sediment can indicate quick changes in glacial presence or activity by the
composition of the percent organic matter in their layers. These frequently are annually resolved
records.
The timing of some advances were constrained by tephra dates, which are dates given to volcanic ash
that can bracket a moraine that indicates a prehistoric glacial position. Although recent volcanic
eruptions and their associated tephras can be dated to the year, they lack the annual resolution to
precisely date a moraine or deposit to the year or decade even of formation. Tephra dates themselves
commonly are radiocarbon dated based surrounding stratigraphy so have the associated +/- 10% errors
of the radiocarbon method. Tephra dates are excellent however, for placing upper or lower limits on
deposits.
Aerial, repeat and historical photos have a similar issue with precision in that they are “snapshots”,
literally, of a single date and can be used as a reference point of to bracket an event as happening before
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R. Franklin
Written Preliminary Exam
Presented to O. Davis
or after the date the photograph was taken. This is an excellent dating technique when a series of
photographs with known dates of capture are available to compare to subsequent glacial positions e.g.
the historical and repeat photographs from Glacier National Park. However, photographs only span the
past century or so and are relatively rare compared with the more ubiquitous natural archives available at
a glacier forefield.
Lichenometry is used for determining the time a surface has been exposed by the colonization rates and
growth curves of various species of lichen at a site. The precision of this technique is quite variable and
is sometimes compromised as in the case of dating the Arapahoe Advance, where the supposed date of a
glacial retreat was used to calibrate the lichen growth curve that was then used to date other advances in
the same region. However, many areas such as the Sierra Nevada have independent lichen growth
curves that are accurate with a precision down to +/- 10 – 50 years. This is excellent control for mid
neoglacial advances.
Although palynology is used traditionally for climate and vegetative reconstructions it is definitely
worth mentioning in conjunction with dating Holocene glacier advances. The temperature and
precipitation fluctuations that accompany glacial activity are documented in the changing pollen
percentages of alpine lake sediments and can be used as supportive evidence for the timing of an
advance or retreat that parallels a change in vegetation. The precision of a palynological study varies
from dubdecadal in recent sediments (using Cesium and 210Pb dating) to multidecadal or centennial for
longer chronologies.
Soil profile development (and rock and mineral weathering) have a very high error associated with them,
on the order of 50 – 100% in some cases and have been interpreted incorrectly so as to provide wrong
dates for the ages of advances, usually providing too young of an age for an older deposit (i.e. dating
14,000 year old Recess Peak deposits as ~3000 year old neoglacial deposits in the Sierra Nevada and
dating ~8000 year old deposits of the front range of Colorado as ~3000 year old “Triple Lakes” deposits.
Table 2.Techniques used in dating Neoglacial glacier activity
Techniques ranked from high to low
Level of Precision when used over Neoglacial period
precision levels (some overlap)
Herbchronology
Dendrochronology
Tephrochronology
Aerial Photography (repeat photographs)
Varve Chronology (lake sediments)
Historical documents, photographs
Lichenometry
Palynology
Radiocarbon dating
Vegetation development
Rock Weathering
Soil Profile development
Can be annually resolved, short ecesis time but record length of 10 –
300 years maximum (“predictive” ability?)
Annually and seasonally resolved, longer ecesis times but millennial
scale records (“predictive” ability)
Variable, date explosion to year of formation, can use as age “bracket”
multimillennial records
Similar to tephra, date to year of photograph, can act as age “bracket”
recent- past century, scarce.
Annually and seasonally resolved, up to decadally resolved,
multimillennial length records
Variable, can bracket or point out events of interest recent, scarce, past
century +/High to mid level precision
Variable, in some cases can be annually resolved with 210Pb dating
but mainly multi-decadal or lower resolution (“predictive”?)
100s – 50,000 y record. Has +/- 10% resolution
Variable, can be more precise if exact ecesis rates are known
Highly variable, very high error- 50 – 100%
Highly Variable on order of 1000s of years, very high error 50 – 100%
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R. Franklin
Written Preliminary Exam
Presented to O. Davis
3. Creative answer:
Dates for glacial activity usually represent the position that a
glacier is at in an advanced state before it recedes. Thus, any
subsequent retreat can only be measured accurately until the
glacier starts advancing again. Then, any distance between
the terminus of the glacier and the position of the earlier
moraine does not represent the true distance needed to
calculate the rate of retreat for the glacier. For example, if
each subsequent advance in the southern Rocky Mountains
(see Figure 1 at right) extended further that the previous
stade, even if we know the absolute positions of the termini
of the “Triple Lakes”, Audubon and Arapahoe Peak
glaciations, we do not know how far up-valley the glacier
retreated before they readvanced. Because the distance is an
unknown in the rate = distance/time equation, rates between
glaciations cannot readily be calculated. A helpful discovery
would be in situ stumps as with the case of the Peyto and
Athabasca Glaciers but these only put time controls on the
rates of advancement. Using this method we can calculate
the rate for the current glacial retreat and previous glacial
advancement.
We can say that a rate of retreat is faster than a previous rate
of advancement, as in the case of the Peyto Glacier. The
distance from the ~3000 BP advance to the LIA termini was
gained in 3000 years as the glacier advanced but was
covered in 300 years as the glacier retreated. This
information is helpful in putting the recent global retreat in
perspective with previous glacial advance.
Figure 1
"Triple Lakes"
~3400 – 2900 BP
less OM in lake sediment
Audubon Advance
Wind river range, Teton, La Sal Mtns,
Colorado
~950 - 2500 BP
If the most recent “LIA” advance was the most extensive
over the period of the neoglacial then this means that it has
taken a shorter period of time to return to the position of
previous advancement. Then we can say that the retreat over
the past 300 years has been the most rapid in the neoglacial.
This then gives the early neoglacial retreat the slowest retreat
rates and the mid neoglacial falls somewhere between.
However, hypothetically we could experience an advance
that could return today’s glaciers to a down-slope position
that would then decrease the overall rate of retreat and
perhaps return it to a rate comparable with previous
neoglacial advances. Interestingly the error involved with
measurements of the early and mid neoglacial advances
(associated with tephrochronology, soil-profile development
and radiocarbon dates) is almost as large a time span as the
entire period of current glacial retreat from LIA positions.
Gannett Peak, Arapahoe Peak
~1700s – 1850
14 C
The rate we are measuring now, hundreds of meters over 100 – 300 years, actually falls within the error bars of
the dates for prior neoglacial retreat. This can give pause to making comparisons between rates of retreat over the
recent Holocene. However the positions of present glacier termini can be contrasted with positions of glacial
termini in the past (without rate comparisons) to assess forcings on current glacial mass balance compared with
those of the past.
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R. Franklin
Written Preliminary Exam
Presented to O. Davis
For example without knowing retreat and advance rates of glaciers such as Quelccaya, and Peyto we can say that
present positions have not been experienced for the past 5000 and 3000 years; and at the summit of Kilamanjaro
we can know that a glacier that has lasted for the past 11,000 years, through the early Holocene Hypsithermal is
now in a negative mass balance that will usher in its demise in this century. Even though these patterns are not as
quantifiable as possible, they do indicate unusual conditions globally for mountain glaciers.
Another interesting trend in figure 1 is that the recent LIA advance and subsequent retreat is much more
synchronous across the whole of western North America than the previous advances of the neoglacial. This may
in part be to do with the better dating control that dendrochronology affords over radiocarbon, soil development
and tephrochronology but it may be an actual effect of the climate system operating over North America.
Table 3
North American Retreat Sequences
Retreat after Late Neoglacial Arapahoe Peak
Advance
Relative Rates of Retreat
Fastest retreat
Retreat after Mid Neoglacial Audubon Advance
Retreat after Early Neoglacial “Triple Lakes”
advance
Slowest retreat
References:
Birkeland, P. W., Cyandell, D. R., and Richmond, G. M. 1971. Status of correlation of Quaternary
stratigraphic units in the western conterminous United States. Quaternary Research Vol. 1: 208227.
Calkin, P.E. Wiles, G.C. and Barclay, D.J. 2001. Holocene coastal glaciation of Alaska. Quaternary
Science Reviews. Vol 20 pp.449 – 461.
Clark, D.H. and Gillespie, A.R. 1997. Timing and significance of late-glacial and Holocene cirque
glaciation in the Sierra Nevada, California. Quaternary International Vols. 38/39 pp. 21 – 38.
Denton, G.H. and W. Karlen. 1973. Holocene climatic variations -- their pattern and possible cause.
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Gillespie, A.R., Porter, S.C. and Atwater, B.F. 2004. The Quaternary period in the United States. In
Developments in Quaternary Science 1. Elsevier, Amsterdam, The Netherlands.
Lamoureux S.F. and Cockburn, J.M.H., 2005. Timing and climatic controls over Neoglacial expansion
in the northern Coast Mountains, British Columbia, The Holocene, 15: 619-624
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Konrad, S.K. and Clark, D.H. 1998. Evidence for an Early Neoglacial Glacier Advance from Rock
Glaciers and Lake Sediments in the Sierra Nevada, California, U.S.A. Arctic and Alpine
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Luckman, B. H., Holdsworth, G., and Osborn, G. D.: 1993, 'Neoglacial Glacier Fluctuations in the
Canadian Rockies', Quaternary Research. 39, 144-153.
Daniel P. McCarthy, D.P. and Luckman, B.H. 1993. Estimating Ecesis for Tree-Ring Dating of
Moraines: A Comparative Study from the Canadian Cordillera. Arctic and Alpine Reseurch,
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Schweingruber FH & Poschlod P. 2005. Growth rings in herbs and shrubs: life span, age determination
and stem anatomy. Forest Snow and Landscape Research 79: 195-415.
Scuderi L. A. 1987. Glacier variations in the sierra Nevada, California, as related to a 1200-year treering chronology. Quaternary Research Vol. 27:220 – 231.
Sigafoos, R.S. and Hendricks, E.L. 1972: Recent activity of glaciers on Mt. Rainier, Washington. U.S.
Geological Survey Professional Paper 387B. Reston VA: U.S Geological Survey.
Wright, Jr., H.E. 1983. Late Quaternary Environments of the United States: Vol. 2: the Holocene.
University of Minnesota Press, Minneapolis, MN.
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