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Landscape Complexity, Soil Development,
and Vegetational Diversity Within a
Sky Island Piedmont:
A Field Trip Guide to Mt. Lemmon and San Pedro Valley
Joseph R. McAuliffe1 and Tony L. Burgess2
Abstract.-This paper focuses on the broad, gently sloping piedmonts
between elevations of approximately 950-1450 m which flank the
mountains of southeastern Arizona and adjacent regions. The first part
briefly reviews general pa,tterns of complexity in geomorphology and
soils, how soil characteristics affect water dynamics, and the responses
of plants to various soil water conditions. The second part is a detailed
road log for a 72 km (45 mile) field trip in the piedmont of the Santa
Catalina Mountains near the towns of Oracle and San Manual in Pima
and Pinal Counties, Arizona. This road log includes eleven interpretive
stops where a variety of complex landscape, soil, and vegetation
relationships are presented. This field guide provides a detailed overview
of the types of landscape and ecological complexity that are found
throughout many piedmonts of the American Southwest.
INTRODUCTION
variability is often encountered at a single elevation that cannot be explained by such
microclimate differences. Such variability is common in the semiarid piedmonts that flank the Sky
Island mountains. In these semiarid environments, moisture is typically the most important
factor limiting plant growth. Subtle differences in
soil conditions greatly affect quantity, timing, and
vertical distribution of available soil moisture.
Differences in soil moisture regimes within this
zone translate into predominance by different life
forms of plants. The considerable edaphic variation within the semiarid piedmonts produces
complex mosaics of several different kinds of
vegetation including woody scrub, grassland, savanna, chaparral, and woodland. In addition,
many of these communities are unstable mixes of
different species that can undergo rapid shifts in
relative dominance in response to disturbances
caused by weather events, fire, or grazing (Martin
1975). What results is a virtual/Ecological Confusion Zone' that is widespread across the semiarid
American Southwest. Our goal in this paper is to
present a basic framework of knowledge regarding landscape evolution, soil development, and
water dynamics that can explain some of these
ecological complexities.
The "Sky Islands" of the American Southwest
are rich reservoirs of biological diversity. The
sharp climatic gradients along these abrupt mountain slopes lead to the juxtaposition of floral and
faunal elements from more than a dozen degrees
latitude to the north and south. Some of the first
ecologists to work in the American Southwest focused on the climatic factors responsible for the
striking zonation of biotic communities ranging
from low elevation desertscrub to the woodlands
and coniferous forests of higher elevations (Merriam 1890, Shreve 1915).
Microclimate differences of different slope exposures often produce abrupt vegetation contrasts
within small areas, contributing to the diversity of
the biota found at any single elevation. Droughtadapted but cold intolerant species extend their
upper elevation ranges on warm southern exposures and more mesophytic species extend
downward into the arid elevations on cooler,
moister northern exposures. Additional ecological
1Desert Botanical Garden, 1201 N. Galvin Pkwy., Phoeniz, AZ 85008.
2The Desert Laboratory, 1675 W. Anklam Rd., Tucson, AZ 85745.
91
Geologic History of Basin and Range
Landscapes
ing of the valley floor has given rise to such stairstepped sequences of piedmont landforms in a
number of areas in Arizona including the San Pedro, Upper Gila, and Upper Verde River valleys.
Topographic differences among alluvial fan surfaces of various ages are considerably more subtle
in valleys lacking such mar ked base level change,
for example, the Sulfur Springs Valley in southeastern Arizona which lacks an external drainage.
Nevertheless, spatially discontinuous burial of
older surfaces by younger alluvium and the erosion of older surfaces contributes to complex
landscape patterns even in piedmonts that contain
relatively smooth, unbroken slopes (Peterson
1981). In addition to alluvial deposits, erosional
surfaces cut into bedrock or older alluvial deposits
(pediment
landforms)
may
exhibit
considerable variability in surface stability and
the length of time that has elapsed since past episodes of erosion.
Soil development is in part dependent on the
passage of time, therefore, knowledge of the ages
and spatial distribution of various parts of the
landscape is a prerequisite to understanding soil
variability. Two soil horizons typically form and
become increasingly strongly developed in noncalcic, gravelly to stony parent materials of fan
deposits in arid and semi-regions of the southwest: a clay-enriched argillic horizon and a calcic
horizon enriched with calcium carbonate (Gile et
al. 1965, Gile and Grossman 1968, Gile 1975, Gile
et al1981, McAuliffe 1994, McAuliffe 1995)(fig. 2).
Argillic horizons are extremely important in these
semi-arid environments because the considerable
water-holding capacity of clay greatly affects infiltration of precipitation, the soil depth at which
water is stored, and the seasonal duration of
water availability (Walter 1973, Noy-Mier 1973,
McAuliffe 1994, 1995, Burgess 1995).
Different spatial and temporal distributions of
soil water contribute to the predominance of different plant life forms. Various plant life forms are
above-ground expressions of different modes of
water use that are generally correlated with different spatial and
temporal
patterns of
belo,v-ground water acquisition. Shallow-rooted
plants including most perennial grasses, succulents, and some small, drought-deciduous shrubs
are subjected to highly variable water supplies in
the uppermost soil horizons. The persistence of
these plants is dependent on the intensive exploitation of a highly seasonal pulses of shallow soil
moisture coupled with a capacity for either
drought-ind uced dormancy or the storage of
water in succulent tissues that enables continued
Within the American Southwest, the repeating
pattern of isolated mountains separated by broad
basins is due to crustal stretching, faulting, and
the downdrop of basins relative to uplift of
ranges. This process began 12-15 million years
ago. Since that time, erosion of upland ranges has
contributed vast amounts of sediment that fill adjacent basins. The surface landscape features of
most piedmonts are typically of Quaternary age
(formed within the last 2 million years) or of late
Tertiary age (e.g., see Pearthree et al. 1988). These
surface features may consist of either constructional landforms (such as alluvial fans) or
degradationallandforms, such as pediments produced through erosion of pre-existing landscapes.
Piedmonts often consist of distinct alluvial fan deposits that vary widely in age and erosional
dissection. Deposition of alluvium within these
landscapes has been spatially and temporally discontinuous, producing mosaics of different-aged
constructional surfaces (Gile 1975, Peterson 1981,
Bull 1991). The morphology and areal extent of
different-aged alluvial fan surfaces varies from
piedmont to piedmont due to a variety of factors.
Where basin floors are in the process of being dissected by erosion due to regional base-level fall,
piedmonts are typically deeply incised and contain stair-stepped sequences of alluvial fan
remnants of various ages (fig. 1). Base-I-evellower-
Figure 1.-Block diagram of a highly dissected piedmont in a basin
that has experienced substantial lowering of base-level due to
Incision of the basin floor by an exterior drainage. The
piedmont consists of a stair-stepped sequence of alluvial fan
remnants with progressively younger surfaces inset within the
topographic confines of dissected, older surfaces. Surfaces
1-3 represent Pleistocene surfaces (3=0Idest, 1 = youngest), H
Is a Holocene surface and F indicates floodplain deposits
associated with the stream of the basin floor. (Adapted from
Peterson 1981).
92
photosynthetic activity into the dry season (fig.
3)(Sala et al. 1982; McAuliffe 1994, 1995). On the
other hand, the deeper root systems of most large
woody plants occupy a soil environment that exhibits less seasonal variation in water availability
(Noy-Mier 1973, Schlesinger et al. 1987, Burgess
1995). The extensive, deeper root systems of these
plants are capable of extracting the more widely
distributed but more constant supplies of water
stored at greater soil depths.
The following field trip guide examines a variety of these relationships between landform
development, soils, and vegetation responses
within a 200 km2 area of the piedmont flanking
the northeast side of the Santa Catalina Mountains
(fig. 4).
Mode of Water Use
Intensive exploitation
•
Peronn;.1
G......
1
~
Succule~"
Extensive exploitation
•
Winter-deciduous
Droughl-deciduous
or evergn,en
woody plants
woody plants
~.*~!~/~~
Figure 3.-Mode of water use as a function of above-ground plant
life form and associated rooting patterns of plants of semiarid
environments.
PART 2: FIELD TRIP GUIDE
Plan two days to complete the entire set of 11
interpretive stops. Stop 5B at the mid-way point
of the trip is an area suitable for overnight camping and is far removed from the main road.
Developed campground facilities are also present
at the Coronado National Forest Peppersauce
campground (mile 5.6), A detailed mileage log is
for the trip is listed below, followed by a discussion of each of the interpretive stops. Plant species
are referred to by scientific names but Appendix 1
also provides a list of common names.
Percent Clay
0
0
40
20
,
60
••
•
•
••
••
I
•
\
••
•
8\
'
A
-- •
-E
20
0
.r:::.
.....
a.
40
Q)
0
60
•
••
,
•••
•
••
••
•
•
.~
I
;
..
I
\
I
•
I
Fieldtrip Mileage Log
mile (km)
0.0 (0.0) - Stop 1 at Ray Spring Hill [on Mt. Lem11l0n Rd. 2.6 miles (4.2 km) southeast of
junction with Oracle business loop of Rte. 77].
General overview of vegetation variability
within a small area and of the field trip area to
the east and south.
0.5 (0.8) - Junction with Webb Road, turn right
(south), remaining on Mt. Lemmon Rd .
1.5 (2.4) - Cross streambed at American Flag
Ranch
1.8 (2.9) - Stop 2 at base of grassy hillslope. Examp Ie of soil with strong argillic horizon formed
on an ancient pediment remnant, Contrast behveen soils of grass-dominated sites and
rrticrophyllous scrub,
2.5 (4.0) - Madrugada Ranch road, continue south
on Mt. Lemmon Rd,
I
I
,,
,
I
•
80
•
Figure 2.-Accumulation of clay as a function of soil age in coarse
gravelly-stony, non-calcic alluvium in a semiarid zone (700-730
m elevation) on the east-facing piedmont of the Tucson
Mountains. A - late- to mid-Holocene surface, B - Middle
Pleistocene surface, C - Early-Middle Pleistocene surface. Soil
data from soil profiles "AU. ''C'', and "0" of McAuliffe (1994),
AppendixC,
93
)I
I'
"
~ ....'1.~.
SANTA CATALINA
PIEDMONT
,'~A'!>.
o
SANTA CATALINA
MOUNTAINS
32 30'
,.._..
~.,.
"..
r
1
km
N
~
2
,...
o
"
,
'
,
..,
~
•. ..,-l
,
I; .
"
J
1
'~24 '.
Figure 4.-Map showing route to the 11 field trip stops. Cross-sections 1,2, and 3 are detailed In Figure 8. Shaded areas "A", "B", and ''COl
Indicate early, middle, and late Pleistocene alluvial fan remnants, respectively, as discussed In text.
2.7 (4.3) - Stop 3 at road cut through granitic pediment surface showing soil structure and root
distributions. Proceed south on Mt. Lemmon
Rd. Climb up ridgeline across the Mogul fault,
from Precambrian granitic rocks (quartz monzonite, commonly called "Oracle granite" to
mixed bedrock terrain.
4.4 (7.1) - Summit of ridgeline mantled by early
Pleistocene alluvium
5.0 (8.0) - Stop 4 at borrow pit on east side of road.
Examination of species-rich vegetation on
limestone slopes and how shallow, cemented
calcic horizons of soils at foot of slopes affect
vegetation. Proceed southwest on Mt. Lemrrlon Rd.
5.6 (9.0) - Coronado National Forest Pepper sauce
Campground. Proceed generally south toward
1t1t. Lemmon. Traverse area of diverse slope
94
25.8 (41.S) - road curves to the north and climbs
onto a middle-Pleistocene alluvial fan remnant
26.6 (42.8) - Stop 6 at road cut immediately north
of Stratton Wash exposing soil profile in middle Pleistocene alluvial fan remnant.
Desertscrub with Acacia constricta, Zinnia acerosa,
Isocoma
tenuisecta,
Cercidium
nlicrophyllum, and Opuntia spp. on soil with
well developed argillic horizon. Example of
grazing-induced conversion of savanna to desertscrub accompanied by erosion of surface
soil horizons.
26.9(43.3) - Road cut at north ma,rgin of middle
Pleistocene fan remnant exposing mantle of
stony Pleistocene alluvium capping finegrained Tertiary basin deposits.
27.2 (43.8) - Stop 7 at road cut on highly dissected,
fine-grained alluvium of latest Pleistocene to
earliest Holocene age. Desertscrub dominated
by Larrea tridentata.
28.1(45.2) - Stop 8 on late Pleistocene alluvial fan
deposit dominated by Cercidium microphyllum and Carnegiea gigantea.
28.5 (45.9) - entrance on east to Black Hills limestone quarry; continue north.
30.3(48.8) - Junction with State Route 76, turn left
(northwest) toward San Manuel. CAUTION-DANGEROUS INTERSECTION.
34.2 (55.0) - McNab Parkway junction and Chevron store in San Manuel; continue north on
Route 76.
34.9 (56.2) - Turn left (southwest) onto Webb
Road. Proceed uphill on mixture of gravelly
fan deposits and young pediment surfaces cut
into Tertiary deposits. Vegetation is a mosaic
of desertscrub and savanna with widespread
invasion by Eragrostis lehmanniana.
38.2 (61.5) - Stop 9 at south-facing hillslope beneath ridge with microwave antennae dishes.
Site of burn in August 1993. Contrasting vegetation responses on different soils. Proceed
",'est and upslope across ridgelike landforms
capped with earliest Pleistocene quartzite alluvium.
39.0 (62.8) - Cross Smelter Wash
39.2 (63.1) - Turn right (northwest) onto pipeline
road. Proceed past gate. REMEMBER TO
CLOSE GATE.
39.5(63.6) - Stop 10 at excavated cut along base of
ridge to west of road. Dissected piedmont
with savanna and scrub. Influence of weathering-resistant diabase dikes on development of
landforms and soils. Sequences of pedogenesis
and erosion have created a context for complex, fine-scaled vegetation patterns. Turn
exposures and lithologies with juxtaposed
grassland, savanna, scrub, and woodland.
Rocks are mostly massive, highly fractured
shales with some hard, argillaceous sandstones and conglomerates. Occasional highly
eroded remnants of late Tertiary and earliest
Pleistocene alluvium.
9.2 (14.8) - Cross Catalina Wash. Climb up slope
onto earliest Pleistocene alluvial fan remnant
covered by savanna with Hilaria belangeri and
Prosopis velutina.
10.0 (16.1) - Stop S on planar surface of earliest
Pleistocene alluvial fan remnant. Contrast
soils and vegetation of remnant surface with
those of adjacent, south-facing erosional
slopes. Proceed in southerly direction toward
Mt.Lemmon.
16.1 (2S.9) - Turn left (east) onto Forest Service
Road 44S0. CAUTION-DANGEROUS INTERSECTION DUE TO HEAVY TRUCK
TRAFFIC FROM MINE. Proceed east and
downhill toward San Pedro River Valley, from
warm temperate woodland, savanna, and
grassland into subtropical (Sonoran) desertscrub.
23.8 (38.3)- Optional turnoff to the south on unimproved road to stop SA. This 2.S mile (4 km)
spur leads to a mesa-like landform mantled by
earliest Pleistocene alluvium and well-developed, clayey soils supporting savanna. The
relative isolation of this site has led to little use
by livestock. A comparison of this site with
Stop 6 indicates the degree to which use of
landscapes by livestock has altered vegetation
compositions. The road to stop SA requires a
high-clearance vehicle, but a 4-wheel drive vehicle is unnecessary. To reach the site, travel
eastward on the unimproved road, cross the
gas pipeline corridor at 0.8 mi (1.3 km) and
continue east. At 1.2 mi (1.9 km) from the exit
from the main road, the road loops to the
south, crosses Alder Wash, climbs the hills
south of the wash, eventually linking with the
gas pipeline corridor. Follow the pipeline road
to a saddle 2.5 mi (4.0 km) from FSR 44S0. Stop
SA is located atop topographic high located to
the immediate east. Retrace route to return to
FSR44S0.
23.8 (38.3) - Return to FSR 44S0 from road to Stop
SA (S miles to Stop SA not included in mileage
total)
24.5 (39.4) - Cross natural gas pipeline corridor
25.3 (40.7) - Turn north on abrupt switchback to
enter canyon of Geesaman Wash
25.6 (41.2) - cross Geesaman Wash
95
landforms and soils and ultimately greatly affect
plant distributions. These relationships are examined in detail at the next stop.
around and retrace route along pipeline road
toward Webb Road.
39.9 (64.2) - Turn right (west) on Webb Road toward Oracle.
41.5(66.8) - Turn right (northwest) onto Mt. Lemmon Rd. toward Oracle.
44.6(71.8) - Junction with business loop 77 at
Hildreth's Market in Oracle, turn left (west)
toward Tucson.
Stop 2: Grassy Hillslope South of American
Flag Ranch, Elev. 1355 m
Parts of a northwest-trending diabase intrusion have weathered and eroded more slowly
than the surrounding quartz monzonite, forming
the hills to the west and northwest. The grassdominated lower hillslope contrasts sharply with
the nlicrophyllous scrub on the weathered quartz
monzonite landscape located to the east of the
road. Immediately west of the road is a contact
between the coarse-grained quartz monzonite and
a fine-grained, deeply weathered diabase (fig.
5A). The hillcrest consists of diabase that is considerably more resistant to weathering.
Weathering-resistant diabase cobbles derived
from upslope have mantled portions of the lower
slope, armoring and protecting parts of the underlying, highly weathered diabase and quartz
monzonite from erosion. Past episodes of clast accumulation on this hillslope apparently stabilized
the surface and enabled formation of strongly developed soils. The hillslope is a mosaic of ancient
armored pediment remnants containing these
well-developed soils and erosional incisions into
Stop 1: Ray Spring Hill, Elev. 1380 m
From the top of the hill about 100 m north of
the road we can view the Santa Catalina Mountains to the southwest, the granitic landscapes of
the Oracle pluton to the west, and the piedmont
sloping eastward to the San Pedro River. Within
view is an elevational gradient from a subhumid
temperate climate supporting a "sky island" forest on the mountain peaks to a semi-arid
subtropical climate on the valley bottom. The field
trip route traverses landscapes between the lower
edge of Madrean woodland at an elevation of
about 1450 m and the uppermost Sonoran Desertscrub at 950 m (fig. 4). Complex surface geology
and landform development have generated correspondingly complex soil patterns across this
piedmont. This edaphic complexity in a semiarid
climate has produced a mosaic of scrub, savanna,
and woodland vegetation: an 'ecological confusion zone' which is widespread at this elevation in
the American Southwest (Burgess 1995).
The ridgetop we are standing on is a diabase
mass covered by an open savanna of scattered
CaiJiandra eriophylla/ Yucca baccata and
Dasylirion wheeleriin a mosaic of grasses including Hilaria belangeri, Bouteloua curtipendula/ B.
eriopoda/ Heteropogon contortus, and Aristida
purpurea var. wrightii. This grass-dominated
vegetation contrasts strongly with the sclerophyllous scrub and woodland of Quercus emoryi/
Arctostaphylos pungens/ and Ceanothus greggii
on the adjacent coarse-grained, light-colored granitic rock called quartz monzonite (often referred
to in this locale as "Oracle granite"). This pluton
of quartz monzonite contains numerous northwest-trending intrusions of dark-colored diabase
and occasionally, light-colored aplite (Wallace
1954, Creasey 1967, Brown 1970). At this stop, the
road cuts through dikes of these intrusive rocks.
Some parts of the diabase intrusion are deeply
weathered whereas other parts are extremely
weathering resistant. These differences in weathering have influenced the development of
Ancient pediment remnant
annored by weatheringresistant clasts derived
from
®
reSistant
diabase
®
\BI
E'
I
ros! I~na
___
In__
c !!on
.
. .
Argillic
horizon
A
Surface armored by
weathering·resistant
diabase clasts
7
- ~.~'~'~~~'~~~~!\'\.~~"'~~"~_~".
S~;:;:., v>-.~~"~: ~,;~. :'.',' ,'; .~
,-
,
J.
Deeply weathered and friable
intrusive lithology
Figure 5.-landscape cross-sections in vicinity of Stop 2, A.
East-west section through hillslope. B. North-south section
along foot of hillslope. Area "A" on the armored pediment
remnant Is dominated by the short, sod-forming grass, Hilaria
belangerl and the small drought-deciduous subshrub
Calliandra eriophylla. At area "8" along erosionai Incisions
where argilliC horizons have been completely truncated, the
grass Bouteloua erlopoda Is dominant together with patches of
B. curtlpendula and Calliandra eriophylla on deeply weathered,
friable diabase.
96
rounded due to weathering, and an abundance of
rounded pea- to marble-sized weathered diabase
fragments indicates that a considerable amount of
time has passed since this surface was originally
mantled with once larger, more angular clasts derived originally from upslope locations. The
weathering and gradual diminution of the surface
armoring of diabase cobbles has contributed to
erosion of parts of the armored pediment and
truncation of the well-developed soils at eroded
margins of the remnants. As the older surface
erodes, clasts accumulate in the incisions (fig. SB).
The accumulation of a substantial mantle of new
diabase cobbles in some of these incisions has led
to the diminishment or cessation of channel cutting which may in turn allow new episodes of
pedogenesis on these more recently formed, lower
surfaces.
The soils with strong argillic horizons are
dominated by Hilaria belangeri and Calliandra
eriophylla. Where the armored pediment remnants have been completely truncated by erosion,
friable, coarse textured soils on the deeply weathered, soft diabase favor Bouteloua eriopoda. Soils
occupied by B. eriopoda have considerably deeper
infiltration and storage of water (fig. 7 A) than do
those occupied by the shallow-rooted H belangeri
(fig. 7B). Quartz monzonite surfaces to the east of
the road with poorly formed soils have little
stored moisture near the surface but substantial
storage along fissures and cleavage planes (fig.
7e). Such soil moisture conditions tend to favor
woody plants with extensive root systems.
the ancient pediment surface. The armored pediment remnants are slightly convex features
elevated slightly above adjacent erosional incisions (fig. 5B). Soils of these armored pediment
remnants contain very strongly developed, reddened argillic horizons (fig. 6). A long history of
surface stability was required for the formation of
these argillic horizons. Soils with this degree of
development suggest an age of at least late- to
mid-Pleistocene. Relatively dense cover by the
sod-forming Hilaria belangeri protects this soil
from erosion much of the time, but when grass
cover is markedly reduced by drought or fire, the
armoring by surface diabase clasts plays a critical
role in stabilizing the surface (see McAuliffe
1995). The diabase clasts found on the surfaces of
these ancient pediment remnants are well-
E
.3.c
c..
Armored Pediment Remnant ~
.........~..-o A
1~
33
Bt1 } .2
Bt2
Bt3
64
~:
:4'
~,~
~
'~.~
<~
" ' ti- .,;' .;-:/
If
.,.
~
ItI
,
J;I.
III....
•
,
69
Rr
R
~:~.~. ~
Eroded Pediment Surface
Stop 3: Granitic pediment surface,
Cox
Eleva 1340
m
The surface of the quartz monzonite pediment
has eroded too fast to allow substantial pedogenesis. The rate of erosion of this pediment surface is
indicated by the difference in elevation (about 2040 m) between the pediment surface and the
east-'west trending ridges located 1-2 km to the
south (Figs. 4, SA). This abrupt topographic transition marks the east- to west-trending Mogul
Fault separating the rapidly weathering, coarsegrained quartz monzonite pluton to the north
from a variety of more weathering resistant lithologies to the south. The uppermost crests of
the elevated ridges to the south of the Mogul
Fault are mantled with coarse, stony alluvium
thought to have been deposited approximately 2
million years ago at the time of the Pliocene-Pleistocene transition. A few ridgeline remnants
Figure 6.-Soil profiles of the armored pediment remnants at Stop
2 (upper) and the quartz monzonite pediment dominated by
woody species at Stop 3. The soil of the armored pediment
remnant has a sandy clay loam A horizon. Textures of underlying argillic horizons are clay loam in the Bt1 horizon,
grading to clay In the Bt2 and Bt3 horizons. Rr Is soft, highly
weathered bedrock grading downward to less weathered bedrock (R). The R horizon is apparently not the parent material
for the overlying soil, as indicated by the stone line of clasts
of a different lithology scattered just above the Bt/Rr boundary. Plant roots (primarily H. belangeri) are common in the A
and Bt horizons, but are scarce below the Bt2 horizon.
The young soil of the quartz monzonite pediment has Q
gravelly sandy loam A horizon. The BC horizon Is highly
weathered and altered bedrock with a gravelly sandy clay
loam texture. The Cox horizon is massive weathered granite
with variably spaced joint and shear fractures. Some clay
accumulation has occured along these fractures. Plant roots
are common In the A horizon, relatively scarce in the BC, and
abundant in joints and fractures within the Cox horizon. Soil
horizon nomenclature follows Birkeland (1984).
97
capped with this earliest Pleistocene alluvium are
located within the main body of the eroded pediment to the north, indicating an extensive
elevated surface mantled by fan deposits once apparently spanned the entire front of the Santa
Catalinas within the area of the field trip (fig. SA).
Large areas of these ancient alluvial deposits have
been preserved only when they have mantled
weathering or erosion-resistant lithologies or have
been in topographic positions that have resisted
rapid erosional incision (further discussion at
Stop 10). Apparently, the area of the quartz monzonite pediment we are presently observing has
experienced a net lowering by erosion of approxinlately 20-40 m during the last two million years.
Soils on this pediment show relatively little
pedogenic alteration, often consisting of a thin,
gravelly sandy loam A horizon directly over BC
and Cox horizons derived from weathered quartz
monzonite (fig, 6B). The weathered Cox horizon
contains variably spaced joint and shear fractures.
Rainfall penetrates along these fractures, creating
a deep, uneven distribution of soil moisture (fig.
@~{:.:
· •• ,,10.
1i~!J.~,.J~ ~;2~: :!;'
©
Depth and darkness of stippling
indicates soil water distribution
and concentration
Figure 8.-landscape cross-sections of field trip area Indicated on
map of Figure 4. A. Cross-section 1i surface features drawn to
scale at a vertical axaggeratlon of 14.5:1. B. Cross-section 2
(diagrammatic representation, not to Icala). C. Croll-Iectlon
3 (diagrammatic, not to Icale).
Deep. coarsetextured soils
B
~
,.
Y.
l'
X
;<
)t
II.
'II
.,J'
X
;..
JC
If
t
A
7C). The extensive root systems of woody shrubs
and subshrubs are better suited to exploit moisture from this edaphic environment than are the
shallow, diffuse root systems of perennial grasses.
Winter-deciduous woody plants, including MimOSc1 biuncifera, Prosopis veJutina, and Acacia
greggii predominate over most of the pediment.
These winter-deciduous species flower and are in
leaf during the extended pre-summer dry period,
indicating that their extensive root systems are
tapped into relatively deep, dependable stores of
moisture found along the deep bedrock fissures.
Another dominant plant, the small, drought-deciduous subshrub Eriogonum wrightii, has
relati.vely shallow roots that occupy shallow fractures which are more prone to seasonal drought.
At slightly higher elevations this substrate supports sclerophyllous chaparral and evergreen oak
woodland.
Lithological variation within the pediment has
contributed to topographic and soil variability.
Occurrences of small areas of darker granodiorite
and thin intrusive dikes of more weathering-resistant diabase have locally impeded erosion of the
Strongly developed
argillic horizon
" ""
Weathered bedrock
(pediment surfaces)
with fissures and
weakly formed soil
Figure 7.-Vertical distributions of moisture in three different solis
following a moderate rain after the onset of a drying cycle. A.
Deep, coarse-textured soils such as those occupied by
Bouteioua eriopoda at stops 2, 5, and 10 have relatively deep
infiltration and storage. B. Due to the high moisture holding
capacity of clay, soils with strong argillic horizons retain most
water near the surface. C. Weathered and fractured bedrock
contains considerable moisture stored at depth within fissures,
but little water in thin soil horizons near the surface.
98
pediment surface, producing slight topographic
convexities. The road cuts through several of
these convexities, revealing the underlying lithological variation. The somewhat greater but
highly localized surface stability present on these
convexities has contributed to more strongly developed soils. These soils often contain
moderately well developed argillic horizons that
support a greater abundance of perennial grasses.
Stop 4: Borrow Pit on West Side of Road,
Elev. 1460 m
The hillslope of weathered Missisippian Escabrosa limestone west of the road supports a
high diversity of plant growth forms and species.
This limestone slope contains the greatest number
of species and plant life forms of any of the stops
along the field trip route. Varied edaphic conditions contribute to this diversity. Mesic edaphic
conditions are created by the presence of deep
fractures and cleavage planes and large solution
pockets filled with loamy soil. Xeric edaphic conditions are present where extremely thin soil
layers mantle massive, unfractured bedrock or in
very small, soil-filled solution pockets (fig. 9).
This mosaic of moisture regimes allows the coexistence of species with very different strategies of
soil moisture extraction and use. The vegetation
contains many species capable of intensively exploiting shallow, highly seasonal supplies of
water, including the many species of perennial
warm-season grasses, agaves, cacti, and
Fouquieria splendens. Conversely, the slope also
contains large evergreen woody shrubs such as
Dodonaea viscosa and Cercocarpus montanus
which indicate the presence of more seasonally
constant, deeper soil moisture sources.
Other factors may also influence the vegetation of limestone landscapes and need further
investigation. The basic pH of limestone-derived
soils may diminish the availability of phosphates
to some plants (McGeorge 1942). The high specific
heat of limestone may ameliorate the effects of
episodes of extreme cold. This allows some plants
and ground-dwelling insects to reach their northern and upper elevation limits on limestone
outcrops (Lindroth 1953).
The borrow pit is located at the foot of the
limestone slope. The east-facing wall of the pit
exposes soils that have developed in carbonaterich materials that have accumulated on top of
bedrock at the foot of the limestone slope. In
arid and semi-arid climates, a high carbonate
Figure 9.-Varied soil microenvironments in limestone terrains of
semiarid landscapes that support a variety of plant life forms.
A shallow, drought-prone soli over bedrock occllJpied by
xerophytic plants capable of intensive, short-duration
exploitation of water; Band C - deep fissure planes and solution
pockets that store considerable moisture p'equh'ed 10L" large
evergreen and winter-deciduous woody plants with Gxtensive
root systems; D - shallow, soil-filled solution pockets occupied
by Intensive water exploiters.
m
content of parent materials impedes the development of a clay-enriched argillic horizons (Gile et
al. 1981), but may accelerate the formation of
strong calcic horizons that eventually become cemented and alter the penetration of soil moisture.
Toward the center of the exposure along the eastfacing wall of the borrow pit, a moderately
strongly cemented calcic horizon is located rela-'
tively deep (up to 60 em). Two large woody
plants, Prosopis velutina and Acacia constricta
are codominants in this relatively deep soil.
However, immediately south of the borrow pit,
the rooting environment is considerably shallower with massively cemented calcic horizons
present at depths of 15-35 cm. In the more xeric
edaphic conditions created by this shallow cemented calcic horizon, A, constTicta is dominant
and P. velutina is rare. The capacity for drought
deciduousness in A. constricta may enable this
woody species to predominate in the slightly
more xeric soil conditions south of the borrow
pit.
Stop 5: Early Pleistocene Alluvial Fan
Remnant~ Elev. 1430 m
The elevated, planar surface that we climbed
after crossing Catalina Wash is a remnant of an
anciE~nt geomorphic surface known as the
99
Martinez Surface (see Morrison 1985). A geomorphic surface is an area of the land surface that
formed during a defined time period and is readily distinguished from adjacent areas by
topographic and stratigraphic relationships (see
Peterson 1981). Well-preserved planar remnants of
the Martinez surface are found throughout southern Arizona, flanking the fronts of many
mountain ranges (Menges and McFadden 1981;
Morrison 19S5). Planar remnants of this surface
typically slope away from the mountain front
with an inclination of 2-5%. In many cases the upper, planar features of the Martinez surface have
been completely removed by erosion, yielding
long, subparallel ridges separated by deep ravines
(fig. SA).
In the study area, the Martinez Surface consists of a relatively thin layer of extremely coarse
alluvium thought to have been deposited approximately 2 million years ago near the time of the
Pliocene-Pleistocene transition. Considerably
thicker Tertiary alluvial deposits underlie the thin
upper mantle of the earliest Pleistocene alluvium
(fig. 8A,B). These underlying Tertiary deposits are
exposed along the road south of the Catalina
Wash crossing. The coarse, earliest Pleistocene alluvium on the surface includes cobble- to
boulder-sized clasts of weathering-resistant
quartzite and metamorphosed conglomerate. One
of the most distinctive lithologies is the Barnes
Conglomerate. These lithologies are exposed in a
3-5 km-wide band located directly south of the
Mogul Fault (the Precambrian Apache group and
the Cambrian Bolsa and Troy Quartzites) (Ariz.
Geol. Soc. 1952, Ariz. Bur. Mines 1959). The presence of remnants of alluvial mantles containing
clasts derived from these Cambrian and Precambrian strata in various places across the quartz
monzonite pediment to the north indicates that
Pleistocene-aged alluvial fans once apparently
flanked the entire mountain front within the area
of the field trip. However, these fan deposits have
been nearly completely removed by erosion in the
area of the wide granitic pediment to the north of
the Mogul Fault (fig. SA; see further discussion at
Stop 10).
Catalina and Stratton Washes located directly
to the north and south of this fan remnant drain
large watersheds in the Santa Catalina Mountains
(fig. 4). These two drainages provided the runoff
and coarse alluvium that contributed to creation
of the original fan deposit in earliest Pleistocene
times. However, since the deposition of the ancient fan, both washes have incised to a depth of
approximately 60 m below the fan's original sur-
face (Figs. 4, 8A). Consequently, the fan remnant
has not been hydrologically connected to the substantial watersheds of the mountain slopes for a
long time and the only runoff across the surfaces
of the planar remnant is derived exclusively from
precipitation falling directly on the limited area of
the remnant (the shaded area labeled AU in fig.
4). The preservation of the wide, planar surface of
this fan remnant is due, in part, to the physical
armoring provided by the large, weathering-resistant metamorphic clasts. The substantially less
energetic runoff originating solely from within the
planar fan remnant is generally incapable of moving the large cobbles and boulders that were
deposited by much larger and powerful stream
systems during the fan's original creation.
Erosional truncation of this surface proceeds principally by slope retreat at the abrupt margins of
remnants overlooking the canyon-like drainages
of major streams. The principal erosion occurring
on planar surfaces is the selective removal of
fines. Large boulders on the surface frequently exhibit a iron oxide-stained collar up to 30 cm above
the current soil surface, indicating the erosional
removal of some of the fine-textured, reddened
soil.
Soils and vegetation. The stability of the planar remnant surface has led to the development of
soils with extremely strongly developed, reddened argillic horizons. Color, clay accumulation,
and structural development of this soil greatly exceed the clayey soils of the armored pediment
remnants of Stop 2, indicating considerable age
differences between these two soils. Below the
thick argillic horizon is a well developed calcic
horizon. The Tertiary deposits beneath the Pleistocene alluvium also contains abundant carbonate,
probably of pedogenic origin.
Under the current climate, rainfall seldom
penetrates deeply into this clay-rich soil, Consequently, grasses, especially the shallow-rooted
Hilaria belangeri, are favored at an elevation
where woodland might otherwise develop.
Shrubby mesquite trees are widespread across the
surface and may persist due to their ability to exploit water stored beneath surfaces of large rocks
and boulders which is inaccessible to the shallow
root systems of grasses.
Soils that mantle the erosional sideslopes differ from those of the upper Martinez surface.
Even within a single slope, soils and associated
vegetation can vary considerably. For example, directly south of this stop, the moderately steep,
south-facing slope (35% inclination), despite continuity in aspect and inclination, contains a sharp
1/
100
transition in soils and dorninant grass species (fig.
10), In parts of the slope located below the intact,
planar tvlartinez surface clay-rich colluvium derived fronL the argillic horizons of the upper
I\1artinez surface rnantles the slope environment
(fig. 10A,B). This clay-enriched soil supports a
very open savanna vegetation dominated by the
grasses Bouteloua curtipendula t Bouteloua filiforrnis" _Hilaria belangeri/ Heteropogon contortus,
abundant LalliandTa eri~phyiia! and widely scattered Prosopis Feiutina/ Acacia constricta, and
Ferocactus wislizenii, This slope continues to the
west but as the upper planar remnant of the
Martinez surface narrows and is eventually
erosionally truncated! soils in downslope locations change markedly. Argillic horizons have
been completely lost from this ridgeline remnant
and calcic horizons are exposed on the surface of
the ridge along the road, In the absence of a
source of clay-rich colluvium, soils on the slopes
below this ridge are deep, gravelly, calcareous
loams (fig. 10), The dominant perennial grasses in
this edaphic setting are Bouteloua eriopoda and
Tridens muticus together with Acacia constricta/
Prosopis velutina/ Calliandra eriophylla, and
Fouquieria splendens.
Profiles of root distributions from the upper
Martinez surface, the clay-enriched slope, and the
calcareous loam slope indicate considerable differences in the vertical distribution of soil
moisture that is related to texture (fig, 11). The
clay-rich soil of the intact, planar Martinez surface
has an extremely low infiltration capacity. In this
soil, the average time required for the infiltration
of 1 em depth of water in dry soil applied in a 15.3
cm diameter ring was 6 minutes, 52 seconds. Additionally, the extremely high water-holding
capacity of the clayey soil retains most of the
water near the surface, contributing to the dominance of shallow-rooted H. belangeri (fig. 11).
Infiltration capacities on adjacent erosional slopes
are considerably greater, ranging from 21-47 seconds for infiltration of 1 em depth of water.
However, where somewhat finer textures and correspondingly higher moisture-holding capacities
are found in subsurface horizons of soils on
clayey colluvium (fig. lOB), water would be stored
more shallowly than in the loose, calcic loam
where a clay-enriched horizon is absent (fig. 10C).
These subtle differences in sideslope soils apparently contribute to the predominance of different
grass species with differing root distributions (fig.
IlB,C).
t
j
N
STOP
5
~
......•.• ,
Qf\¥Y".~'.':--'''''
,-e~~$tf?(:)/: ,
~~
,.,
/'
\'C
c___(,~·
~.;::- .•. ~~ '\"'
..
''-
-,-J. _/,~
\cPw~
Number of Roots Intersecting 10 cm 2 Area
10
0
5
0
5
5
Dominant
Species'
Figure 10.~Block diagram and landscape cross sections showing
iocation and features of the three different soil environments
(A, B, C) discussed in the text. The 6t horizon is a clay argillic
horizon; Bkm is a massively cemented calcic horizon which is
highly weathered and degraded at the surface,
Hilaria belangeri
Boule/oua
curlipendula
B. eriopoda
Figure 11.-Root densities of soils from the three different landscape
positions (A, B, C) at Stop 5 indicated in Figure 10. Each data
point consists of counts of the numbers of roots intersecting
three separate 2 cm X 5 cm sample areas in freshly excavated
suil pits,
101
Notice the relatively small sizes of Cercidium
microphyllum/ Prosopis vel utina, and Acacia
greggii. The small stature of these species may be
due to limited infiltration of precipitation to substantial depth in the clayey soil. A similar
condition exists in the clayey soils at Stop 6, but
contrasts strongly with Stop 8 where more permeable soils are present.
Optional Stop SA: Lower Elevational Remnant
of Martinez Surface, Elev. 1060 mThe top of the
mesa-like landform to the east of the saddle where
we are par ked is an isolated, lower elevational
remnant of the Martinez surface. Notice the
quartzitic composition of the coarse alluvium capping the remnant is identical to that of the
Martinez surface at Stop 5. Finer-grained, calcareous Tertiary deposits underlie the earliest
Pleistocene alluvium. Soils with strongly developed argillic horizons mantle the planar surface of
the remnant. This surface was once continuous
with that of Davis Mesa, located 5 kilometers to
the west (fig. 4). Extensive erosion of lower parts
of the piedmont have long since removed the once
extensive Martinez surface with the exception of
this isolated remnant and another narrow
ridgeline located 1 km to the northwest.
This isolated remnant is 350 m lower in elevation and 12 km further away from the mountain
front than the site at Stop 5. Both factors contribute to increased aridity (lower precipitation and
higher temperatures) of the lower site and plant
species characteristic of subtropical Sonoran desertscrub are present. Acacia constricta/
Cercidium microphyllum, and Prosopis velutina
are the dominant woody species, but these small
trees and a diverse array of other woody and succulent plants are inserted in a nearly ubiquitous
matrix of perennial grasses. Relatively steep escarpments surround the remnant with the
exception of the west end near our parking place.
This relative inaccessibility has contributed to little use of the mesa top by domestic stock and
considerable cover by perennial grasses has been
maintained. This condition contrasts strongly
with Stop 6 where a long history of heavy use by
livestock apparently has virtually eliminated perennnial grasses from similar, dayey soils. Hilaria
belangeri is the dominant grass and is abundant
in spaces between trees and shrubs, although it
does not cover the soil surface as completely as it
does in higher elevations. Bouteloua triEda is locally dominant in some places where H belangeri
is absent. In addition to these two short grass species, the taller grasses Aristida ternipes/ A.
purpurea/ Setaria macrostachya/ Digitaria cali/ornica/ Bothriochloa barbinodis, and Muhlenbergia
porteri are common. The taller, highly palatable
grass species are frequently found in the open and
are ungrazed, indicating lack of use of the area by
livestock. Other grasses present include Hiliaria
mutica/ Bouteloua curtipendula/ Heteropogon
contortus, and occasionally, the introduced South
African Eragrostis lehmanniana.
Stop 6: Middle Pleistocene Alluvial Fan,
Elev. 990 m
North of Geesaman Wash, we began to cross a
series of alluvial fan surfaces that are lower in
projected elevation and younger than the
Martinez surface (fig. 8B). The road cut at this
stop shows a fairly old soil containing a well-developed" reddened argillic horizon above a
strongly cemented calcic horizon. A soil with this
strength of development suggests the fan deposit
dates to as old as the mid-Pleistocene. The surface
soil horizon has a clay loam texture that impedes
rapid infiltration. Presence of soil pedestals beneath some perennial plants and a varnish line on
SOine stones above the present soil surface indicates a history of recent erosion of the A horizon.
A few species of perennial grasses found at
Stop SA are found at Stop 6, but they are exceedingly rare, typically persisting beneath the
protective, spiny canopies of acacias or cactL Before the late nineteenth century, the site probably
supported a savanna with considerably greater
grass cover, similar to that of Stop SA. The complete absence of Hilaria belangeri from the site is
an anomaly, as this grass species and others are
present on another Pleistocene fan surface at a
lower elevation (927 m), 6 km directly north of
Stop 6. Loss of grass cover at Stop 6 may be one
factor that has contributed to the accelerated surface
erosion,
potentially
producing
a
self-enhancing feedback loop of soil degradation
that further inhibits recovery of grasses, even if all
livestock grazing on the site were to end (see
Schlesinger et al. 1990). The abundance of the
short-lived, drought-deciduous subshrub Zinnia
acerosa may be a vegetation response that followed grass decline. This shrub is uncomlTIOn in
the grass-occupied site at Stop SA. An independent line of evidence indicating a long history
of use by livestock is the great abundance of
Opuntia fulgida. The easily detached, spine-covered stem joints are readily spread when they
impale the hide of livestock, leading to local increases in areas of livestock concentration. An
102
An argillic horizon is present, but is less strongly
developed than in the soil of Stop 6, indicating a
late Pleistocene age for the deposit. An additional
contrast between this site and Stop 6 is the presence of a far more permeable, loamy A horizon
that facilitates infiltration of precipitation. Perennial grasses are more abundant here than at Stop 6
and Cercidium microphyllum/ Prosopis velutina,
and Acacia constricta achieve canopy heights and
diameters approximately 25% greater and canopy
volumes more than twice as great as they do on
the mid-Pleistocene surface. The greater size of
these woody species at Stop 8 is probably related
to the enhanced infiltration and deeper storage of
precipitation in soils at this stop. The vegetation
shows a mixture of growth forms and an abundance of stem succulents that is typical of the
Arizona Upland Subdivision of the Sonoran Desert (Turner and Brown 1982). Vegetation includes
woody plants (C microphyllum/ Lycium berlandien~
L. fremontiJ~ Ephedra fasciculata/
Fouquieria splendens, and Janusia gracilis) and
abundant Carnegiea gigantea. However, the relatively high number of grass species, the presence
of Zinnia acerosa/ Isocoma ten uisecta, and Gutierrezia sarothrae, and the absence of the subshrub
Ambrosia deltoidea indicate that this site is close
to the mesic and frigid upper limit of Sonoran Desertscrub.
earthen watering tank, located 0.5 km to the south
of our stop and immediately west of the road, encourages the persistent use of this area by stock,
despite the general absence of quality forage.
Note the small size of the woody plants Cercidium microphyllum/ Prosopis velutina, and
Acacia constricta; we will compare their sizes
with those at Stop 8.
Stop 7: Fine-Grained Latest Pleistocene to
Earliest Holocene Alluvial Deposit
The gravelly layer that forms the surface of
this alluvial terrace is probably a latest Pleistocene
or earliest Holocene deposit (fig. 8B). The small
drainage basin that supplied these fine-grained,
calcareous sediments extends only 2 km to the
west of the road in an area of highly dissected,
calcareous Tertiary deposits. The absence of an argillic horizon is due to two factors: (1) insufficient
time has passed for considerable pedogenic
change and (2) abundant carbonates in the original parent material inhibit clay translocation and
accumulation within an argillic horizon (Gile et al.
1981). Examination of the soil reveals some structural development of the B horizon (cambic B
horizon). Larrea tridentata is clearly dominant
here, in marked contrast to its virtual absence on
the clayey soil at Stop 6. The exclusion of this
shrub from soils with strongly developed argillic
horizons is a widespread phenomenon in the
more moist parts of the Sonoran Desert
(McAuliffe 1994) and is apparently related in
some way to the control of water infiltration by
the argillic horizon. Some of the individuals of L.
tridentata on this terrace have basal diameters exceeding one meter, indicating these shrubs may
have ages of at least several centuries (see Vasek
1980, McAuliffe 1991, 1994). The sizes and probable ages of some of these shrubs indicates that
creosotebush probably was a dominant plant in
this location before the area began to be heavily
used by livestock.
Stop 9: Southern Hillslope Exposure Below
Microwave Antennae Dishes, Elev. 1210 m
The slope immediately north of the road was
almost completely burned in the summer of 1993.
Widespread fires occurred in this area as a result
of high vegetative production that was stimulated
by an unusually wet winter and spring in conjunction with an EI Nino event in the Pacific. The
crest of the slope is capped by a remnant of a
Pleistocene-aged alluvial deposit containing
weathering-resistant quartzite clasts. The slope
contains a variety of soils and associated dominant grasses. Calcareous loams derived from
degraded calcic horizons of the remnant Pleistocene surface are present near the ridgecrest and
are dominated by Bouteloua eriopoda. Sideslope
soils containing weakly developed argillic horizons support stands of Heteropogon contortus/
Hilaria belangeri/ Bouteloua /iliformis/ B. curtipendula/ B. rothrockii, and Bothriochloa
barbinodis. Small Holocene alluvial fan deposits
are located at the foot of the slope, directly below
fluves cut into the hillslope (fig. 12). The young,
Stop 8: late Pleistocene Alluvial Fan,
Elev. 960 m
The stratigraphic relationship of this alluvial
fan and soil characteristics indicates it is younger
than the mid-Pleistocene fan of Stop 6 (fig. 8C).
103
(1983) document the damaging effect that fire can
have on B. eriopoda.
deep, sandy loam soils of these fans supported
Aristida purpurea/ Muhlenbergia porteri, and
some Bouteloua eriopoda before the fire. The
shrub Acacia constricta is present across the entire
hillslope.
The various grass species that predominated
in different parts of the slope exhibited various
responses to the fire of 1993, On the small alluvial
fans, most perennial grass clumps did not resprout. By late fall, 1993, these fans were covered
by a dense cover of Bouteloua rothrockii and
Panicum hirtica ule that germinated after the burn.
Most of the Acacia constricta rapidly and vigorously resprouted. On the calcareous soils near the
ridgetop there are patches of Bouteloua eriopoda
that escaped the burn. In adjacent burned sites,
few B. eriopoda survived, and in late August
1993, many burned, dead clumps of B. eriopoda
were already on small pedestals created by the
rapid erosion of soil by summer rains. In midslope
positions,
HeteTopogon contortus/
Bouteloua curtipendula, and the other grass species rapidly resprouted and exhibited vigorous
regrowth. The different responses of various grass
species to fire is related to the location of carbohydrate storage within the grasses. The culms serve
as principal organs for storage of reserve carbohydrate in both Bouteloua eriopoda and
Muhlenbergia porteri; both of these species are
typically damaged or killed by fire. Conversely,
grasses that store reserve carbohydrates in a
crown located below ground generally respond
favorably to fire.
Those who advocate widespread use of fire to
maintain grass dominance in these landscapes
should pay close attention to soils and the identity
of the grasses. Grass species growing on clayey
soils will probably respond favorably to burning.
However, on coarser-textured soils dominated by
B. eriopoda and Acacia constricta, shrubs may actually show better recovery than perennial grasses
after an intense burn. Cable (1965) and Martin
Stop 10: Pipeline Road 0.3 Miles Northwest
of Webb Rd., Elev. 1240 m
The ridges directly east of us are quartz monzonite capped by remnants of an older, early
Pleistocene alluvial fan surface (fig. 13). Large
stream-rounded boulders of quartzite and occasional Barnes conglomerate are found along the
ridge crests. In this area, no transversely lever surfaces remain of this early Pleistocene fan.
Geomorphologists refer to these highly eroded,
somewhat humpbacked remnants of ancient fan
deposits as "ballenas" (Spanish for "whales") (Peterson 1981). Soils of these ridgeline remnants
have been considerably truncated and argillic horizons are absent, Surface soils are highly
calcareous due to the exposure and degradation of
calcic horizons that originally would have been
positioned beneath argillic horizonso Crests of
these ridges are elevated approximately 25-30 In
above adjacent drainages. The advanced degree of
spalling, splitting, and weathering of large
streaIn-rounded quartzite boulders along the
ridgE! suggest considerable age of the remnants of
the alluvial deposits. The alluviuln capping these
separated ridges is all that remains in this area of
a middle- or early- Pleistocene alluvial fan, possibly eqivalent to the Martinez surface (fig. SA).
Between 1.5 and 2.5 miles north of this stOPf notice a set of four additional, even more elevated
ballenas. The crests of these ballenas are approximately 30 m higher in elevation than the ballenas
Remnants of Earliest
Pleistocene Alluvial Fan
(Weathering' resistant _ _ _ _
T
Qua""'e)
N~/A
,<
11)"
C\l
i
P,e-PI.,,'ocene Pedlmen' EI.,.lIon
,..--~;-;-...
E ... '." •
'"
~
-:'-".~~/pedlmentremnants---"""
~ *.,.
_f.O!~e~bastnJl-o<).~-·, .- ..
-
......
Quartz, .. ..
'"
.. tMonzo~lte
~
,. . . ~
~
©
CV" ".. '" "'
~
..",:
""n-
0
""
't
~
_ ~ ....
''',."
Mid·late Pleistocene
...
. . . "-
"Quartz
Monzonite J<
it
'*
~
~
11
Figure 13.-North-south cross-section through landscape east of
pipeline road at stop 10. The south-facing slope (A) has a very
open grass-dominated vegetation dominated by Bouteioua
eriopoda and Calliandra erlophylla with iesse~ amounts of
Opuntia engelmannii, IErioneuron pulchellum, and Tridens
muticus. The clayey soil of the armored pediment remnant (8)
supports dense Hilaria belangerl with Calliandra erlophylla and
scattered Acacia constricta. The lowest slope (C) contains
young, poorly formed soils dominated by A. constricta and B.
Ilniopodao
Figure 12.-Block diagram of south-facing hillslope along Webb
Road below microwave antennae. Area "c" represents the
hillcrest containing calcic, loamy soils, "s" represents
sideslopes with moderate argillic horizons and "f" indicates
small Holocene-aged fan deposits.
104
immediately east of us. The more elevated set is
also capped with coarse quartzitic alluvium and
may represent remnants of either an earliest Pleistocene or an even older Tertiary deposit (fig. SA).
However, the ages of the various landforms in this
vicinity have yet to be satisfactorily deciphered.
The persistence of these ballenas of quartz
monzonite capped with remnants of ancient alluvial deposits within the area of the broad quartz
monzonite pediment (figs. 4,SA) is apparently due
to the location of a weathering- and erosion-resistant diabase dike directly west of these landforms.
This dike forms the prominent north- to southtrending ridge immediately west of the pipeline
road. The crest of this ridge lacks any quartzite
clasts or other indications of a former alluvial
mantle (such as exposed, degraded calcic horizons), indicating that at the time of deposition of
the early Pleistocene alluvium which now caps
the ballenas to the east, the dike apparently
formed a topographic high above the fan surface.
This intrusive dike and hills associated with it extend approximately one kilometer to both the
north and south of Webb Rd. The presence of this
erosion-resistant landscape feature apparently impeded the subsequent ability of fluvial systems
shaping the pediment surface to completely remove early Pleistocene alluvial landscape features
to the immediate east, whereas to the south, the
quartz monzonite pediment has been more uniformly planed down by erosion.
The slopes of the ballenas to the east show a
complex history of periods of landscape stability
punctuated by erosion (fig. 13). These landscapeshaping processes have given rise to considerable
variation in soils and associated vegetation within
single hillslopes. Directly to the east, the middle
portion of the adjacent north- and south-facing
slopes possess small, more gently inclined shoulders dominated by Hilaria belangeri and
Calliandra eriophylla. These landscape features
are remnants of a local pediment surface cut into
underlying quartz monzonite and graded to the
former level of the adjacent drainage (fig. 13).
These pediment remnants are armored with cobble-sized quartzite colluvium derived from the
early Pleistocene alluvium that caps the ridges.
This armor of extremely weathering-resistant cobbles provided substantial protection from erosion
of the underlying, highly weathered quartz monzonite. Soils of small pediment remnants are
virtually identical to those of the armored pediment remnants examined at Stop 2 and contain
strongly developed, clayey argillic horizons above
highly weathered quartz monzonite. Quartzite
S
Armored Pediment
-'E
B'eQ)
Remn~
2-
Holocene
Cut & AI
/
N
Exposed Quartz
Monzonite
............,.
~t; 0-
en
0
I
20
40
60
80
Horizontal Scale (m)
Figure 14.-North-south cross-section of east-facing exposure
showing contrasting soils along the pipeline road at Stop 10.
Vertical scale Is exaggerated as shown. Dominant species on
soils with strong argillic horizons of the armored pediment
remnant are Hilaria belangerl and Calliandra erlophylla. 111e
young soil on the Holocene alluvium Is clearly dominated by
Bouteloua erlopoda, with lesser amounts of B. rothrockll,
Erlogonum wrightii and C. eriophylla. The weakly formed solis on
recently exposed and weathered quartz monzonite are
dominated by E. wrlghtil, and scattered Acacia greggii, Krameria
parvifolia, and Mimosa bluncifera with a minor grass component
that includes Bouteloua hirsuta, B. eriopoda, and B. curtipendula.
A diagrammatic comparison of typical soli moisture distributions
In soils from these three mlcroenvlronments is shown In Figure
7.
cobbles found at the interface of the lowermost
argillic horizon and underlying highly weathered
bedrock indicate that initial accumulation of
quartzite cobbles on otherwise highly erodable
quartz monzonite may have initially contributed
to the surface stability required for lengthy pedogenesis. Loamy soils lacking argillic horizons
located above, below, and to the sides of the armored pediment remnants lack H belangeri and
instead typically contain more deeply rooted
Bouteloua eriopoda and Acacia constricta.
Similar small-scale landform complexity is
present on the slope of the diabase ridge immediately west of the pipeline. A cross-section of
landscape features and soils along the foot of the
slope is clearly visible in the lengthy vertical exposure on the west side of the pipeline (fig. 14).
Weathering-resistant clasts derived from upslope
locations have contributed to the formation and
preservation of an armored pediment remnant at
the same elevational level as the pediment remnants on the slopes of the ballenas to the
immediate east. This armored pediment has been
incised, yielding a slightly convex remnant approximately 15 m across which is dominated by
Hilaria belangeri and Calliandra eriophylla. Adjacent parts of the hillslope with weakly formed
soils over highly weathered quartz monzonite are
dominated by various deeper-rooted shrubs, including Eriogonum wrightii Acacia greggii and
Mimosa biunci/era. Bordering the north side of
the armored pediment remnant is a Holoceneaged cut-and-fill feature containing a dark sandy
loam alluvium (fig. 14) similar to the soils of the
small fan deposits discussed at Stop 9. Bouteloua
105
Crea~ey, S. D. 1967. ~eology of the Mammoth Quadrange,
eriopoda is the dominant species on this deep,
young, coarse-textured soil.
This hills lope and exposure at our final stop
encapsulate many of the themes of the trip. The
temporal and spatial distribution of soil moisture
varies considerably from one soil to another. In
this semiarid climate the soil moisture regime determines which species and growth forms are
most likely to predominate. A knowledge of geom.orph.ology provides a predictive capacity
rega,rdlng th~ spatial ~d topographic positioning
of dIfferent kInds of soIls that otherwise are easily
overlooked. If we are to interpret the ecology of
these extremely complex landscapes, we must understand the formation and patterning of its
landforms and soils, and how they ultimately influence the distribution and availability of water.
Pmal County, ArIzona. U. S. Geological Survey Bulletin 1218. 134p.
Gile, L. H. 1975. Causes of soil boundaries in an arid
region. I. Age and parent materials. Soil Science of
America Proceedings 39:316-323.
Gile, L. H. and R. B. Grossman. 1968. Morphology of the
argillic horizon in desert soils of southern New Mexico.Soil Science 106:6-15.
Gile, L. H.,J. W. Hawley, and R. B. Grossman. 1981. Soils
and geomorphology in the Basin and Range area of
southern New Mexico - guidebook to the Desert Project. Memoir 39. New Mexico Bureau of Mines and
Mineral Resources, Socorro, New Mexico. 222 p.
Gile, L. H" F. F. Peterson, and R. B. Grossman. 1966.
Morphological and genetic sequences of carbonate
accumulation in desert soils. Soil Science 101:347360.
Lindroth,C.H.1953.Someattempts toward experimental
zoogeography.Ecology34:657-666.
Martin, S. C. 1975. Ecology and management of southwestern semidesert grass-shrub ranges: the status of
our knowledge. US.D.A. Forest Service Research Paper RM-156. Rocky Mountain Forest and Range
E ~periment Sta tion, Fort Collins, Colorado. 38 p.
MartIn, S. C.1983. Responses of semi-desert grasses and
shrubs to fall burning. Journal of Range Management
36:604-610.
McAuliffe,]. R. 1991. Demographic shifts and plant succession along a late Holocene soil chronosequence in
the Sonoran Desert of Baja California. Journal of Arid
Environments 20:165-178.
McAuliffe,J.R.1994. Landscape evolution, soil formation,
and ecological patterns and processes in Sonoran Descrtbajadas.EcologicaIMonographs64:111-148.
McAuliffe,]. R.1995. Landscape evolution, soil formation,
and Arizona's desert grasslands. In: M. McClaran (ed,)
The desert grassland. University of Arizona Press,
Tucson, Arizona. (In press).
McGeorge, W. T.1942.Studiesonplantfood availability in
a!kali~e-calcare?us soils: seedli.ng tests and soil analyS]s. Artzona AgrIcultural Experiment Station Technical
Bulletin94.
Menges, C.M. and L.D. McFadden. 1981. Evidence for a
latest Miocene to Pliocene transition from baSin-range
tectonic to post-tectonic landscape evolution. Arizona
Geological Society Digest 13:151-160.
Merriam, G. H.1890. Results of a biological survey of the
San Francisco Mountains region and desert of the Little
Colorado in Arizona. USDA, North American Fauna 3
Washington, D.C .136 p.
'
Morrison, R. B. 1985. Pliocene/Quaternary geology, geornorphology, and tectonics of Arizona. Pp. 123-146 in
D. L. Weide (ed.). Soils and quaternary geology of the
southwestern United States. Geological Society of
America Special Paper 203.
Noy··Mier, 1.1973. Desert ecosystems: environments and
producers. Annual Review of Ecology and Systematics
4:25-51.
ACKNOWLEDGEMENTS
We sincerely thank Mr. Joe Goff, owner and
operator of the U-Circle Ranch for the hospitality
he extended while we worked on his land and
grazing allotnlents. We also thank Dr. R. B. Scarborough for visiting the stops with us and
providing valuable information on local geology.
Dr. P. Pearthree reviewed the manuscript.
LITERATURE CITED
Arizona Bureau of Mine:: 1959. Geologic map of Pinal
County, Arizona. Arizona Bureau of Mines, Tucson,
Arizona.
Arizona ~eol~gical Society. 1~52. Guide book for field trip
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Amenca, Cordilleran Section Meeting, April 10-14
1952.
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'
Bull, W. B. 1991. Geomorphic responses to climatic change.
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106
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107
APPENDIX 1. SCIENTIFIC AND COMMON NAMES OF PLANTS
GRASSES
Digitaria californica
Eragrostis lehmanniana
Erioneuron pulchellum
Heteropogon contortus
Hilaria belangeri
Hilaria mutica
Muhlenbergia porteri
Panicum hirtica ule
Setaria macrostachya
Tridens muticus
Aristida purpurea
Purple three-awn
Aristida purpurea var. wrightii Three-awn
Aristida ternipes
Spidergrass
Bothriochloa barbinodis
Cane beardgrass
Bouteloua curtipendula
Sideoats grama
Bouteloua eriopoda
Black grama
Bouteloua filiformis
Slender grama
Bouteloua hirsuta
Hairy grama
Bouteloua rothrockii
Rothrock grama
Bouteloua trifida
Red grama
Arizona cottontop
Lehmann love grass
Fluff grass
Tanglehead
Curly mesquite grass
Tobosa grass
Bush muhly
Annual panic grass
Bristlegrass
Slim tri dens
TREES AND LARGE SHRUBS
Acacia constricta
Acacia greggii
Arctostaphylos pungens
Ceanothus greggii
Cercidium microphyllum
Cercocarpus montanus
Dodonaea viscosa
Ephedra fasciculata
Fouquieria splendens
Larrea tridentata
Lycium berlandieri
Lycium fremontii
Mimosa biuncifera
Prosopis velutina
Quercus emoryi
White-thorn acacia
Catclawacacia
Manzanita
Desert ceanothus
Foothill paloverde
Mountain mahogany
Hop bush
Mormon tea
Ocotillo
Creosotebush
Wolfberry
Wolfberry
Wait-a-minute
Velvet mesquite
Emory oak
SUBSHRUBS
Ambrosia deltoidea
Calliandra eriophylla
Eriogonum wrightii
Gutierrezia sarothrae
Isocoma tenuisecta
Janusia gracilis
Krameria parvifolia
Zinnia acerosa
Triangleleaf bursage
Fairy duster, False mesquite
Wright's buckwheat
Broom snakeweed
Burro-weed
Slender janusia, samara vine
Range ratany
Desert zinnia
SUCCULENTS AND ROSETTES
Carnegiea gigantea
Dasylirion wheeled
Ferocactus wislizenii
OpuIltia engelmannii
Opuntia fulgida
Yucca baccata
Saguaro
Sotol, Desert spoon
Fish-hook barrel cactus
108
Engelmann's prickly pear
Chain-fruit cholla
Banana yucca