Era Period Epoch Years Major
before geological
present events
time
Cenozoic Quaternary Recent 10,000 Cascade Range
and Sierra
Nevada uplifted
Pleistocene
1,000,000
Tertiary Pliocene
12,000,000
Miocene
26,000,000
Oligocene
38,000,000
Eocene
54,000,000 Rocky Mountains
formed
Paleocene
65,000,000
Mesozoic Cretaceous
136,000,000
Jurassic
195,000,000
Triassic
250,000,000 Appalachian
folding
Paleozoic Permian
280,000,000
Pennsylvanian
320,000,000
Mississippian
345,000,000
Devonian
395,000,000
Silurian
440,000,000
Ordovician
500,000,000
Cambrian
570,000,000
Precambrian
[Illustration: Summer thunderstorm in the Rincon Mountain Section.
The Catalina Mountains rise in the background.]
climate:
the vital factor
Climate is the chief arbiter of life on earth. Each plant and animal,
including man, has tolerance limits for heat and water below or above
which it dies. The Sonoran desert, with its low and uncertain rainfall
and high summer temperatures, thus presents one of earth’s most taxing
environments. For most desert plants and animals, the main problem
presented by this climatic combination is a scarcity of water. Why, we
might ask, is this part of the globe so dry?
Essentially, the dryness of the Sonoran and other southwestern deserts
results from their geographic situation. First, these lands lie east of
a persistent high-pressure cell over the eastern Pacific—part of a belt
of high-pressure cells which encircles the earth at this latitude.
High-pressure systems in the Northern Hemisphere have descending,
warming, dry air in their middles and along their eastern sides. The air
that flows eastward from the “East Pacific high” thus contains little
moisture, and dictates desert conditions over much of the Southwest.
Furthermore, the mountains that more or less ring these deserts
intercept much of the meager incoming moisture, by forcing the air to
rise and cool so that its moisture condenses and falls to the ground.
But during two periods of the year, the desert does receive rain. In
winter, the East Pacific high shifts southward with the sun, reducing
its effects on the Southwest. Now the low pressure storm systems, which
at this season move eastward across the continent, can sometimes dip far
enough south to bring rain to the Sonoran Desert. These rains are
usually gentle, and cover large areas. Their clouds may cover the sky
for several days at a time.
By April, the East Pacific high has returned far enough northward to
resume its control of southwestern weather. Now the fore-summer drought
sets in, intensified by gradually rising temperatures. Day after day the
sun climbs higher in the sky, baking the earth and testing the endurance
of desert life. Then, sometime in late June or early July, clouds begin
forming in the afternoons over the mountains; finally, one day they
build up to great thunderheads. The drought ends suddenly as rain pours
down and lightning cracks the sky. Down the canyons and along the desert
washes rages the water, brown with soil. In a short time, the storm is
over, and the water disappears into the ground. Just 1 mile away, rain
may not have fallen.
These intense, local summer rains, so different from the winter ones,
have a different origin as well. They are spawned by the Atlantic’s
Bermuda high, which has shifted northwestward and sent moist air from
the Atlantic and Gulf of Mexico flowing off its western edge toward the
deserts. This air rises quickly when it strikes hot, high mountains such
as the Rincons, and its moisture condenses to fall as rain.
Some time in September the rains of summer stop, as the Bermuda High
moves south again, removing its influence and returning the East Pacific
high to a position of dominance. The sun and wind again wring moisture
from the soil, but with diminishing effect. The thermometer no longer
reaches the 100’s as it did in June, July, and August. In November or
December, the winter rains begin. Water stays longer in the soil,
because temperatures now range between 30° and 80°. Life in the desert
begins a new cycle. This rainy period usually lasts into March.
On the mountain slopes, of course, the same seasonal patterns of heat
and precipitation occur, but they are tempered or enhanced by elevation.
With increasing elevation, temperatures decline and precipitation
increases. When July temperatures at headquarters are averaging in the
nineties, they will be in the sixties on top of the Rincons. In the
cactus forest at the foot of Tanque Verde Ridge only about 10 to 12
inches of rain fall in a year, while Manning Camp, at 8,000 feet, gets
about 18 to 25 inches. These up-slope gradients of temperature and
precipitation, as we shall see, have pronounced effects on plant and
animal life.
[Illustration: Gila monster.]
[Illustration: Banded augen gneiss on Cactus Forest Drive. Javelinas
formerly took refuge from the sun in the dens under these
overhanging rocks.]
rocks:
foundation and soilmakers
Soils are derived from the fragmentation and decomposition of rocks.
Combinations of soils and climate, varying from place to place, create
an almost infinite number of environments with differences so subtle and
small as to make it appropriate to refer to them as microenvironments.
Though these differences be minor, they are often sufficient to create
niches each of which becomes a habitat for a particular group of plants
and animals. Thus it is that the saguaros and other desert vegetation in
Saguaro National Monument and vicinity are found in particular
environmental or ecological niches where just the right combinations of
soils, moisture, sunlight, temperature and other factors are present.
Since the rocks provide the foundation and the source of soils that
support the plant and animal life, it is helpful to understand something
about the origin and evolution of the rocks and the landscape of which
they are a part. Clues to this story are to be found in the composition
of the rocks and their relationships to one another.
Rocks of the three major classes—igneous, metamorphic, and
sedimentary—are found within the monument. The igneous rocks include
granites and various kinds of lava flows, together with some intrusive
dikes and veins. The metamorphic rocks are represented by gneiss and
schist. Lastly, sedimentary rocks occur as limestone, sandstone, and
alluvial fill material. Each of these great classes of rocks was formed
in a different manner, and these differences reveal the nature of the
events which are a part of the geologic history of the monument and
surrounding area.
Catalina Gneiss (pronounced NICE) is the predominant rock that visitors
see in the Rincon Mountains and the Tanque Verde Ridge; exposures of
granite and schist are also found. Gneiss is a coarse-grained
metamorphic rock resembling granite, having a banded appearance, and
consisting of alternating layers of different minerals such as feldspar,
quartz, mica, and hornblende. The banding and texture reveal that the
gneiss, now exposed by uplift and erosion, was formed from parent rocks
deep below the earth’s surface during the Precambrian Era more than a
half billion years ago. There the parent rocks, subjected to pressure
and heat, melted, flowed, and crystallized, before resolidification.
Though the gneiss of the Rincon Mountains is ancient, the uplift that
raised it to its present height is thought to have occurred rather
recently (within the past 24 million years), during the period when most
of the Sonoran Desert ranges apparently were formed. In age, these
mountains thus would fall somewhere between the older Rockies and the
younger Sierra Nevada.
The schist, which underlies much of the Cactus Forest, where it is
exposed along washes, was probably formed during the Cretaceous period
more than 65 million years ago. Like the gneiss, the schist is a
metamorphic rock, having been formed by the transformation of parent
rocks at depth under great pressure and heat. Due mainly to differences
in the composition of the parent rock, together with its mica content,
the rock that resulted was platy and cleavable (accounting for its
classification as schist).
Granite, which forms Wasson and Amole peaks in the Tucson Mountain
Section, is an igneous rock that (like the metamorphic gneiss and
schist) originated at depths below the earth’s surface. The granite was
formed by the solidification of molten rock material that moved upward
en masse from greater depths, rather than by the alteration of ancestral
rocks. Erosion of the uplifted land mass has not only stripped away the
overburden but has developed valleys deeply incised into the granite
itself.
Volcanic rocks in the form of rhyolite, andesite, and basalt flows also
are exposed, chiefly in the Tucson Mountain section of the monument.
These are all extrusive igneous rocks composed of magma that solidified
after reaching the earth’s surface through vents or fissures. There are
differences in the lava flows, reflecting differences in composition,
temperature, and other conditions of the magma from which they were
formed. The rhyolite is lighter in color than andesite, and it is
somewhat richer in feldspar. Basalt, on the other hand, is dark, is
deficient in feldspar and quartz, and contains relatively large amounts
of the darker minerals such as hornblende, pyroxene, and olivene. Lava
flows of the rhyolite and andesite variety occur in the Cactus Forest
locality of the Rincon Mountain Section. Cat Mountain Rhyolite is the
name given to a rhyolite flow of Tertiary age that forms the topmost
layer in much of the Tucson Mountain Section. A small exposure of basalt
is also found here.
Limestone, sandstone, and shale formations are also exposed at various
places within the monument. They are reliable indicators of seas that
covered the area during one or more times of the geologic past.
Comparisons with the limestone formations farther to the south in the
Colossal Cave region indicate that the limestone in the monument was
formed as far back as late in the Paleozoic Era (345 million years or
more ago). This age is suggested by types of fossils, including
fragments of crablike trilobites and crinoid (sea lily) stems, found in
limestones of the same age near Colossal Cave.
The Recreation Redbeds of the Tucson Mountain Section is one of the
important sandstone members exposed in the monument. Geologists believe
that its deposition occurred early in the Cretaceous Period more than 65
million years ago. Above the Recreation Redbeds lies a formation
consisting mainly of limestone and commonly known as the Tucson Mountain
“Chaos”—an appropriate name, in view of the geologists’ meager knowledge
of it. Within the Chaos formation, believed to be lower to Middle
Tertiary in age, are limestone blocks of greater age than the rocks
within which they are entombed. No generally accepted explanations have
been advanced as to how these relationships developed.
The processes that destroy mountains continue concurrently with those
that build them. Temperature changes, weathering, and downslope creep
under the influence of gravity are among the agents which destroy the
rocks and eventually convert them into soil. Though these processes work
rapidly from a geological standpoint, in terms of the average human life
span, they progress at an imperceptible rate. Violent thunderstorms and
cloudbursts, however, cause a massive, often spectacular movement of
boulders, gravel, sand, and silt by torrential streams.
In desert regions, most of the runoff from storms sinks into the slopes,
dropping its burden along the way. Big rocks are dropped early as the
carrying power of the water diminishes with the speed of flow; smaller
fragments travel farther. The finest material is carried far out into
the basins between mountain ranges, gradually filling them. (The
alluvial material in the Tucson Basin is estimated to be 2,000 to 5,000
feet thick.) Thus desert mountains tend to bury themselves in their own
debris.
Alluvial fans (fan-shaped deposits built by rivers flowing from
mountains into lowlands) form at the mouths of canyons. Sometimes the
alluvial fans of adjacent canyons coalesce, forming the long, sweeping
slope known as a bajada (pronounced ba-HA-da). In other places the
eroded bedrock extends outward from the bases of desert mountains,
forming “pediments,” which are usually covered by a veneer of alluvial
material. Pediments of this kind stretch from the lower slopes of the
Rincon Mountains. In the Tucson Mountains the alluvial material deepens
rapidly toward the Avra Valley because the bedrock at the base of the
mountain dips steeply downward.
Serious gaps remain in the story of the origin and evolution of the
landscape in Saguaro National Monument. Moreover, it is difficult to
establish the exact sequence in which the various events occurred. The
composition, texture, and relationships of the rocks, however, do reveal
much about the nature of the events and the processes that were
involved.
[Illustration: During summer, mule deer and javelina frequent King
Canyon, which has the only known permanent spring in the Tucson
Mountains. The predominant rock in this major canyon northeast of
the Arizona-Sonora Desert Museum is called Amole Arkose; it is of
sedimentary origin and was deposited during the Cretaceous period.]
[Illustration: GEOLOGICAL CROSS-SECTION OF SANTA CRUZ VALLEY]