Showing:
Valley Fill
Wasson Peak
_Granite_
_Folded and tilted Sedimentary and Volcanic Rock_
_Santa Cruz River_
_Flood Plain_
_Valley Fill_
Tucson
_Pantano Wash_
Pediment
Tanque Verde Ridge
Mica Mountain
_Gneiss_
The presence of the limestones and sandstones indicates that that area
was submerged below the water of ancient seas one or more times in the
geologic past. The gneiss, schist, and granite bespeak deep-seated
metamorphism and magmatic intrusion, which gave these rocks the form,
composition, and texture they possess today. The lava flows are
indicative of volcanic activity that was a part of the extensive
volcanism that occurred in this part of southern Arizona. And, finally,
the great alluvial fans and bajadas suggest that these mountain ranges
could bury themselves in their own products of erosion unless the
mountain-building processes in future eons continue at a faster pace
than the wearing-down processes. This evidence enables us to perceive
today’s landscape as but a transitional phase in the drama of change
that will continue for milleniums.
[Illustration: Chiminea Creek, originating in the higher elevations
of the Rincon Mountains, forms a ribbonlike oasis of hydrophytic
plants, including large trees such as sycamore, walnut, and
cottonwood.]
plant and animal zones
Mountains are biologically exciting because their elevation produces, in
a short vertical distance, the same climatic changes that occur over
long latitudinal distances. In Arizona, average temperatures drop about
4° with each 1,000-foot rise in elevation, and precipitation increases
about 4 to 5 inches. At sea level, you would have to travel about 900
miles north to experience the climatic change found in going from the
lowest part of the monument (2,200 feet) to the top of Mica Mountain
(8,600 feet). Plant-and-animal communities change along these climatic
gradients.
In southern Arizona, the sequence of vegetation types begins with desert
scrub at the lowest elevations, and ranges through desert grassland, oak
woodland and chaparral, oak-pine woodland, ponderosa pine forest, and
Douglas-fir forest, to Engelmann spruce forest on top of the highest
mountains. In the national monument, we have all these types except the
last—although desert grassland is poorly represented here because the
steep slopes squeeze it into a narrow altitudinal band; and Douglas-fir
forest is restricted to small areas in canyons and on north-facing
slopes at high elevations. A few drainageways (notably Chiminea Canyon,
on the south side of the monument) support patches of deciduous trees
classified as riparian woodland.
These vegetation zones should not be visualized as nice neat bands on
the mountainsides, however, for roughness of topography and differing
tolerances of plant species usually lead to gradual changes from one
zone to the next. Because of the greater exposure to drying heat and
sunlight, each belt occupies a higher range of elevation on south-facing
slopes than on north-facing ones. And along draws, where conditions are
wetter and cooler than on ridges at the same elevations, the vegetation
zones finger down to lower elevations; on ridges the opposite is true.
Animals are less restricted than plants to particular vegetation belts,
but they, too, show a zonation with altitude. You must look, for
instance, in the paloverde-saguaro community (part of the desert scrub
type) for kangaroo rats and cactus wrens; in the oak-pine woodland for
Mexican jays; and in the ponderosa pine forest for whitetail deer and
tassel-eared squirrels.
In the Tucson Mountains, only the desert scrub type is well represented,
although the highest ridges support a suggestion of desert grassland. A
small patch of shrub live oaks, relicts of a wetter period, huddles on
the north side of Wasson Peak. Zonation is poorly developed here because
the mountains are low and small in mass. (Studies have shown that
mountains of smaller mass tend to have warmer and drier climates than
bulkier mountains of the same height.)