Plate 2.—Regional description. This text stays at a general level, describing the
region as a whole. These broad generalizations are later amplified.
The Colorado Plateau is a region of high plateaus, cut by-
deep canyons, and studded by isolated mountains. It is lo
cated in the "four corners" area of Arizona, New Mexico,
Colorado and Utah (see Fig. 1.1) and covers some 130,000 sq.
mi. It is a dry, sparsely settled region where lack of water
precludes large scale agriculture and settlement, but its
wealth of natural resources is bringing rapid development.
RELIEF - The region is underlain by nearly horizontal beds
of sandstone, shale and limestone, which form elevated plat
eaus. Many isolated areas have been uplifted forming high
mountains, which have either retained their sedimentary cov
ering or have weathered until basaltic laccoliths remain.
The upper sedimentary strata of the plateaus are resistant
to erosion and their surfaces have a mature drainage system
of winding draws and shallow lakes. V.here these strata have
been worn through erosion has accelerated creating canyons.
These exposed faces are more vulnerable to weathering so
canyon walls and fault scarps are crumbling and retreating.
There has been considerable volcanic acivity In the region
and many bf the. Jilateaus have high volcanic mountains tower
ing above them, and the surrounding area is covered by cin
der cones, lava flows and volcanic necks.
DRAINAGE - The region is drained by the Colorado River and
its tributaries. The swift current of the Colorado carries
a heavy load of silt which keeps its channel cutting ever
deeper. The main tributaries, the Little Colorado, the San
Juan, and the Green Rivers flow inward into a wide basin in
the center of the region and then join the Colorado just be
fore it enters the Grand Canyon. Perennial streams are rare.
CLIMATE - The region has a semi-aria climate, which is ex
tremely variable and_ strongly influenced by elevation. Elev
ation creates climatff- zones, with a subarctic climate on the
higher mountains, which rise above 12,000 ft., a temperate
climate on the high plateaus from 8,000 to 7,000 ft., and a
semi-desert climate below 7,000 ft. The mountains receive
large amounts of precipitation, ranging up to 40" annually,
which is about equally divided between summer rains and win
ter snows. The high plateaus receivea20" - 30" annually,
and lower elevations receive between 5" and 20". Droughts
of long duration are common. High summer temperatures in
the low elevations cause strong updrafts, creating local
convectional storms. The storms generally form over the
higher elevations then drift over other areas. This gives
the higher elevations almost daily rains during the summer,
but the lowlands have a progressively smaller chance of
getting the benefit of these drifting storms the farther
they are from the high country. There are strong winds in
spring which disturb the formation of updrafts, and as a
result there is a period of drought through June each year.
COLORADO PLATEAD
SOILS - The soils of the region are generally thin, poorly
developed residual clays and sandy clays resulting froa the
wasting of sedimentary rock. Due to the light rainfall
many of the soils at lower elevations are alkaline, produc
ing little more than alkali tolerant shrubs. Slow soil
movement before the disturbance of vegetation filled many
basins with deep, fertile deposits of soil, but since the
advent of white men and their herds heavy grazing has reduc
ed the vegetation cover and many of these deposits are now
cut by deep gullies. Other basin soils have been irrigated
and are extremely productive. Most of the volcanic areas
are so recent that soils have not yet formed, except where
fine cinders and ash fell. This material acts as a mulch
and makes agriculture possible in some unirrigated places.
NATURAL VEGETATION - The mountains and high plateaus of the
region have a zone of heavy coniferous forests. The forests
are composed of spruce and fir on the mountain tops, with
open stands of pine covering the high plateaus. At lower
elevations there is a zone of open savanna composed of low
pinon, Juniper and grasslands. Open rangelands of sparse
weeds and shrubs cover many low plateaus and basins, with
barren lands at the lowest elevations.
RURAL CULTURE - Livestock raising is one of the principal
occupations and Isolated ranches are found throughout the
region. Large numbers of cattle and sheep are grazed on the
high plateaus and mountains in summer and on the low plat
eaus and basins in winter. Many marks of this activity are
visible, such as earth tanks, windmills, corrals and fences.
Indian reservations occupy large areas and have very primi
tive settlements. Some are pueblos of adobe houses, while
other areas have scattered circular huts called "hogans".
Many people are engaged in timber cutting and sawmilling or
in fuel wood and post cutting. Oil and gas production, and
more recently the processing of uranium, is rapidly expanding.
Small settlements are often found around these processing
plants, but more often settlements are supported by a com
bination of activities and are located at transport Junctions.
URBAN CULTURE - Large settlements have sprung up near irrig
ation and road and rail junctions, which has placed most
of them in the low basins where these occur. These settle
ments are laid out in a characteristic gridiron pattern,
oriented to the railroad, with large blocxs and wide streets,
without shade trees.
TRANSPORTATION - There are only four rail lines in the region
and they follow routes through the lower basins where they are
paralleled by paved highways. An open net of other paved roads
branches off from these axes, and large areas are served by low
standard dirt roads. Rut roads and trails serve the mountains.
REPORT OF COMMISSION VII GVII-85
