VIII. PIMAN INDIANS OF THE SONORAN DESERT
499
Cubabi], and the Desierto de Altar. The Sierra del Pinacate, rising to 1291
meters above sea level, dominates all (Ives, 1964b).
The soil in these mountains and valleys, like that throughout the
Sonoran Desert, is probably formed chiefly by the disintegrating action
of 16°-21°C rainwater falling on basaltic rocks heated to 66°-71°C by
the sun's rays. Low-lying hills and rocks are also ground to soil by the
effects of sand and wind erosion (Shreve and Wiggins, 1964, p. 22).
According to Shreve and Wiggins ( 1964, p. 22) :
The general features of the relation of land forms and physiographic processes
to soil history are the same in arid and humid climates. A series of soils ranging from mountainside or talus slope through outwash slope and bajada to
playa or flood plain will exhibit a decreasing number of large stones and an
increasingly finer texture. Because of torrential rains and the lack of continuous cover of vegetation, there is a more ready downgrade movement of
coarse material in the arid climate, and a consequent coarser texture in the
soils of such relatively mature surfaces as the lower bajadas.
Where the gradients of upper mountain slopes are steeper, as they are
in western Pimeria Alta, the bajadas are shorter and, because of sheetflood
erosion, more likely to be devoid of soils capable of supporting much plant
life. Humus in these desert soils is normally less than 1 %, and such organic
matter as it may contain is quickly oxidized and decomposed. The intermontane basins, which are flat, receive insufficient rainfall and runoff to
allow their stream beds to carry the water to the sea, with the result that
the valleys are like land-locked basins. Furthermore, this means that the
soluble salts found in desert soils that are flushed out by rainfall are ultimately deposited in these basins where they become a detriment to the
growth of most plants (Shreve and Wiggins, 1964, pp. 24-25).
c. Plant Life. Because nearly all of western Pimeria Alta lies within
Shreve's Lower Colorado Valley vegetational subdivision, what Shreve says
concerning this province is apropos here (Shreve and Wiggins, 1964, pp.
49-50 and 57-68). He points out that about 85% of the area, excluding
the delta of the Colorado River, is made up of plains and bajadas of low
gradient and that these, in turn, have a plant population which is comprised of 90-95% Larrea tridentata (creosote bush) and Franseria dumosa
(white bursage; burro weed). It is this Larrea-Franseria combination
which earns the label "microphyllus desert" for the Lower Colorado
Valley.
The remaining microenvironments within the subdivision—the margins
of drainageways in plains and lower bajadas, upper bajadas, sandy plains
and dunes, malpais fields and volcanic hills, older volcanics, and granitic
mountains and hills—support a richer vegetation. Mesquite (Prosopis
499
Cubabi], and the Desierto de Altar. The Sierra del Pinacate, rising to 1291
meters above sea level, dominates all (Ives, 1964b).
The soil in these mountains and valleys, like that throughout the
Sonoran Desert, is probably formed chiefly by the disintegrating action
of 16°-21°C rainwater falling on basaltic rocks heated to 66°-71°C by
the sun's rays. Low-lying hills and rocks are also ground to soil by the
effects of sand and wind erosion (Shreve and Wiggins, 1964, p. 22).
According to Shreve and Wiggins ( 1964, p. 22) :
The general features of the relation of land forms and physiographic processes
to soil history are the same in arid and humid climates. A series of soils ranging from mountainside or talus slope through outwash slope and bajada to
playa or flood plain will exhibit a decreasing number of large stones and an
increasingly finer texture. Because of torrential rains and the lack of continuous cover of vegetation, there is a more ready downgrade movement of
coarse material in the arid climate, and a consequent coarser texture in the
soils of such relatively mature surfaces as the lower bajadas.
Where the gradients of upper mountain slopes are steeper, as they are
in western Pimeria Alta, the bajadas are shorter and, because of sheetflood
erosion, more likely to be devoid of soils capable of supporting much plant
life. Humus in these desert soils is normally less than 1 %, and such organic
matter as it may contain is quickly oxidized and decomposed. The intermontane basins, which are flat, receive insufficient rainfall and runoff to
allow their stream beds to carry the water to the sea, with the result that
the valleys are like land-locked basins. Furthermore, this means that the
soluble salts found in desert soils that are flushed out by rainfall are ultimately deposited in these basins where they become a detriment to the
growth of most plants (Shreve and Wiggins, 1964, pp. 24-25).
c. Plant Life. Because nearly all of western Pimeria Alta lies within
Shreve's Lower Colorado Valley vegetational subdivision, what Shreve says
concerning this province is apropos here (Shreve and Wiggins, 1964, pp.
49-50 and 57-68). He points out that about 85% of the area, excluding
the delta of the Colorado River, is made up of plains and bajadas of low
gradient and that these, in turn, have a plant population which is comprised of 90-95% Larrea tridentata (creosote bush) and Franseria dumosa
(white bursage; burro weed). It is this Larrea-Franseria combination
which earns the label "microphyllus desert" for the Lower Colorado
Valley.
The remaining microenvironments within the subdivision—the margins
of drainageways in plains and lower bajadas, upper bajadas, sandy plains
and dunes, malpais fields and volcanic hills, older volcanics, and granitic
mountains and hills—support a richer vegetation. Mesquite (Prosopis