498
BERNARD L. FONTANA
respectively (Hastings and Turner, 1965, p. 16; Simmons, 1965, p. 10).
Although western Pimeria Alta is nearly frost-free, as the above figures for
Yuma and Ajo indicate, temperatures below the freezing point have occasionally been recorded.
The amounts, regional variations, and seasons of precipitation and temperature fluctuations determine in large part the efficient use of rainfall
which can be made by native vegetation. Ecologists have defined "effective
precipitation" as that which "penetrates the soil surface far enough to become available to plants, without percolating on through" (Hastings and
Turner, 1965, p. 15). Furthermore, it is clear that the amount of water
thus becoming available to plants also depends on the nature of the soil,
the gradient of slopes, and on the amount of runoff as contrasted to the
amount of penetration. The physical geology and topography of the region,
therefore, are also important considerations.
b. Physiography. Virtually all of Pimeria Alta, physiographically speaking, lies within the so-called Desert Region of the Basin and Range Province (Wilson, 1962, Fig. 13). This is comprised of a series of alternating
mountain ranges and valleys, both generally lying diagonally in a southeast
to northwest direction and both generally increasing in altitude above sea
level moving from west to east.
The arid west of Pimeria Alta is made up of rows of mountain ranges
and hills rising sharply out of their intermontane valleys. In Arizona, these
"rows" include the Gila-Tinajas Altas mountains (960 meters maximum
elevation), Baker Peaks-Copper-Cabeza Prieta-Tule mountains (890
meters), Sierra Pinta-O'Neil-Davidson Hills (850 meters), MohawkBryan-Agua Dulce mountains (890 meters), Aztec Hills-Aguila-Granite
mountains (533 meters), Growler-Bates-Puerto Blanco-Sonoyta mountains (1013 meters), Crater-Childs-Little Ajo mountains (710 meters),
and the Batamote-Redondo-Ajo mountains (1403 meters). Adjacent to
these lines of mountains and hills lie the Yuma Desert (150 meters),
Lechuguilla Desert (215 meters), Mohawk Valley-Tule Desert (250-275
meters), San Cristobal Valley-Growler Valley-La Abra Plain (350
meters), and Childs Valley-Valley of the Ajo-Sonoyta Valley (460
meters). The "valleys" and "plains" are deserts in every sense of the word,
and even the highest of these peaks never rise above the lower Sonoran
life zone (see Fig. 1 ).
The topography of northwesternmost Sonora is relatively simple. Immediately east of the Colorado River is the low-lying Sonoran Mesa, bounded
on its east by the Great Sand Dunes. To the east of these dunes, sometimes
called "El Gran Desierto" (Powell, 1964, 1966), lie the crater-pocked
Sierra del Pinacate, a few isolated mountain clusters [Sierra Pinta, Sierra
de San Francisco, Sierra del Pozo (Sierra de los Tanques), Sierra del
BERNARD L. FONTANA
respectively (Hastings and Turner, 1965, p. 16; Simmons, 1965, p. 10).
Although western Pimeria Alta is nearly frost-free, as the above figures for
Yuma and Ajo indicate, temperatures below the freezing point have occasionally been recorded.
The amounts, regional variations, and seasons of precipitation and temperature fluctuations determine in large part the efficient use of rainfall
which can be made by native vegetation. Ecologists have defined "effective
precipitation" as that which "penetrates the soil surface far enough to become available to plants, without percolating on through" (Hastings and
Turner, 1965, p. 15). Furthermore, it is clear that the amount of water
thus becoming available to plants also depends on the nature of the soil,
the gradient of slopes, and on the amount of runoff as contrasted to the
amount of penetration. The physical geology and topography of the region,
therefore, are also important considerations.
b. Physiography. Virtually all of Pimeria Alta, physiographically speaking, lies within the so-called Desert Region of the Basin and Range Province (Wilson, 1962, Fig. 13). This is comprised of a series of alternating
mountain ranges and valleys, both generally lying diagonally in a southeast
to northwest direction and both generally increasing in altitude above sea
level moving from west to east.
The arid west of Pimeria Alta is made up of rows of mountain ranges
and hills rising sharply out of their intermontane valleys. In Arizona, these
"rows" include the Gila-Tinajas Altas mountains (960 meters maximum
elevation), Baker Peaks-Copper-Cabeza Prieta-Tule mountains (890
meters), Sierra Pinta-O'Neil-Davidson Hills (850 meters), MohawkBryan-Agua Dulce mountains (890 meters), Aztec Hills-Aguila-Granite
mountains (533 meters), Growler-Bates-Puerto Blanco-Sonoyta mountains (1013 meters), Crater-Childs-Little Ajo mountains (710 meters),
and the Batamote-Redondo-Ajo mountains (1403 meters). Adjacent to
these lines of mountains and hills lie the Yuma Desert (150 meters),
Lechuguilla Desert (215 meters), Mohawk Valley-Tule Desert (250-275
meters), San Cristobal Valley-Growler Valley-La Abra Plain (350
meters), and Childs Valley-Valley of the Ajo-Sonoyta Valley (460
meters). The "valleys" and "plains" are deserts in every sense of the word,
and even the highest of these peaks never rise above the lower Sonoran
life zone (see Fig. 1 ).
The topography of northwesternmost Sonora is relatively simple. Immediately east of the Colorado River is the low-lying Sonoran Mesa, bounded
on its east by the Great Sand Dunes. To the east of these dunes, sometimes
called "El Gran Desierto" (Powell, 1964, 1966), lie the crater-pocked
Sierra del Pinacate, a few isolated mountain clusters [Sierra Pinta, Sierra
de San Francisco, Sierra del Pozo (Sierra de los Tanques), Sierra del