34 
GEOLOGY: C. KEYES 
thin sheet of gathered storm-waters — the play a. There is next the long 
smooth slope of low inclination, having about a 2% gradient. This is 
followed by a short, steeper slope, with a 4% grade, to which the name 
hajada is properly given. Fourth is the mountainous periphery. 
In the literature on the arid regions the bajada-belt is usually treated 
as one of the most conspicuous and important drainage features, as 
formed by prodigious outwash from the peripheral highlands of basins 
of centripetal drainage, and as consisting of a series of great coalescing 
delta-fans. To this interpretation several strong objections arise. 
Discordant facts greatly outweigh the supporting evidence. The 
piedmonts of western deserts most frequently described chance to be 
on the margins of the Great Basin where the lofty Wasatch range and 
the still loftier Sierra Nevada produce effects which are not at all typi- 
cal of the true Basin-ranges. In other places the steeper parts of the 
intermont plains, or bajadas, often mark belts of resistant rocks and are 
almost devoid of soil. In still other cases the extension of the even- 
sloping bajadas up into the valleys of mountain-arroyos is manifestly 
the direct result of rapid and tremendous drifting of soils from the low- 
land plains, rather than of the gravitational flow of detrital materials 
from the adjacent highlands. The volume of finer rock-waste brought 
down from a desert-range by the infrequent storm-waters is not by any 
means what might be expected; it is, in reality, phenomenally small. 
As more fully stated elsewhere the yearly amount washed down by the 
rains may be swept away by the winds in a single day. 
In spite of the fact, then, that the hajada is often a belt of thick, 
adobe soils, of drifting sands, or of sporadic outwash from the nearby 
mountains, it is also still oftener true that it is an area of the most 
indurated rock, so free from soil that, as W. J. McGee describes, "the 
horses' shoes beat on the planed granite, and schist and other hard rocks 
in traversing the plain 3 or 5 miles from the mountains." Then, again, 
there are typical bajadas widely separated from desert ranges by deep 
longitudinal valleys which hug the mountain bases — conditions under 
which there is no possible chance for the outwash from the highland to 
reach the plain. The Sandia, Manzano and Caballos sierras, in central 
New Mexico, are a few of the many notable examples. 
The terracing of arroya-courses in the piedmont belt appears usually 
to be merely the outcome of a vigorous contest which is constantly waged 
between the local, temporary aggrading of wind-driven soils or sands 
on the one hand, and on the other hand by the weak degrading action 
of the infrequently running mountain torrent. The phenomena of 
bajada-terracing is not, as urged by some physiographers, a necessary 
