BASE-LEVEL OF EOLIAN EROSION 663 
account of the inflow of phreatic waters. ‘This seems to be also the 
true explanation of the presence of many of the salinas and playas 
which are of such frequent occurrence throughout the arid country. 
The surface waters derived from the violent but sporadic “cloud- 
bursts”? contribute to playa-formation' in many cases, but in the 
majority of instances the ground-waters doubtless have very much 
the greater influence. 
A noteworthy instance is the Sandoval bolson, or Estancia 
plains, in central New Mexico. This is the highest and driest 
bolson of the Mexican tableland within the limits of the United 
States.? Its surface is 6,000 feet above the sea. Its center is occupied 
by a great chain of dry and bitter lakes. In all of its vast area, and 
during a period of 400 years since the earliest Spanish invasion, only 
two small springs of potable water were known within its confines. 
Recently, it was inferred from the general character of the great 
basin, its geologic structure, and the location of two springs, that 
ground-water level at certain points must come very near the surface. 
Proceeding upon this hypothesis several test-wells were put down 
and the inference found to be correct. At once there was excavated 
an area of several acres in extent for reservoir purposes. Now there 
stands a fine large body of soft water, the surface of which comes 
within a few feet of that of the surrounding plain. Around the lakelet 
a prosperous town has sprung up. 
As the geographic cycle in an arid region goes on and the whole 
face of the country becomes worn down to the condition of a pene- 
plain there finally must come a time when the general ground-water 
level nearly coincides with that of the plains-surface and deflation 
can proceed no farther. ‘This level which is perfectly independent 
of sea-level can never be very far below it. The base-leveled area 
in an arid region may thus compare favorably in size and character 
with any of the peneplains in humid climates. The base-level of 
eolian erosion under conditions of an arid climate seems as com- 
pletely controlled and as sharply delimited by definite physical factors 
as is normal peneplanation. 
t Am. Jour. Sci. (4), XVI (1903), 377- 
2 Journal of Geology, XVI (1908), 434. 
