6 BULLETIN 5, U. S. Enact AGRICULTURE. 
among the most interesting of present: -day geologic problems, but 
it can not be pursued here. It will suffice to note briefly a few of 
the effects of it and the preceding history upon the topography, and 
especially upon the formation of inclosed basins. In the long period 
during which the Great Basin has been cut off from the sea the 
proeional waste of its mountains has been accumulating in its valleys 
until all are now filled very deeply with such alluvial débris. The 
character of all is the same. Where the mountain reaches the plain 
it is surrounded by a broad alluvial slope or ‘‘apron,” which stretches 
outward with ever-decreasing slope until it merges with the apron - 
of another mountain or into the practically level’ plain which forms 
the deepest depression of most of the valleys. This plain ee = 
a tiny lake, but more commonly it has only a clay flat or ‘‘playa,” 
on which waters gather in wet weather or after storms, but which is 
usually dry. This succession of mountain slope, apron, gradually 
flattening plain, and playa is typical of all the desert basins. The 
playa is the place of concentration of all the present drainage and 
the playa is usually more or less saline, depending upon the amount 
and character of this drainage and the time during which it has 
been received. | 
The alluvial fillmg of the valleys is not of itself of much impor- 
tance to this Inquiry, but one phase of it is very much so. Where 
canyons cut back into a mountain range the discharge of detritus is 
more concentrated and the normal apron grows into an alluvial cone 
or fan which may extend many miles into the valley. If two moun- | 
tain ranges face each other across a trough-like valley (as they usually 
do in this region), and if a canyon in one range chances to discharge 
opposite a canyon in the other, the fans which they build may ulti- 
mately merge in the center of the valley and gradually build a ridge 
or dam which rises few or many feet above the general valley level. 
By this process of “ alluvial damming” a valley trough may be cut off 
at one end or both, or split into sections by dams composed entirely 
of alluvial mountain waste. Obviously this is possible only where 
the climate is arid. If the rainfall and run-off are sufficient to 
maintain a vigorous through-flowing stream the fans can not merge. 
The detritus will be carried entirely out of the valley, or graded to 
slopes which permit free egress of the waters. But it is probable 
that the Great Basin and its environs have been essentially arid ever 
since the early Tertiary and the processes of fan-building and fan- 
merging have been everywhere at work. Many valleys structurally 
open to the sea have been dammed im this way and many of the | 
basins whose major limits are structurally defined have been divided 
by one or many of these alluvial dams. 
Some of the alluvial dams are very ancient, some are very recent. 
The period of lake expansion was, of course, a period of vigorous 
