6 BULLETIN 54_, U. S. DEPAETMENT OF AGEICULTUEE. 



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 

 erosional waste of its mountains has been accumulating in its valleys 

 untU aU are now filled very deeply with such alluvial debris. The 

 character of aU 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 may carry 

 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 

 flattemng 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 filling of the valleys is not of rtseH 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 graduall}^ 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-buUding and fan- 

 merging have been everj^where at work. Many valleys structurally 

 open to the sea have been dammed in 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 



