4 BULLETIN 54^ U. S. DEPAETMENT OF AGBICULTURE. 



In Paleozoic and Pre-Cambrian time the area which is now the 

 Great Basin was alternately above and below the sea, finally attain- 

 ing in late Carboniferous time an emergence which was to be perma- 

 nent. Its Triassic and Jurassic history is recorded only in fragments. 

 Apparently it was largely and more or less continuously above the sea 

 and was probably eroded to a low and mature relief. With the end of 

 the Jurassic came the birth of the Sierra Nevada and with it the move- 

 ments by which the basin was first outHned. The forces and the yield- 

 ing of which the nascent Sierra were the expression did not spend 

 themselves in this alone, but extended far to the east. At first by fold- 

 ing, later by profound and complex faulting, the former region of 

 inconspicuous relief was broken into a series of troughs and ranges 

 limited on the east by the westward-facing scarp of the Wasatch, as 

 on the west by the Sierra. The more prominent lines of fracture 

 being north and south, and the accompanying crustal displacement 

 mainly by monoclinal tilting, there originated the series of north-and- 

 south trough valleys and of parallel, monoclinal ranges so character- 

 istic of the Great Basin. 



Extensive faulting is Hkely to be pictured as cataclysmic, and one 

 is tempted to think of the Great Basin as breaking in a day, like a 

 dropped platter, from its original unity into the hundreds of structural 

 fragments that now compose it. This is radically wrong. The present 

 structure of the basin has grown very gradually. The movement initi- 

 ated at the close of the Jurassic has continued ever since and is still 

 in progress. So slow, indeed, has been the development of the relief 

 that many streams have been able to maintain what seem to be their 

 Jurassic channels and have cut the rising ranges as fast as they arose. 

 This did not always happen, and sometimes the streams were turned. 

 It would seem that different displacements were of different ages and 

 have grown with differing rapidities. 



Neither must it be imagined that the structure is completely simple 

 and regular. The general parallehsm of valleys and ranges is quite 

 unmistakable, but details are much more complex. Ranges sink and 

 bend and merge with other ranges; valleys join to other valleys and 

 are cut by transverse uplifts; all to make a structure of extreme com- 

 plexity, but through which the original simplicity may stiU be dis- 

 cerned. 



It is impossible to say just when in tliis slow structural develop- 

 ment the region became a "basin; " probably not for a long time after 

 the structure had begun to take form. The whole of the Cretaceous 

 and the early part of the Tertiary seems to have been a period of open 

 seaward drainage and energetic erosion— -an erosion which has severely 

 modified many of the ranges. In the early Tertiary this erosional 

 period was closed (though not necessarily Math causal relation) by a 

 period of intense and long-continued vulcanism which is only now 



