EVOLUTION OF MODERN SURFACE FEATURES 39 



region has been uplifted several thousand feet since the early 

 Tertiary, and at least some of the ranges have undergone actual, 

 rather than merely relative, uplift. Structures resulting from crustal 

 tension are no doubt present in the region, but they may have 

 resulted from components of a more pervasive crustal compression. 

 This compression, however, manifested itself in a different guise 

 from that which deformed the eugeosynclinal and miogeosynclinal 

 rocks at an earlier period. 



Time Relations of Basin and Range Structure. In the generalized 

 picture sketched above. Basin and Range topography and structure 

 were presented as an accomplished fact, although their development 

 through time was hinted by the varying degrees of erosion observed 

 in different ranges. But the present topography and structure were 

 long in the making, and when one attempts to trace their develop- 

 ment through time, the picture at once becomes more complex 

 (Longwell, 1950, p. 427). 



For example, it was stated that the low-standing blocks, or desert 

 basins, were largely filled by detritus eroded from the adjoining 

 ranges, but these deposits formed during a considerable span of 

 Tertiary time, in which the geography changed as a result of con- 

 tinuing crustal movements. Earlier basin deposits were derived 

 from ranges in a state of growth different from the present ranges 

 and perhaps even in different positions; in places the deposits were 

 spread over the sites of ranges that developed later. With further 

 movements, the earlier deposits were faulted, tilted, and eroded, and 

 those in which resistant lavas were embedded were raised in places 

 to mountainous heights. Later Tertiary and Quaternary deposits 

 bear a closer relation to modern geography, although even these are 

 more or less deformed and eroded. 



Sierra Nevada Topography and Structure. The Sierra Nevada, 

 which lies west of the Great Basin, is a single massive block 400 

 miles long and 80 miles wide, whose crest attains alpine heights, yet 

 its form and structure differ only in degree from the mountain 

 blocks of the Basin and Range province, and its development was 

 closely related to at least the Great Basin segment of that province. 

 The Sierra Nevada block may have been shaped by the great masses 

 of granitic rocks embedded in its deformed eugeosynclinal strata, as 

 these extend along its eastern side for most of its length. 



The Sierra Nevada faces the Great Basin on the east in a series of 



