TOPOGRAPHIC FEATURES OF THE DESERT BASINS. 9 



is beyond the scope of the present report, and which it is desired 

 to avoid. It is thought best, therefore, to designate this period 

 simply by the name of the great lake which best illustrates its 

 history, and to refer to it as the Lahontan period. This is meant to 

 include the whole period of deciphered lake history from the initial 

 rise to the end of the second or final desiccation. No implication is 

 intended as to the internal character of this period, and no specific 

 names are applied to its various divisions. 



THE UNDRAINED AREAS OF THE UNITED STATES. 



It has already been noted that the Great Basin is not a unit. Its 

 parallel mountain ranges cut it into numerous more or less connected 

 valleys, and about haKway across the basin from east to west is one 

 range in particular^-the White Pine-Ruby Eange — which has formed 

 a major parting of the waters of the basin. East of this range is the 

 Bonneville Basin, whose deepest depression was occupied by the 

 ancient lake of that name and whose valleys now drain to its rem- 

 nant — the Great Salt Lake of Utah. West of the range the Hum- 

 boldt River cuts across the northern ends of the north-south ranges 

 and discharges into the Carson Sink, once the home of the ancient 

 Lake Lahontan. The basin of this lake then included not only the 

 drainage of the Humboldt River, but also that of the Carson, Truckee, 

 and Walker Rivers, the two latter of which have since been cut off 

 by desiccation. These, with various smaller basins tributary to the 

 early lake, form the Lahontan Basm. 



North of the whole of the Great Basin and south of the eastern 

 or Bonneville section of it the ranges and trough valleys which char- 

 acterize it merge into wide, dissected plateaus, that of the Columbia 

 and Snake River lavas on the north and that of the Colorado Plateau 

 on the south. The southern limit of Lahontan is very different. 

 The great trough valleys which characterize the core of the .Great 

 Basin are diverse in their slope, some draining northward and some 

 southward. Most often, however, they drain both ways from an 

 alluvial divide somewhere near the center. Thus the troughs forming 

 the eastern part of the Lahontan Basin drain into the Humboldt 

 River from their northern portions, while their southern extremities 

 slope and drain either toward smaller basins also inclosed or toward 

 some tributary of the Colorado River. Farther to the west the south- 

 ern boundary of Lahontan is a transitional area of irregular cross 

 uplift in which are a number of small basins, conveniently grouped 

 with those of the Nevada trough valleys that chance to be inclosed. 

 West and southwest of these is the great trough system of California, 

 containing the Owens, Searles, and Panamint Valleys and their 

 smaller analogues, and the great basin of Death Valley, to which 



19750°— Bull. 54—14 2 



