56 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTURE. 



across it, usually without much vertical displacement on either side. 

 As the ramf all was then (or later) much greater than at present, these 

 cracks determined stream channels and became eroded to considerable 

 depths and with a steep-walled, canyon topography. Later ice- 

 dammed lakes occupied these valleys and supplied the alluvium 

 which forms the present flat bottoms. With the disappearance of 

 these lakes the valleys again became stream channels but apparently 

 not for long. Desiccation intervened and the once through-flowing 

 streams were split into a series of pools or playas. This is the present 

 condition of the coulees. Some of the lakes overflow and are fresh, 

 others do so seldom if ever and are brackish or saline. In Grand 

 Coulee is one — Soap Lake — which is a nearly saturated brine and 

 contains an extraordinarily large proportion of carbonate of soda. 

 But interesting as is this history of the coulees, it indicates clearly 

 the recency of the lakes which occupy them, and therefore their unim- 

 portance to the present inquiry. None of their areas have been 

 computed. 



THE PONDS OP THE GREAT PLAINS. 



The western half of the Mississippi Valley is a great apron sloping 

 imperceptibly upward to the mass of the. Rockies. Over this in 

 Quaternary time stretched a complexly dendritic drainage system, its 

 finger tips reaching to the crest of the mountains and to every ridge 

 and hill between, so that each township had its river and every acre 

 its rill. But advancing aridity has respected this greatest river 

 system no more than the lesser ones to the west. Its streams have 

 been clogged and truncated and its remotest and slenderest tendrils 

 withdrawn, until to-day there is a large area at the foot of the Rockies 

 which has nearly no drainage at all. In all this region alluvial dams 

 and sand dunes (the latter much more than the former) have advanced 

 upon the defenseless drainage, damming the little streamlets in a 

 dozen places, cutting off here and there a tributary of more consider- 

 able size, creating tiny and tinier basins now numbered by the thou- 

 sands. These dot the whole plains region of Nebraska and Wyoming, 

 the northwestern corner of Kansas, the eastern fourth of Colorado, 

 the dune areas of southwestern Kansas, and the great plains of the 

 Pecos Valley and the Llano Estacado, but they are perhaps best 

 exemplified in the Sand Hills of Nebraska.^ Here alluvium and 

 dunes and have conspired against the drainage and with entire suc- 

 cess. The region is a wilderness of rolling hills, originally dunes but 

 now fixed by vegetation with the intermediate valleys dotted with 

 lakes varying in area from a few acres to 2 or 3 square miles. There 

 is usually an annual fluctuation in level of 1 or 2 feet from a maxi- 



1 For much of the information here given I am indebted to Prof. Uaymond J. Pool, of the University of 

 Nebraska. 



