56 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTURE. 
across it, usually without much vertical displacement on either side. 
As the rainfall 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 coulées. Some of the lakes overflow and are fresh, 
others do so seldom if ever and are brackish or saline. In Grand 
Coulée 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 coulées, 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 OF 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withdrawn, until to-day there is a large area at the foot of the Rockies 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.t 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. Raymond J. Pool, of the University of 
Nebraska. 
