282 University of Kansas Geological Survey. 
plains area was covered by a fresh water lake in which the Tertiary 
material accumulated. He says: 
“At the close of the Eocene a large part of the plains area, from 
middle ixansas indefinitely northward, became depressed and re- 
ceived the drainage which now forms the western affluents of the 
Mississippi, the Missouri, the Red river, and other of the British 
Columbia rivers, forming a wide sheet of water. 
“For this I propose the name Sioux Lake. 
“Unfortunately the Fortieth Parallel area only covers a very 
slight exposure of the series of the Miocene beds which accumulated 
in Sioux Lake, to which, long since, Hayden gave the name of the 
White River group.” 
From that date-—1878—to the present time the literature of the 
plains, with one exception, seems to show no objections to this state- 
ment. On the contrary the general application of the lake deposits 
have been carried southward, covering the whole of the state. In 
his discussion of the origin of the Tertiary in eastern Colorado along 
the Arkansas river in 1896 Gilbert! deviates from the customary 
method and decides that they were fluviatile in character rather 
than lacustrine. After speaking of the erosion which was set up by 
the uplifting of the mountain area he says: 
“Kventually the process of erosion was completely arrested and 
processes of decomposition took its place. This change was brought 
about by some modification of conditions which is not yet clearly 
understood. Perhaps the plains region was depressed at the west, 
and the slopes thus rendered so gentle that the streams could no 
longer carry off the detritus which came from the mountains, and 
it was deposited on the way. Perhaps a barrier was lifted at the 
east so that the base level stood higher. Whatever the cause the 
streams which flowed from the mountains onto the plains and thence 
eastward across the plains, ceased to carve valleys in the region of 
the plains and began to deposit sediment. When they had filled 
their channels so that their beds lay higher than the neighboring 
country, they broke through their banks, shifting their courses to 
new positions, and they thus came to flow in succession over all 
parts of the plains and to distribute their deposit widely, so that 
boxe, hey, joy 2-5, 
