POST-KANSAN EROSION 
85 
these and possibly other streams must be taken into considera- 
tion in gaining the correct conception of the length of post- 
Kansan time. The well-known fact that the Kansan drift-plain 
has been quite thoroughly dissected to a mature stage of erosion 
is evidence in itself of the great age of the drift. But it would 
seem that if these valleys have been cut by streams which were 
superimposed here and there upon buried rock divides, the rate 
of erosion must have been considerably retarded, possibly giv- 
ing sufficient time for considerable decomposition of the Kan- 
san drift over the great featureless, low-gradient plain before 
any considerable areas had been dissected. 
The writer has had in mind extending his studies to include 
the Missouri and other large tributaries of the Mississippi river 
in order to determine what factors were operative in affecting 
the rate of dissection of the older drifts, but his call to other 
fields makes this survey impossible. The Missouri field is an 
inviting one for if if is found that the Missouri river is super- 
imposed at Glasgow, Missouri, as is suggested by the narrow 
width of the valley there as compared with that above, as shown 
on the topographic map, and as suggested by Todd’s descrip- 
tions of the buried channel to the southwest in the vicinity of 
Salt Springs, and by his descriptions of the drift materials in 
the valley walls of the Missouri above Glasgow, and if it is found 
for example, to be post-Kansan, then obviously this would affect 
the rate of dissection of much of the area upstream, which would 
involve southern and southwestern Iowa, It is to be notea mat 
the Chariton river joins the Missouri just above the rock chan- 
nel at Glasgow, that the mouth of the Grand river is a little 
farther up, and the Platte a little farther, still. If it is found 
that each one of these streams is superimposed at several places 
along its course on old rock divides this retarding factor would 
be still more important as applied to large areas. The results 
of such a study would promise to be of importance in gaining 
the proper conception of the erosional and weathering conditions 
existing during the Pleistocene. In so doing the exact dates of 
' superimposition should be ascertained. 
Department of Geology, 
University of Washington. 
