Drift in South Dakota. — Todd. 97 
(b) The drift in northeastern Nebraska, though suggest- 
ing previous advance of an ice-sheet, is, nevertheless, from its 
thinness and its relation to the Altamont moraine, thought to 
be due, in part, to marginal waters ; with a possible sub-glacial 
origin for a portion of it resulting from an extreme advance of 
the ice-sheet, slightly, antedating that moraine. Because this 
conclusion seemed to disagree with those derived from other 
regions, the writer's results of several years' work in the Mis- 
souri valley have been withheld from publication for several 
years. * 
/. Inference from the Trough of the Missouri , River. — Since 
1884, it has been generally recognized that the relation of the 
outer moraine and its drainage channels and attendant deposits, 
to the Missouri river, and the narrowness of the channel of the 
latter above Yankton, with the reflection of pre-glacial topog- 
raphy in the ice movements, all indicate that the Missouri river 
was displaced from the James river valley, and forced to take 
its present course above Yankton, by the advance of the ice- 
sheet of the Wisconsin. Now the inference mentioned is this: 
that if the Missouri was so displaced by the Wisconsin advance 
(and this hypothesis certainly furnishes the best explanation of 
the known facts), then the James river valley was occupied l)y 
that stream previous to that time, at least during the so-called 
Kansan stage. (Possibly some of its upper tributaries may 
have discharged to the northeast in pre-glacial times.) If so, 
we can hardly conceive any sub-glacial till occurring in or west 
of the axis of that valley or in the Missouri valley above Sioux 
City. That the James river valley and that of the Missouri 
river below Yankton, are really identical is indicated by their 
widths and depths and relations to the drift. If this were not 
true, we must believe that both the James valley and the wide 
Missouri valley below Yankton are of pre-glacial origin 
to their present depths; that the Missouri was displaced by the 
Kansan advance; that it nnist have had another channel below 
Niobrara or Yankton in that epocli, and that that channel has 
been so filled that it is unrecognizable, while the Missouri be- 
low the latter place has been kept unfilled in some inconceiv- 
able way during the recession of the Kansan ice and particular- 
*They have been recently published as Bulletin No. 158 but without 
theoretical cxiilanation. 
