134 PROCEEDINGS OF THE BOSTON MEETING 



CERTAIN DIVERSE INTERPRETATIONS OF PLEISTOCENE IN THE DAKOTAS 



BY JAMES E. TODD 



{Abstract) 



Ever since the first study of the Pleistocene deposits of the Missouri River 

 in the Dakotas the opinion has been held persistently that that stream holds 

 its course through that region on account of the diverting influence of the 

 continental ice-sheet as it pushed in from the north and northeast. 



Dr. A. G. Leonard, State Geologist of North Dakota, came to a different 

 conclusion, from his studies in the western part of the State. His conclu- 

 sions were published briefly in the Bulletin of the Geological Society of 

 America, volume 27, His view was given more in detail in the Bismarck 

 folio in 1912. He concludes, first, that the Missouri River has flowed in 

 approximately its present course in North Dakota ever since the middle of the 

 Tertiary, and that the ice-sheet had little effect upon its course. Second, its 

 valley had already been noted down to the present plain of drainage, and 

 that it was partly filled during the latter part of the Wisconsin stage of the 

 Pleistocene. It is the purpose of this paper to consider his reasons and show 

 what conclusions the present knowledge of the subject calls for. 



The first significance of a fact that he considers decisive is that glacial 

 boulders were found near Bismarck resting on bedrock at the bottom of the 

 present river channel, 125 feet below the surface and 80 feet below the 

 surface of the river. The locality was on a river terrace rising 40 feet 

 above the river. It apparently was formed as a terrace of Apple Creek 

 where it joined the Missouri River, and apparently in its maximum flow, 

 during the latter part of the formation of the Altamont moraine, which is so 

 approximately developed a few miles east of that point. He concludes that 

 the river had cut down to the depth where the boulders were found before the 

 formation of the moraine, that the boulders were part of the early portion 

 of the deposit, and that since that time the valley has been filled at least .50 

 feet. This, it may be remarked in passing, would require that the river was 

 relatively 80 feet lower than at present, which is considerably lower than any 

 conditions of this channel farther toward the south or east ; for, according 

 to the conclusions of South Dakota geologists, the Missouri at this point was 

 much higher than at present, making Dr. Leonard's conclusion directly opposed 

 to that just stated. 



He apparently comes to this conclusion by ignoring or underrating the 

 depth of scour exerted by a flooded river. Cases will be cited showing that 

 this scour, to a depth of 130 feet below the stream, has frequently existed. 

 Another fact is the occurrence of till or boulder clay 30 feet or more in thick- 

 ness resting upon Cretaceous clay. The argument which he draws from this 

 is shown to be inconclusive, as is also a point which he considers proved, that 

 the broad, mature valley north of Manden is preglacial. The strip of country 

 between the Altamont moraine and the margin of the drift, 25 to 100 miles 

 •"jirther west, seems to have been occupied, both in North and South Dakota, 



