PROVISIONAL HISTORY OF ORIGIN OF THE MISSOURI 489 



the older deposits there to be early Wisconsin.^^ Leonard considers the 

 older drift in jSTorth Dakota, especially that west of the Missouri, as 

 Kansan, but also recognizes till east of the Missouri, outside the Altamont 

 moraine, to be early Wisconsin. 



It seems not improbable that some of the morainic drift west of the 

 Missouri belongs really to the early part of the Altamont moraine, as, 

 for example, in eastern McKenzie County. It is generally agreed that 

 the ice which was concerned in this case was also from the Keewatin 

 center west of Hudson Bay, and that it was of the AVisconsin stage ; and 

 it seems probable that considerable of the drift outside of the Altamont 

 moraine is to be included also under the term "early Wisconsin,'' and 

 that the great prominence of extra morainic till in Montana, and to a less 

 degree in North Dakota, is to be attributed to the lower temperature of 

 the ice at that higher latitude. 



GLACIAL ACTIVITY INCREASES WITH TEMPERATURE 



A principle that is generally recognized is that higher temperature 

 promotes greater activity in glaciers — that is, that the velocity is much 

 greater in summer time than in winter time. Moreover, much more 

 water is discharged in all streams supplied with water from the ice, 

 forcing them to put on a- torrential phase with maximum. A corollary 

 from this principle would be that glaciers on the south tend to receive 

 more heat, and consequently move more rapidly on that side. For some- 

 what similar reason^ glaciers seem to be more active on their south and 

 west sides because of the greater influence of the sun's heat. 



DURING THE ADVANCE OF THE WISCONSIN ICE-SHEET 



It will be readily seen that, so far as North and South Dakota are 

 concerned, the ice approaches them from the northeast, and it is evident 

 that it must have moved more rapidly in a southward direction ; so that 

 it may have, perhaps, reached the southern line of South Dakota nearly 

 as early as the western part of Montana, and that its appearance opposite 

 the angle northeast of Bismarck may have been considerably earlier than 

 at the points already specified. At any rate, we may suppose that the age 

 of the Altamont moraine is about the same throughout, although older 

 portions — that is, near the outside and at higher levels, unless apt to be 

 removed — may have been accumulating hundreds or thousands of years 

 earlier than the inner portion of the same moraine. 



Previous to the formation of the Altamont moraine, we may suppose 

 that the valleys of all streams flowing north or east would become ob- 



"U. S. Geol. Siirv.. Professional Papers, No. 50, p. 52. 



