548 FSB-WISCONSIN GLACIAL DRIFT IX MONTANA 



1,600 feet lower than the drift capping Kennedy Bidge, a few miles 

 farther southwest. 



In response to a letter of inquiry concerning this bed (B) of north- 

 eastern drift, Doctor Calhoun writes in part as follows : s 



"Along this river, in the vicinity of Sloan Ranch, there had been a great 

 deal of slumping and. with the time and help that I had at my command. I 

 could not take the trouble to make any excavations. I was satisfied in my own 

 mind that the lower bed B was really part of the older drift sheet, but. 

 although I looked for many days. I found nothing else in any other river 

 valley to substantiate this. The age of the boulders in the two sheets, as 

 shown by the amount of weathering to which they had been subjected, was 

 very different and. if I remember rightly, as far as I could make out from the 

 badly weathered specimens, the rock material was also different. I left the 

 section very much pleased with what I thought was evidence that could not be 

 disputed as to the existence of two different ice-sheets from the northeast, but 

 after many weeks of search I came to the conclusion that it would not do to 

 claim the existence of another sheet upon the evidence of one section. 



"The layer B was a layer of boulders — some of them igneous and every one 

 badly decomposed. In this layer there was not one sound rock of igneous 

 origin, although there were quartzite gravels in the formation which were not 

 decomposed. These decomposed igneous rocks occurred in considerable num- 

 bers. I am not sure whether this deposit was till or material let down by 

 erosion. If it were till, it was very stony till. I found no other deposit in the 

 area which contained badly decomposed igneous rock of undoubtedly north- 

 eastern origin.'* 



From Doctor Calhoun's statement it seems to be not at all certain that 

 the deposit is unmodified till left in its present position by the melting 

 of the continental glacier. This being the case, the inference is war- 

 ranted that it was let down by erosion from a higher level, perhaps from 

 a horizon corresponding to the high tracts where the pre-TTisconsin moun- 

 tain drift now occurs, and that its occurrence now at so low a level has 

 no direct bearing on the amount of erosion accomplished and length of 

 time elapsing since the deposition of the pre-Wisconsin drift. 



It is not probable or. as it seems to us, possible, on account of the topo- 

 graphic relations, that the Keewatin ice was depositing the bed of drift 

 (B) at so low a level in Saint Mary Valley in the vicinity of Sloans 

 Ranch at approximately the same time that the mountain glaciers were 

 depositing the drift on the high-level tracts. 



Farther north, however, on Belly Biver near Lethb ridge and on Old- 

 man Biver as far west as Brocket (figure 1). there are considerable de- 

 posits of unmodified glacial till, a part of which evidently is and a part 

 of which may be of pre-Wiseonsin age, exposed in the bluffs, which are 



8 Personal communications, November 25 and November 30, 1912. 



