628 W. A. JOHNSTON 



and that it overrode three prominent moraines which marked halts 

 or readvances of the ice front. Upham supposed that the Herman 

 beach represented the whole time of the formation of the several 

 moraines and of the retreat of the ice front for at least 250 miles, 

 in spite of the fact that he found the beach to be not very massive 

 and not very notably stronger in the southern than in the northern 

 portion. Recognizing this difficulty, Chamberlin suggested that 

 "the whole history of Lake Agassiz may not have fallen within the 

 period of stationary or rising crustal movement but that the early 

 part of it may have taken place during the latter portion of the period 

 within which the crust was being depressed." 1 In this way it may 

 be supposed that shore lines were formed at early stages of the lake 

 but were later submerged. The uppermost Herman beach would 

 have been formed at the time of maximum submergence. It would 

 be all of one age and would represent a comparatively short time. 



Another difficulty arises from the character of the deposits laid 

 down in the basin of the lake. Upham held that the greater part 

 of these deposits were derived from the ice sheet and its inclosed 

 drift — a necessary inference from his interpretation of the history 

 of the lake. But he found that " bowlders are absent or exceedingly 

 rare in the beaches, deltas, and finer lacustrine sediments." 2 If it 

 is true that a series of moraines was deposited in the lake, and if 

 the sediments of the lake basin were largely derived from the ice 

 sheet, it seems highly probable that berg deposits would form an 

 important part and that bowlders would be included in the sedi- 

 ments. 



A serious difficulty also arises if Upham's interpretation of the 

 mode of origin of the sediments which occur in Red River valley 

 is accepted. Upham held that these sediments are recent fluvial 

 deposits laid down in local depressions and on flood plains of streams 

 after the disappearance of Lake Agassiz. The deposits, he states, 

 "have commonly greater thickness and extent than the underlying 

 silt of glacial Lake Agassiz." 3 In the southern portion of the basin 

 they are in places underlain at considerable depths by "sheets of 

 turf," 4 etc., apparently indicating the presence of an old soil. The 



1 "The Glacial Lake Agassiz," U.S. Geol. Survey, Monograph 25, p. 245. 



2 Ibid., pp. 183 and 201. 3 Ibid., p. 202. 4 Ibid., p. 253. 



