IOWA ACADEMY OF SCIENCES. 17 



TBE SHORE-LINES OF ANCIENT GLACIAL LAKES. 



BT PROF. J. E. TODD. 



As most are aware, there art areas of drift external to any terminal moraines, 

 the origin of which is still in dispute. On general principles, it would be expected 

 that numerous lakes would have frequently occurred during the Ice Age. As the 

 ice advanced, streaflis would frequently be dammed, and their channels more or 

 less changed, and the weight of the ice, with its chilling effect, in level areas would 

 not infrequently produce a subsidence toward the ice, which would often become 

 filled with the floods escaping from the ice. 



Geike, in his "Ice Age," last edition, draws a graphic picture of such lakes in 

 central North America. Inferences derived from the Merjelen Sea, and similar 

 lakes in the Alps, Greenland and the Himalayas, strongly urge the probability of 

 much larger ones of the same kind during ancient times. Such have been found 

 in side moraines upon the more recent drift. Lake Agassiz, and of the Blue Earth 

 region in Minnesota, and Lake Hakota and James Lake in Dakota, readily come 

 to mind in tnis connection. But can similar lakes be recognized in the much 

 eroded and fragmentary deposits external to the great terminal moraines? Some, 

 as one with whom I was talking a few j-ears ago, when discussing Prof. Wright's 

 hypothesis of Lake Ohio, said, " Glacial lakes are a delusion and a snare," and yet 

 the same person has mapped sueh a lake in central Wisconsin. Others would refer 

 most of the extra-morainic drift to this cause. 



One difficulty, and one which some consider insuperable, is the absence of dis- 

 tinct barriers and shore-lines and old water levels. The beaches of Lake Agassiz 

 have been readily traced, but where are there any such traceable about Lake Ohio 

 or Lake Missouri, or anywhere upon what has been called the older drift? The even, 

 flat topography impresses one with lacustrine character in traversing the Blue Earth 

 region, or that between Scotland and Mitchell, S. D.; but we can readily see that 

 if a lake has been of transitory duration it would fail of producing a plain. 



Before dwelling on a few recent observations, which it is the main purpose of 

 this paper to present, let us consider briefly a few reasons for the common obscu- 

 rity of the shore-lines of old glacial lakes. 



1. The surface of such lakes would usually be very inconstant. The ice would 

 have been a very uncertain barrier. The chance of depositing a beach or cutting 

 a cliff would therefore have been small. 



2. The accumulation of shore deposits would not only be slight, but being 

 made largely by floating ice would be quite unequally distributed, especially in 

 wide and shallow lakes. Prevalent winds would drive the drift-laden ice to cer- 

 tain shores much more than to others. If the lakes contained islands, the more 

 remote shores might receive no erratics. 



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