EVIDENCES OF GLACIAL LAKES. 195 



glaciers, as in the Himalayan Range, or on an ice-sheet, as observed by 

 Nordenskjold in Greenland. 



Very abundant and extensive development of glacial lakes attended 

 the recession of the ice-sheet in- the northern United States and in Canada, 

 being due to the temporary damming of the waters of glacial melting and 

 of rains on areas where the land has a northward descent. While the ice- 

 sheet was melting away from south to north on such a slope, free drainage 

 was prevented, and a lake was formed, overflowing across the lowest point 

 of what is now the southern watershed of the basin. Many of these lakes 

 were of small extent and short duration, being soon, by the continued re- 

 treat of the ice, merged into larger glacial lakes, or permitted to flow away 

 where basins sloping northward are tributary to main river courses drain- 

 ing southward. Professor Chamberliu has well written of these lakes 

 fringing the ice-sheet: 



They vary iu areal extent from trivial valleys blocked by ice to the broad 

 expanses of the great basius. If an attempt were made to enumerate all Instances, 

 great and small, and all stages, earlier and later, the list of localities and deposits 

 would swell, not by scores and hundreds, but by thousands.^ 



EVIDENCES OF GLACIAL LAKES. 



Five principal evidences of the former existence of glacial lakes are 

 found, namely: (1) Their channels of outlet over the present watersheds; 

 (2) cliff's eroded along some portions of the shores by the lake waves; (3) 

 beach ridges of gravel and sand, often on the larger glacial lakes extend- 

 ing continuously through long distances; (4) delta deposits, mostly gravel 

 and sand, formed by inflowing streams; and (5) fine sediments spread 

 widely over the lacustrine area. A few words of general description may 

 be given to each of these before proceeding to notice the areas of some of 

 the more important glacial lakes formed by the waning North American 

 ice-sheet, and to trace in detail the stages of the growth of Lake Agassiz 

 and its final reduction to the present Lake Winnipeg. 



Outlets. — Among the eAddences of glacial lakes, the one most invari- 

 ably recognizable and most definite in its testimoii}^ is the outlet showing 



1 Proc. A. A. A. S., Vol. XXXV, for 1886, p. 208. 



