430 H. L. FAIRCHILD — GLACIAL GENESEE LAKES. 



were decomposed and tlie region was deeply scored and eroded b}^ atmos- 

 pheric forces and stream action, 'i'lie main features of the present to})og- 

 raphy were impressed upon it during later geologic time, possibly since 

 the Cretaceous. 



EPISODE OF LAKES BY ICE-ADVANCE. 



With the invasion of the ice-sheet of the Glacial period the northward 

 drainage was blocked and the waters were ponded in the main and tribu- 

 tary valleys and compelled to overflow southward. The levels of these 

 static waters were succe.ssireli/ higher a^ the ice closed the outlets from the 

 lower toward the higher elevations of the region. The de^^osits in the 

 glacial lakes may have lieen considerable, derived from the ice-drainage 

 as well as from the land. The subsequent abrasion 1)}^ the lieavy ice-sheet 

 would largely remove or modify these early lake deposits, yet traces of 

 them may probabl}^ be recognized, although little may ever l)e known 

 of the character of these lakes. 



E PI so DE OF G L A CIA TIO N. 



The ice-invasion resulted in largely removing the disintegrated rocks, 

 and may have cut the solid rock upon the saliencies, especiall}'^ on the 

 stoss sides of the hills. However, the erosion of rock strata was probably 

 not great in the Genesee valle^^ the work of the ice being chiefly a smooth- 

 ing of the eminences and an acccMituation of the nortli and soutli forms 

 and a fllling of sections of the valley with heavy de})osits of wreckage 

 derived from n^^'ions lying northward. 



EPISODE OF LAKES BY ICE-RETREAT. 



The northward retreat of the ice-wall resultiMl in a second series of 

 glacial lakes at successively lower levels, which are the special subject of 

 this paper and will be described l)el()w. 



EPISODE OF MORA IX AL LAKES. 



Subsequent to the retreat of the ice and the <lraining or lowering of the 

 glacial lakes by removal of the ice-dam numerous morainal lakes were 

 left in the main and tril)utary valleys, held up' by the l^arriers of drift. 

 All these moraine-dammed waters have been drained, many outlets cut- 

 ting through rock upon the slopes of the ancient valleys (see page 427). 

 The lacustrine phenomena of these moral nic lakes are left commingled 

 with the phenomena of the ice-dammed lakes and the subsequent streams. 



ERA OF rOS TG LA CIA L S (IB A ERIA L E ROSION. 



In every section of all the valleys the latest aqueous action is the work 

 of streams. The river anrl all its tril)utaries found their valleys more or 



