ORIGIN AND EVIDENCP:S OF GLxVCIAL LAKES. o5o 



water, overflowing soutliward. A preliminary description of these ice- 

 dammed lakes is the purpose of the present writing. 



These glacial lakes, the predecessors of the present " Finger lakes," 

 were vastl}'^ larger than their successors, and of more than twice their 

 depth. Those described in detail in this i)aper were all forced to i)our 

 their overflow into the southern Susquehanna drainage. Ice-dammed 

 lakes also occupied many valleys in which today no water is ponded. 

 This was probably the case with the upper valley of the Genesee river 

 and with the two stronger valleys adjacent to either side of the Genesee. 

 There were two valleys in similar relation to Canandaigua lake, with 

 several valleys east and others west of the area under consideration, and 

 probabl}" other less important elevated valleys which held glacial lakes. 



Some of these lakes have a complicated history and their relationships 

 are somewhat intricate. By the northward retreat of the ice-Avall new 

 and lower outlets were opened, the directions of flow were tluis changed? 

 the water bodies were dismembered and the diff'erentiated waters at suc- 

 cessively lower levels had varying relations. 



EVIDENCES. 



The descriptions of several extinct lakes, given later, will 2:)resent in- 

 ductively the proofs of the former existence of such elevated waters, but 

 the argument and evidences may here be summarized as follows : 



1. Theoretical considerations. Granted a sheet of glacial ice or ice- 

 lobes filling the valleys, the southern margins retreating by melting, and 

 it must be admitted, unless the capacity of ice to serve as a dam be 

 denied, that the uncovered north-sloping southern ends of the valleys 

 would fill with water. 



2. Lacustrine silts in the valleys, enclosing materials dropped from 

 floating ice. 



3. Shoreline markings, as sea-cliffs and terraces of wave erosion and 

 wave construction. 



4. Abandoned stream channels, with tlieir ])cculiar conformation and 

 unmistakable characters, south of all the important cols. They are cut 

 out of the ice-drift, and some are strongly developed and of large capacity. 



5. Valley-fillings of fluviatile sand or gravel accumulated on lower 

 ground by the old streams. 



G. Stream deltas. They furnish the most consi)icuous and universal 

 proofs and are most depended upon in this paper for tlie water altitudes- 

 They are ])r(>min(*nt ui)on nearly every stream emptying into the liikv- 

 basin.s, and their terraces are often visible for miles. 



