224 Frank Carney 



fore, represents the limit of erosion in the plain determined by the 

 two lines. 



Several of the other factors briefly discussed doubtless had some 

 part in producing the Great Lakes. The most inclusive theory of 

 origin is that stated by Chamberlin: "The basins of the Great 

 Lakes are regarded as due to the joint agencies of preglacial 

 erosion, glacial corrasion, glacial accumulations, blocking up out- 

 lets, depression due to ice occupancy and general crust movements, 

 together with possible unascertained agencies."" 



OLD SHORE LINES IN OHIO 



Three ancestral lakes of Erie formerly stretched across our 

 state. Each of these left such evidences of its existence as the pres- 

 ent lake is now engaged in producing. If you visit Lake Erie 

 at any point, you will observe that the water is either cutting into 

 the shore, which is either till or solid rock, or else it is piling up 

 material which the waves have carried. In the former case the 

 waves are undercutting a cliff; in the latter, constructing a beach. 

 These earlier lakes must have done similar work ; what they accom- 

 plished, either in cliff-cutting or in beach construction, depended 

 upon their duration. Wherever the former lakes made cliffs in 

 rock, the result was more enduring than if the cliffs had been cut 

 in unconsolidated material. Wherever they built beaches of 

 cobble and gravels, this material has held its shape longer than 

 beaches constructed of clay and sand. The sharpness of develop- 

 ment in the shore lines of former lakes is conditioned by the lapse 

 of time since they were made. We would expect, therefore, in 

 case these three shore lines represent lakes which lasted through 

 equal time units, that the oldest one would now be the least 

 distinct ; it has suffered longest from weathering. On the sup- 

 position that there has not been an appreciable change of climate 

 in this area during postglacial time, this method of determining 

 the relative ages of old shore lines is fairl}- accurate. 



Origin of shore lines. A lake occupying a basin either of rock 

 or unconsolidated material, or a basin consisting of both, w^ill 

 alter its basin somewhat through solution. A body of water 



1^ Proceedijigs American Association Advancement of Science, vol. xxxii, (1883), 

 p. 212. 



