GEOLOGY OF OHIO. 



Fourth. The final retirement of the glaciers over the Canadian high- 

 lands; the gradual subsidence of the water in the lake basin until it 

 reached its present level, and occupied only the deeper portions, forming 

 our modern chain of lakes. In this descent, the shore line was marked 

 at different intervals with terraces and beaches. 



In Southern Ohio, the succession of events was somewhat different, in- 

 as much as the Forest bed, an old soil, with stumps, logs, and leaves, there 

 rests on the Bowlder clay, and is covered locally by an upper stratum of 

 Till, which is the product of glacier or iceberg action. No evidence of 

 this has been found in the lake basin, where there is no forest bed — 

 unless it be represented by the drifted vegetable matter which is some- 

 times found in the Erie clay, but which is not covered by a second 

 bowlder clay. So far as yet observed, the facts seem to indicate that the 

 basin of Lake Erie was filled with water or ice during all the time that 

 the alternations of temperature recorded in Southern Ohio we,e taking 

 place. 



Professor Winchell is represented, by Professor Geikie (Great Ice Age, 

 page 462), as claiming the existence of a forest bed, overlain by a second 

 Bowlder iclay, in north-western Ohio, but this is probably due to a mis- 

 understanding of Professor Winchell's language, when he had reference 

 to the north-western States. No forest bed has been seen or heard of by 

 the writer in north-western Ohio, and nothing of the kind is reported by 

 Mr. G. K. Gilbert, or Mr. J. H. Klippart, in their papers on the Drift of 

 the Maumee Valley, nor by Professor Winchell, in his reports on the 

 counties surveyed by him. 



Prof. Winchell, in his " Drift Deposits of the North-west," attributes 

 the excavation of the basins of the great lakes to the great continental 

 glacier cutting locally deep into beds of soft rock. His language is as 

 follows : '■ Southward, prolongations of the ice sheet follow the north- 

 south outcropping edges of argillaceous formations. Lake Michigan lies 

 in one of these troughs ; Lake Huron lies in another. Lakes Erie and 

 Ontario are only shallow basins dug out of soft rooks by ice that passed 

 south-westwardly. The shale bed that gave rise to Lake Ontario als 

 determined the location of Georgian Bay and Green Bay. The basin of 

 Lake Erie is much shallower toward the w^est end than toward the east, 

 and it finally runs out altogether by reason of the westward attenuation 

 and finally entire disappearance of the Salina formation in which it is 

 largely excavated. The ice was then thrust up on to harder rocks that 

 form the basis of north-western Ohio and north-eastern Indiana. Lake 

 Michigan was terminated southwardly by the eastern trend of the rocky 

 outcrops at an angle that the ice could not follow." 



