G. F. Wright — Unity -of the Glacial Epoch. 371 



times, such as is supposed to have brought on the period would 

 seem to provide a natural explanation. But Mr. Leverett, 

 under guidance of Professor Chamberlin's theory of the course 

 of events in the Upper Ohio is compelled to resort to the 

 hypothesis that a part of this lowering of the channels of these 

 streams to the northward was done by subglacial currents of 

 water forced by hydraulic pressure to run up hill to find exit. 

 (See this Journal, Sept. 1891, p. 209.) A theory driven to such 

 extremities cannot be said to be altogether free from difficulty. 



A simple statement of the rival theory concerning the course 

 of events in the Upper Ohio Valley is its own best defence. I 

 suppose that the erosion during early Tertiary times had pro- 

 ceeded so far that base levels had become established, and that 

 the rock shelves at Bellevue and Parker mark the flood plain 

 of the river at that time. During the close of the Tertiary 

 period the land underwent elevation until it stood much 

 higher than now over all the northern part of the United 

 States and Southern Canada. During this time the rivers 

 lowered their beds to the extent shown by their present rock 

 bottoms. The channel of the Ohio was a product of that 

 period. The differential northerly elevation permitted the 

 erosion of the northern tributaries of the Ohio spoken of. 

 Their present attitude is the result of subsequent differential 

 subsidence. This subsidence occurred in connection with the 

 climax of glacial conditions when the ice extended so as to dam 

 up the Ohio at Cincinnati. This dam cooperated with the 

 slack drainage attending the differential subsidence to produce 

 many of the phenomena which Prof. Chamberlin attributes to 

 a first glacial period. Floating ice came down the Allegheny 

 River in great quantities and drifted into the oxbows and 

 upon the shelves of the earlier epoch of erosion, and left the 

 glacial material which is found in such places as Parker and 

 Bellevue. Similar conditions in the Monongahela favored the 

 deposits of pebbles and silt at the high levels described by Pro- 

 fessor I. C. White. The deposits in Teases Yalley between the 

 Kanawha and the Ohio in "West Virginia probably belong to 

 a somewhat later stage when the barrier below Cincinnati had 

 been worn down to a considerable extent. For it should be 

 observed, that the preglacial channel of the Ohio at Cincinnati 

 toward Hamilton was permanently closed by glacial deposits. 



The Columbia terraces of the Susquehanna and the Dela- 

 ware were deposited under somewhat similar conditions. 

 There was a depression of the coast level of about 200 feet in 

 New Jersey allowing the great ice rafts loaded with Medina 

 bowlders which came down the river to tide level to be floated 

 over the lowlands of the state south and east of Trenton, and 

 to be stranded along the shore on the west side of Philadelphia 



