370 G. F. Wright — Unity of the Glacial Epoch. 



the earlier glacial period during its slackened drainage. But he 

 does not seem to have duly considered the facts which I have 

 presented making probable an obstruction of the channel of the 

 Ohio near Madison, Indiana, in Jefferson and Ripley Counties 

 which might well account for the facts in that part of the 

 state most like those in Southern Ohio. (See Bull. U. S. Geol. 

 Surv.j 58, pp. 65, 66.) Something more than similar microscop- 

 ical results must be relied on to demonstrate chronological 

 identity of deposits. 



The theory of a somewhat prolonged obstruction of the 

 channel at Cincinnati by ice has received strong confirmation 

 in Professor James's investigations, going to show that the pre- 

 glacial channel of the Ohio ran at Cincinnati still farther north, 

 following up the valley of Mill Creek until it joined the Big 

 Miami near JIamilton. At any rate it is certain that the Ohio 

 did not in preglacial times flow in its present channel from 

 Cincinnati to the mouth of the Great Miami, for I am in- 

 formed by Mr. Charles J. Bates, inspector of masonry for the 

 Cincinnati Southern Railroad whose bridge crosses the river 

 just below the city, that bedded rock was found by him a few 

 feet below the present bottom of the river extending across its 

 whole width. But at Hamilton in the valley of the Miami 

 where the depression of Mill Creek joins it, the rock bottom 

 is as much as ninety feet below the level of the bottom of the 

 Ohio at Cincinnati. This northern bend of the river in pre- 

 glacial times adds greatly to the argument from direct evidence 

 of a prolonged, ice dam there. The gorge for several miles 

 below Cincinnati is comparatively narrow and. its erosion in 

 good part may perhaps measure postglacial time. 



In all this discussion it should be borne in mind that we 

 cannot assume an absolutely constant level of the land in the 

 region of the Upper Ohio Valley. Indeed, the subsidence 

 proved by the Champlain deposits to exist at the close of the 

 ice age involved a differential in the depression of the land to 

 the north which is very suggestive, while another class of facts, 

 equally suggestive of considerable changes of level in connec- 

 tion with the glacial period, appears in the northerly slope of 

 the bed of many northern tributaries of the Upper Ohio and 

 Allegheny rivers. The Shenango, Mahoning and Beaver 

 rivers, and French, Oil and Conewango creeks, all have a rock 

 bottom which slopes to the north, or away from their present 

 outlets. Mr. Carll and some others have argued from this 

 that there must have been a system of northern outlets into 

 Lake Erie in preglacial times. (2nd Penn. Geol. Surv. iii, pp. 

 330 to 366.) The actual existence of such northern outlets 

 has, however, not been proved by direct evidence. The theory 

 of a general elevation of the country to the north in preglacial 



