176 G. F. Wright — Continuity of the Glacial Period. 



gorge-like channels down through the rock to a depth of 250 

 or 300 feet below their former level. Simultaneously with the 

 culmination of this uplift, and with the conclusion of this long 

 work of erosion, Canadian ice reached the headwaters of the 

 Allegheny, and a work of deposition began which proceeded 

 as long as the ice continued to advance, and probably for some 

 time after it began to retreat. The upper part of the Alle- 

 gheny River became filled with debris up to the summit of the 

 high-level bowldery terrace extending over the rock shelves, 

 and the sedimentation gradually advanced along the narrow 

 trough until it was entirely filled as far down as the mouth of 

 the Beaver, and perhaps to Wheeling, and possibly still farther. 

 Subsequently the larger part of this material was eroded, and 

 carried down into the Mississippi Yalley. If there were two 

 distinct Glacial epochs, the minimum measure of time between 

 them is not that of the rock erosion of the gorge, but the 

 erosion of this loose material. 



!Now in studying this problem we cannot wholly ignore the 

 Cincinnati ice-dam, even though that hypothesis may have 

 been somewhat overworked in former years. I need not repeat 

 the positive evidence of this dam elsewhere presented* any far- 

 ther than to say that the arguments adduced by Professor 

 James,f proving that the preglacial course of the Ohio River 

 was around by Hamilton, and thence down the Great Miami,;}; 

 adds immensely to the significance of its obstruction during the 

 Glacial period. The existence of such an obstruction must 

 have greatly facilitated the silting up of the gorges of the 

 upper Ohio Valley, and have had a great influence in modifying 

 the character of the deposits. An immense amount of fine 

 material which otherwise must immediately have been swept 

 onwards to the Mississippi would settle in the slack water 

 depths of this narrow winding lake and serve as the basis to 

 support the coarser deposits that were brought clown over 

 them. Some positive evidence npon this point has been fur- 

 nished me by Mr. R. C. Hice. In digging for the abutments 

 of the bridge across the Big Beaver at Ellwood,§ just above 

 Homewood, the workmen penetrated sixty or more feet below 

 the present bottom of the river, and found it filled uniformly 

 with a very fine deposit such as could settle only in the stillest 

 of water. In the portions of the Ohio Yalley nearer Cincin- 

 nati the ice dam removes the necessity of supposing so exten- 



*See especially Bulletin of U. S. Geol. Sur., No. 58; Ice Age of North America, 

 pp. 326-350. 



f See Journal of the Cincinnati Society of Natural History, July-October, 1888, 

 pp. 96-101 ; also Amercan Geologist, 1893, pp. 199-202. 



% See map in Fig. 1. 



§ See map, Fig. 2. 



