SURFACE GEOLOGY. 7 



slowly followed and occupied the place of the retreating glacier to the 

 rim of the lake basin, beyond which extensive bodies of water and ice 

 prevented their advance northward. 



7th. When the forest growth had spread over most of the Drift area 

 south of the lakes, and had occupied it for hundreds and perhaps thous- 

 ands of years, a submergence of the continent took place, which brought 

 the waters of the Gulf of Mexico up the valley of the Mississippi until this 

 formed an arm of the sea, which reached and covered all the lower half 

 of our State. In this submergence the clays, sand, and gravel overlying 

 the peat beds in southern Ohio, the lacustrine clays of northern Ohio, 

 and finally the Loess of the Mississippi valley were deposited. These 

 filled and obliterated many of the valleys of the Forest Bed era, as the 

 Erie clay had done those of the pre-glacial date. 



8th. During the submergence that covered the Forest Bed with clay, 

 sand, and gravel, icebergs floated from the Canadian highlands, bringing 

 with them gravel, bowlders, and blocks of granite, greenstone, mica 

 slate, silicious slate, etc., and scattered them broadcast over all the sub- 

 merged area. Some of these icebergs seem to have stranded at various 

 points on the northern slope of the watershed, especially near its sum- 

 mit, and, melting there, to have left large accumulations of bowlders and 

 gravel. 



9th. In this last submergence, portions of the highlands of Ohio were 

 low islands and shallows, exposed to the full action of shore waves, by 

 which the drift accumulations w«re assorted, the clay washed out, the 

 gravel and bowlders well rounded, and many of the gravel hills and 

 sand banks (kames) of the summit of the watershed were produced. 



10th. With the subsidence of the waters of the last submergence of 

 the Drift period, certain great waste-weirs, or lines of drainage, were es- 

 tablished in the gaps in the watershed, which ultimately separated the 

 river systems of the St. Lawrence and the Ohio. Through these waste- 

 weirs strong currents of water poured, which transported and deposited 

 vast quantities of gravel and bowlders in certain lines or belts leading 

 to the Ohio valley. These great drainage lines were through the valleys 

 of the Wabash, Miami, Scioto, Muskingum, and the Beaver. 



11th. The retirement of the sea at the close of the Drift period took 

 place very gradually, with intervals of rest and recession. In these in- 

 tervals the terraces of our river valleys were formed, by the arrest of their 

 flow and the deposition of the materials they transported in the dead 

 water which partially filled these valleys. Hence this is denominated 

 the Terrace epoch, the last chapter in the Drift history. 



12th. The Ohio valley was nearly emptied, while the lake-basin was 



