SURFACE GEOLOGY. a 
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 .ull action of shore waves, by 
which the drift accumulations were 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 
