6 GEOLOGY OF OHIO. 



lands on to the plain which bordered them toward the south and west, 

 all the basins of our great lakes were excavated ; the ice in each case 

 moving in the line of the major axis of the lake, from the north and east 

 toward the south and west. 



3d. At the commencement of this ice period this continent must have 

 stood several hundred feet higher than now. This is proved by the great 

 system of buried river channels, and by the deeply excavated troughs of 

 the Hudson, Mississippi, Columbia, the Golden Gate, etc., etc., which 

 could never have been cut by the streams that now occupy them, unless 

 when flowing with greater rapidity and at a lower level than they now do. 



4th. The ice period — the period of the greatest cold and of the most 

 extensive glaciers, also a period of continental elevation and of active 

 erosion — was followed by a water period, a period of ameliorating climate, 

 of retreating glaciers, of subsidence, and of deposition. In the culminat- 

 ing epoch of this period the climate was much warmer than before, the con- 

 tinent was depressed 500 feet or more below its present level, the glaciers 

 had retreated northward, and were replaced in the basin of the great lakes 

 by an inland sea of fresh water. The first deposit of this period was that 

 of the bowlder clay. This represents the fine material excavated and 

 ground up by the glacier. It is most abundant where glaciers move over 

 soft sedimentary rock, such as shale and limestone, and as such rocks 

 filled most of the great excavated basin north of Ohio, the bowlder clay 

 is naturally the most conspicuous of our Drift deposits. In New England 

 and other countries where granite and other hard and silicious meta- 

 morphic rocks abound, the product of glacial erosion is sand, gravel, and 

 bowlders. As the great ice-sheet retreated northward it thrust out and 

 left behind it a succession of heaps of bowlder clay, which now form a 

 nearly continuous sheet over the glaciated surface. 



5th. When the retreating ice-sheet had passed the great watershed of 

 Ohio, basins of water began to form along its margin, and in these the 

 finer portion of the flour ground by it, for a time suspended, was ul- 

 timately deposited as the laminated clays, which succeed the bowlder 

 clay, and form the upper subdivision of the Erie clay. This, in Ohio, is 

 usually stratified in thin leaves, or lamellae, and contains no bowlders. 



6th. After the retreat of the ice-sheet from Ohio, Indiana, and Illinois, 

 a considerable portion of the surface it had occupied and had left covered 

 with debris was overgrown with a forest, composed largely of coniferous 

 trees. This forest growth continued long enough to form a carbonaceous 

 soil, and in many places beds of peat many feet in thickness. In this 

 peat the remains of the mammoth, mastodon, and the giant beaver have 

 been found, and we thus learn that they inhabited the forests which 



