52 GEOLOGY OF OHIO. 



high surface level in the water that succeeded them. As the glaciers 

 melted away, one and another outlet was opened for the water, and these 

 outlets were certainly deepened in the lapse of time. It is also possible 

 that warping of the earth's crust may have changed the relative altitudes 

 of different portions of the margin of the lake basin. We know that 

 deeply buried channels connect the basin of Lake Michigan with the 

 Ohio and Mississippi, but they were probably excavated previous to or 

 during the ice period, and were subsequently filled and obliterated by 

 the later deposits of the Drift. The present divide between the waters 

 of Lake Michigan and the Mississippi is so low that water would pass 

 over it if the channel of the Niagara below the falls were filled, and the 

 river were forced to flow, as it once did, into Lake Ontario from the sum- 

 mit of Queenstown heights. It is, therefore, evident that the margin of 

 Lake Michigan has been depressed, or that the Straits of Mackinaw were 

 closed by ice, earth, or rock, when Lake Erie stood, as we know it did 

 stand, several hundred feet higher than now. Great changes must have 

 taken place, also, at the Niagara outlet since the old shore lines that 

 encircle Lake Erie were marked out, as the restoration of the barrier at 

 Queenstown heights would not bring the surface of the Lake up to the 

 lowest of the old beaches. There is no doubt that an old channel, more 

 than 200 feet deep, connects the rock basin of Lake Erie with that of 

 Lake Ontario, and the latter with the Hudson ; but these old channels 

 were filled with Drift deposits long before the lake ridges were made, 

 and by the heaping up of Drift material the drainage was turned into 

 new channels, along the line of lowest levels. It happened that this 

 line ran over a spur which projects into the basin of the great lakes, and 

 this spur, partially cut away, now forms the rocky barrier over which 

 Niagara pours. The establishment of this line of drainage is a very 

 modern affair, for all the sequence of Drift phenomena, even to the form- 

 ation of the successive lake ridges, preceded it. But its antiquity, as 

 compared with the reach of human history, is shown by the profound 

 gorge which has been slowly excavated since the waters of Lake Erie 

 began to flow over this barrier. 



Whatever was the condition of other portions of the country bordering 

 the great lakes, we have incontestible proof that in Ohio the water of 

 Lake Erie once covered all the northern counties, and reached up to the 

 passes in the divide, and that subsequently the water level descended 

 step by step, resting at certain intervals, while the shore waves heaped 

 up beaches along the gentle slopes, cut the more abrupt declivities into 

 terraces, and washed prominent rocky headlands into water-worn and 



