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 Hrie 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 Hrie 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 
