Page:Popular Science Monthly Volume 38.djvu/765

Rh The investigation of this matter was made the subject of a special paper by Prof. E. W. Claypole, and from his pamphlet we glean some interesting facts.

Lake Ohio, as this body of water produced by the ice-dam was called, extended four hundred miles up the valley and was in places two hundred miles broad. Its waters covered the present site of Pittsburg to a depth of three hundred feet. Backing up the Monongahela River, it carved the terraces already mentioned, so that these represent the shores of this ancient lake in the mountains of Pennsylvania. Its northern boundary was formed partly by the ice wall itself and partly by the irregular outline of the high land it could not overflow. A few isolated patches projected as islands above its surface. On the south, long fiords existed in place of the former tributaries, and from the lower end of one of these was the probable outlet for the water. This, however, is still a mooted question, and though it is probable that much found its way through a low pass in the water-shed between the valleys of the Licking and the Kentucky Rivers, it is also likely that a part followed the foot of the ice and reached the Ohio Valley again some thirty or forty miles below the present site of Cincinnati.

How long Lake Ohio was in existence it is, of course, impossible to say. Various facts, however, indicate a life of many hundreds, perhaps thousands of years. So long as the dam existed, Lake Ohio held its own; but, when the ice began its retreat, the fate of the lake was sealed. As year after year the foundations of the dam were weakened, the pressure of the water was with greater and greater difficulty withstood. The heat of summer sapped its strength, but this was again renewed by the winter's cold; but, when the cold of winter was insufficient to supply the waste of summer, the end was really at hand. As Prof. Claypole says: "Possibly the change was gradual and the dam and the lake went gently down together. Possibly, but not probably, this was the case. Far more likely is it that the melting was rapid and that it sapped the strength of the dam faster than it lowered the water. This will be more probable when we consider the immense area to be drained. The catastrophe was then inevitable—the dam broke, and all the accumulated water of Lake Ohio was poured through the gap. Days and even weeks must have passed before it was all gone; but at last its bed was dry. The upper Ohio Valley was free from water, and Lake Ohio had passed away."

This conflict of ice and water must have been frequently repeated, for the cold of winter would have repaired the damage of the summer; so that year after year, for how long one can not tell, the conflict was renewed. Says Prof. Claypole: "This