Page:Popular Science Monthly Volume 45.djvu/208

194 Hamilton, adds greatly to the significance of the ice obstruction at that point, for it extends the distance of it about fifty miles, and the distance covered by the ice beyond the original river bed twenty miles.

While every attempt to calculate the chronology of the Glacial period is necessarily but approximate, still we can get from certain data a pretty good idea of the relative periods of time occupied by different stages of the advance and retreat of the ice. It is clear that the obstruction of the Ohio near Cincinnati continued during the whole time occupied by the advance of the ice from Hamilton to its farthest point, ten miles southwest of Cincinnati—that is, during the advance of the ice front over a space of about thirty miles and until its retreat to Cincinnati again. The only statement approaching to definiteness which we are warranted in making concerning the rate of this advance is that it was probably the slowest which we should assign to any part of the movement of the great continental ice-sheet; for, being near the extreme point of extension, the equilibrium of forces must have been very nearly established, and the momentum of the glacier from the north was constantly diminished at the front by the increased rapidity with which a more genial climate was melting the ice. So to speak, the glacier was here getting upon doubtful territory and had carefully to consider every forward step, until finally, having reached the height of the Kentucky hills, the balance was turned, and the retreat began. It is altogether probable that this close balancing of forces resulted in an exceptionally slow movement from Hamilton to Cincinnati, causing the glacier to occupy many centuries, or even thousands of years, in that part of the march.

Something of a measure of this time is perhaps to be found in the erosion of the cross-cut from Cincinnati to the mouth of the Great Miami, which must have begun as soon as the obstruction of the valley near Hamilton first occurred. The length of this new channel of erosion is from twelve to fifteen miles; but how much of the work had been previously done by the small streams formed by the local drainage it is difficult now to calculate. Many such questions remain to reward the labors of local investigators. The general impression which I have received from a study of the facts is that a period of several thousand years may have been occupied by the ice-front in its advance from Hamilton to the farthest point in Kentucky and its subsequent retreat to the north side of the river.

But it is not to be supposed that this period was by any means one of dull uniformity in the history of that region, for upon the first formation of the dam at the bend of the old river at Hamilton, raising the water to the height of the rock obstruction across