194- THE POPULAR SCIENCE MONTHLY. 



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 cer- 

 tain data a pretty good idea of the relative periods of time occu- 

 pied by different stages of the advance and retreat of the ice. It 

 is clear that the obstruction of the Ohio near Cincinnati con- 

 tinued during the whole time occupied by the advance of the ice 

 from Hamilton to its farthest point, ten miles southwest of Cin- 

 cinnati 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 alto- 

 gether probable that this close balancing of forces resulted in an 

 exceptionally slow movement from Hamilton to Cincinnati, caus- 

 ing 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 investi- 

 gators. The general impression which I have received from a 

 study of the facts is that a period of several thousand years maj^ 

 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 Hamil- 

 ton, raising the water to the height of the rock obstruction across 



