T. C. Chamberlin — Diversity of the Glacial Period. 195 



deposition over the country generally to conditions of free 

 drainage pouring down through valleys cut to considerable 

 depths below the silt horizon (Leverett). 



In this region, therefore, between the border of the drift 

 and the outermost moraine there is decisive evidence of a till- 

 depositing period, an interval of soil formation, a silt-deposit- 

 ing period, an interval of radical drainage-change and a 

 moraine-producing advance of the ice. This is a very different 

 collocation of formations from that which seems to be implied 

 by Prof. Wright's language (pp. 353-356 and 357). It is very 

 far from being a simple moraine-and-" fringe " combination. 



In Eastern Ohio and Western Pennsylvania the outer mo- 

 raine runs much nearer the attenuated border of the old drift 

 and the intervening silts are obscure or absent, or confined to 

 the remnants of the old base levels, so that it is not so surpris- 

 ing that the vanishing edge of the old drift should here be 

 mistaken for a dependency of the outer terminal moraine, but 

 that the relationship is really the same as it is demonstrably to 

 the westward, I hold to now be beyond serious question. 



These points bear upon the explanation of the glacial gravels 

 that lie upon the high terraces in the Allegheny and adjacent 

 valleys. These terraces I have maintained (Bulletin 58, IT. S. 

 Geol. Surv., pp. 20-38) were produced at a time of base level 

 degradation, which in its later stages was contemporaneous 

 with the earlier ice incursion whose Waters bore gravels down 

 the Allegheny, Upper Ohio and some adjacent streams, and 

 formed the 40 or 50 feet of capping which lies upon the rock 

 benches that constitute the body of the terraces. I have 

 argued that subsequent to this the land was elevated and the 

 lower newer steep sided gorges of the Allegheny and neigh- 

 boring streams were cut to a depth that may be roundly stated 

 as 250 feet, and that subsequent to this the later ice incursion 

 formed the outer moraine of the region. From the outer side 

 of this moraine glacial streams bore their sands and gravels 

 down the Allegheny gorge cut during the interglacial inter- 

 val. The evidence of this, gathered in a joint study by Mr. 

 Gilbert and myself, may be found in Bulletin 58, U. S. Geol. 

 Surv., pp. 32-36. I therefore argue that between the time 

 when the glacial gravels were deposited on the high terraces 

 and the incursion of the later ice there was a cutting of the 

 gorge to the depth of 250 feet roundly speaking. I regard 

 this gorge-cutting a,s a minimum measure of the interval be- 

 tween the two ice incursions. Prof. Wright does not admit 

 the force of this evidence because he interprets the occurrence 

 of the glacial gravels on the high terraces as the work of ice 

 floating from the edge of the glacier on the surface of the 

 hypothetical lake formed by the supposed Cincinnati ice dam. 



