U'l T. C. I EA.MBERLIN — THE INTERGLACIAL INTERVAL. 



upper branches during the interval between the two epochs. This great ero- 

 sion represents the interval between the formation of the silt< of the earlier 

 glacial epoch and the filling in of the valley deposits of the later glacial epoch, 

 which now demand our attention. If we go hack on the glaciated area to the 

 moraines which mark the limit of the later glacial incursions we find, start- 

 ing from the outer side of these moraines, valley streams of gravel formed 

 contemporaneously with these ice incursions. Tracing these gravel streams 

 along their courses we find that they run down into and partially till the 

 channels cut in the interglacial interval. On the upper Mississippi, on 

 the Chippewa, on the Wisconsin, and on other tributary rivers we find 

 gravel trains heading on the outer edge of the outer moraine of the later 

 epoch. Passing down through the interglacial trenches there are found 

 represented in the lower Misssissippi valley (as I think we may safely say 

 from recently gathered evidence) equivalent deposits in the bottom of the 

 Mississippi overlain, of course, by the more recent deposits. The work of 

 the earlier glacial epoch in the lower Mississippi I conceive to be the deposit 

 of the loess and loess-like silts ; that of the interglacial epoch the erosion of 

 the great trench in which the Mississippi bottoms now lie; and that of the 

 later glacial epoch the partial filling of this trench. The trenching is 

 the measure of the interglacial interval, or at least is a partial measure of it, 

 Tfu Evidence drain/ from the Ohio and Allegheny Rivers. — if we pass 

 to the upper Ohio and Allegheny valleys we find phenomena that fall 

 into close correspondence with the foregoing. There are high shoulders 

 and terraces at various points which bear upon themselves glacial river 

 gravels. One of the most decisive, found in the vicinity of Parkersburg, 

 has been described by Mr. Chance and others. Here an old channel runs 

 hack from the present course and, curving around a group of hills, returns, 

 forming an "ox how." In this old channel, glacial river gravels are found, 

 showing that it was occupied contemporaneously with some stage id' the 

 glacial period. This abandoned channel is about two hundred feet above 

 the present Allegheny river. Mr. Chance tells us there is about fifty feet of 

 drift in the presenf valley bottom; so between this upper river bed and 



the bottom of the present rock bed there is evidence of an erosion of 250 



feet, two hundred of which, in round numbers, are cut through Carboniferous 

 Btrata. Similar and corroborative facts show themselves along the course 



of the river above and below, and along the Monongahela and the upper 

 Ohio. 



[f we trace the old channel of the Allegheny northward by means of 

 remna .1 shoulders and terraces, we find that it lies considerably above the 

 altitude of the terminal moraines of the later epoch, anil also much above 

 the gravel t rain- that head on the outer side of these moraines and run down 

 through the trench above indicated. Ii therefore becomes a necessary in- 



