86 
Wisconsin Academy of Sciences , Arts and Letters. 
It appears therefore that at this point a deep trench was cut in the flood- 
plain of which the old terraces are the remnants before the formation of 
the later moraine and of the valley deposits that sprang from it. 
If we pass over to the Delaware valley we find analagous facts which 
are more familiar through the writings of several geologists. Many years 
ago Professor Lewis called attention to the earlier and later deposits of that 
region, though he did not give them the interpretation I shall place upon 
them here, which coincides essentially with that of McGee. As we follow 
up the valley toward Belvidere where the moraine crosses the Delaware, 
we find old terraces reaching up to about 240 or 250 feet upon which are 
rounded pebbles and glaciated stones, indicating an origin in the earlier 
stage of glaciation. Cutting through these old plains and the rock below 
we find the deep trench in which the later deposits have been placed. 
These later gravel deposits, originating with the moraine at a height of 
somewhat above 150 feet, rapidly decline to about 85 feet a few miles above 
Lewisburg, opposite a point where the older terrace rises to about 250 
feet. The measure of the interval here is some 250 to 800 feet of rock 
cutting. 
It would appear therefore that while there are local variations, there is a 
general correspondence between the amount of erosive work done by the 
lower Mississippi, by the upper Ohio and Alleghaney, by the Susquehanna 
and by the Delaware rivers respectively. The facts indicate that the alti¬ 
tude of the continent was low in the closing stages of the earlier glacial 
epoch; became higher in the interglacial interval, and after sufficient time 
elapsed for these great erosions to take place, the glacial waters of the 
later epoch poured their valley deposits down the trenches formed in the 
interval. The cutting of these trenches rudely measures the length of this 
interval or at least the length of the actively erosive part of it. 
