MAN AND THE GLACIAL PERIOD. 637 



At any rate, in the Delaware Valley we find bowlder-bear- 

 ing clay rising to a height of one hundred and tifty or more 

 feet above the present high-water level. In the Lehigh 

 Valley, at Bethlehem, a few miles above its junction with 

 the Delaware, and several miles south of the limit of the ice- 

 field, Professor Lewis and myself found this bowlder-bearing 

 clay containing scratched pebbles and lying unconformably 

 upon thick deposits of coarse stratified gravel at a height 

 of one hundred and eighty feet above the river. Farther 

 up the Lehigh Valley also, near Weissport, we ascertained 

 the limit of ice-carried bowlders to be one hundred and 

 eighty feet above the river. 



We are probably safe in assuming that these floods, de 

 positing clay and bowlders at the height above mentioned, 

 mark both the period of greatest depression during the Gla- 

 cial epoch and that when the ice was most rapidly melting 

 away. Of course, the deposition of what Professor Lewis 

 styles " red gravel," and the high gravels at Bethlehem, oc- 

 curred earlier, since the clay overlies them. 



It is evident that the deposition of this bowlder-bearing 

 clay is separated from that of the implement bearing gravel 

 at Trenton by a period of considerable physical changes, if 

 not of vast time. 



Considering, now, this Trenton gravel, we find it to be 

 limited at the head of tide-water to a level of about forty 

 feet, and diminishing in height relatively to the river both 

 as one ascends and as one descends the channel, until at 

 Yardleyville, a few miles above Trenton, it merges into the 

 terrace which maintains a pretty uniform height of fifteen or 

 twenty feet above the river all the way to the Water-Gap. 

 Above the Water-Gap the gravel terraces rise to a much 

 greater height. At Stroudsburg a second terrace stands 

 seventy-five feet above the first terrace, which is about fifteen 

 feet above Broadhead Creek. But this upper terrace is 

 kame-like in its structure, and hence would be explained in 

 part by the lingering presence of the glacier itself. 



The descent of the river-vallev from Belvidere, where the 



