300 PROCEEDINGS OF THE ACADEMY OF [1880. 



Above Yardleyville, therefore, we have to deal with but two sur- 

 face formations, — the boulder-bearing brick clay, often much 

 eroded, and the Trenton gravel, confined to the bottom of the 

 valley and showing but little erosion. It will be well to bear in 

 mind the distinction between these two formations, — the one of 

 glacial, the other of post-glacial age. The writer has traced them 

 as far up as the Water Gap, past the great terminal moraine into 

 -laciated regions. It is interesting to note that while the modified 

 moraine material close to the river at Belvidere is in some points 

 similar to the Trenton gravel, and is the source of part of that for- 

 mation , the moraine on the Lehigh River at Stemton and at other 

 inland localities contains pebbles and boulders very similar to 

 those of the Philadelphia brick-clay. 



Throughout the whole course of the Trenton gravel it is observed 

 that it lies within a channel previously excavated down to the 

 rock through the boulder-bearing brick claj- and its red gravel, 

 which, as shown in a former paper, belong to the Champlain epoch. 

 The Trenton gravel is therefore, later than the Glacial and Cham- 

 plain epochs ; and this is a fact which, when considered in connec- 

 tion with the human relics found in this gravel and the consequent 

 antiquit}^ of man, it will be most important to remember. 



Having now sketched the character and position of the Trenton 

 gravel along the Delaware valley, we are prepared to examine the 

 formation as exposed at the locality whose name we have chosen 

 to distinguish it. 



Trenton is in a position where naturally the largest amount of a 

 river gravel would be deposited, and where its best exposures 

 would be exhibited. It is at the point where a long, narrow valley 

 with precipitous banks and continuous downward slope, opens out 

 into a wide alluvial plain at a lower level. It is here that the 

 rocky floor of the river suddenly descends to ocean level and even 

 sinks below it, forming the limit of tidewater. Thus any drift 

 material which the flooded river swept down its channel would 

 here, upon meeting tidewater, be in great part deposited. Large 

 boulders which had been rolled down the inclined floor of the 

 upper valley would here stop in their course, and all be heaped up 

 with the coarser gravel by the more slowly flowing water except 

 sn<3h few as cakes of floating ice could carr^^ oceanward. On the 

 other hand the finer gravel and sand would be deposited farther 

 down the river. 



