o04 PROCEEDINGS OF THE ACADEMY OF 1880.] 



by the Trenton gravel is remarkably level and free from hillocks 

 or ravines. The change in topography may be well seen in the 

 neighborhood of Trenton, and can be noticed almost anywhere 

 along the valley. This fact alone would indicate a more recent 

 age than that of the clays and gravels of the Champlain epoch. 

 This difference is much more marked when comparison is made 

 Avith the oceanic gravels. 



The actual time necessary for the Delaware to cut down to the 

 rock through 50 feet of this gravel at Trenton is by no means 

 great. Numerous facts have been adduced by geological writers 

 and by engineers to show how rapidly a stream of water can wear 

 through loose gravel material. When it is noted that the gravel 

 cliff at Trenton has been made, not b}" a straight downward cut, 

 but by a side wearing away as at a Ijank, and when it is remem- 

 liered that the erosive power of the Delaware was formerly very 

 much greater than it is now, it will be conceded that the presence 

 of the clitf at Trenton will not necessarily infer its high antiquity. 

 From what is known of the action of running water upon gravel, 

 it is thought that the time necessary to produce the erosion now 

 observed might be reckoned b}^ hundreds rather than by thou- 

 sands of 3'ears. While the gravel was of course formed in a 

 previous time, the rapid action of the flood which deposited it, 

 shown in many places by the character of the gravel, indicates 

 that the time necessary for its deposition need not have been long. 



Having now shown that the Trenton gravel is a true river 

 deposit of modern age, it will be of interest to inquire how such a 

 flood as we have proved to exist could have originated. No flood 

 within the historical epoch has been known to at all approach in 

 magnitude that which deposited the Trenton gravel. No boulders 

 of the size found in and upon that gravel are ever carried down 

 the river by recent ice-cakes. In fact, at Trenton and below, the 

 boulders of this gravel are often much larger than any in the 

 Champlain gravel of that part of the valley. 



We have seen that at the time of the Trenton gravel flood, the 

 lower part of Philadelphia, the whole of Bristol and Tullytown, and 

 almost all of Trenton were submerged. That the climate was then 

 cold is indicated not only by the suggestion that there were then prob- 

 ably very large masses of bouldei'-bearing ice floating in the river, 

 but also by the fact that, as the writer is informed by Dr. C. C. 

 Abbott, bones of Arctic animals (walrus, reindeer, mastodon). 



