1880.] NATURAfi SCIENCES OF PHILADELPHIA. 30o 



It has been noticed that from Trenton to Philadelphia the creeks 

 flowing into the river Delaware have a steep south bank, while the 

 ground north of the creek is flat. The writer finds that the flat 

 ground north of the creek is made of Trenton gravel, while the 

 southern bank is made of older formations which have been for- 

 merl}^ cut awaj^ by water action coming from the north. Thus, 

 the steep south bank of the Neshaminy is made of " Philadelphia 

 red gravel " of Champlain age, while a flat plain of Trenton gravel 

 lies south of the creek. The same configuration of the banks of 

 creeks on the New Jersey shore has been noticed by Prof. Cook. 

 By assuming that the river at the time of the deposition of this 

 gravel was of larger volume than now, this fact is of ready explana- 

 tion. The southern bank of the creek, often of Cretaceous or 

 Tertiarj" strata, in each case formed the shore of the ancient river, 

 and was worn away into a steep bank by the flood from the north. 

 Similar in cause and effect are the present banks of the Delaware, 

 which are steep on the outside of each curve of the river, and flat 

 and covered with recent alluvium on the inside. 



Another fact showing river action is the frequent occurrence of 

 exposures of " flow and plunge structure " in this gravel. In these 

 the la3^ers are seen to dip up stream, as would be expected by 

 downward flowing water. It is interesting to find, on the other 

 hand, that the same structure in the Tertiary gravels, both of 

 Pennsylvania and New Jersey, shows layers dipping southeast, as 

 though deposited by incoming oceanic tides. 



Another instance of the fluviatile character of the Trenton gravel 

 is found in the peculiar topography which it sometimes exhibits. 

 Frequently, instead of forming a flat plain, it forms higher ground 

 close to the present river channel than it does near its ancient 

 bank. Moreover, not only does the ground .thus slope downward 

 on retreating from the river, but the boulders become smaller and 

 less abundant. Both of these facts are in accordance wdth the 

 laws of river deposits. In a time of flood the rapidly flowing- 

 water in the main channel, bearing detritus, is checked by the more 

 quiet waters at the side of the river, and is forced to deposit its 

 gravel and boulders as a kind of bank. 



In determining the comparative age of the Trenton gravel, a guide 

 may be found in the amount of its erosion. In this respect a 

 marked contrast exists between this and more ancient gravels. 

 Unlike the land covered bj' older surface formations, that covered 



