1880.J NATURAL SCIENCES 01' PUlLAtlfiLi'fllA. SOl 



This is precisely what occurs at Trenton. The material, Which 

 at Philadelphia is generally- fine, grows coarser as the river is 

 ascended, until at Trenton we find often immense boulders im- 

 bedded at all angles in the gravel. Moreover, the river has here 

 cut entirely through the gravel down to the rock, exposing at 

 one place a cliff of gravel 50 feet high. At Philadelphia, on the 

 other hand, as we have seen, the river still flows on the top of the 

 gravel. This fact may also be accounted for. Having heaped up a 

 mass of detritus in the old river channel as an obstruction 

 at the mouth of the gorge, the river, so soon as its volume dimin- 

 ished, would immediatel}' begin wearing away a new channel for 

 itself down to ocean level. This would be readil}^ accomplished 

 through the loose material, and would be stopped only when 

 rock was reached. On the other hand, that gravel wliich had 

 been deposited at places farther down the river where its bottom 

 was below ocean level, would remain un-eroded or nearly so. Wheii 

 the river had attained the level of the ocean there would be no 

 occasion to cut a deep channel, and it would therefore flow on top 

 of the gravel which it had deposited. It is necessary that this 

 point should be understood, as other geologists have brouglit for- 

 ward various theories to explain the high bank of gravel at Tren- 

 ton. The fact of the river having cut through the gravel at 

 Trenton, while at Philadelphia it flows upon it, is due to the con- 

 figuration of the rock floor of the river, which at Trenton rises 

 above ocean level, and at Philadelphia lies nearl}^ 100 feet below it. 



In addition to the exposure upon the river bank, where the 

 whole depth of the formation is seen, the long railroad cuts 

 made by the Pennsjdvania R. R. at Trenton, afford excellent 

 sections of the gravel. It exhibits the distinctive characteristics 

 of a true river deposit, and is very different from the gravels which 

 are found at higher levels. It contains no clay;. its pebbles are 

 made of the rocks of the river bed and are flattened, and the strati- 

 fication of the whole deposit is well seen in the alternations of 

 sand and gravel. It extends several miles back from the present 

 river, covering the low ground along the Assunpink Creek, and 

 indicating the existence here of a former bay or arm of the Dela- 

 ware. This bay was shaped somewhat like a horseshoe, which had 

 one extremity in Trenton at the hill above the canal, and which 

 washed the base of the hill north of the Assunpink Creek, and, 

 extending about three miles back from Trenton, and sweeping 



