MAX AND THE GLACIAL PERIOD. 639 



tineutal glacier to the north of the Catskills. The time re T 

 quired for the river under present conditions to erode the 

 channel it now occupies was of much greater duration. The 

 following is the probable course of events : 



1, The Philadelphia brick-clay was deposited during the 

 height of the Glacial epoch, when the Delaware Valley was 

 considerably depressed below its present level. This is Mc- 

 Gee's Columbia period. 



2; Toward the close of that period, when the land bad 

 resumed its present level and the ice had nearly all dis- 

 appeared south of the Catskills, the still swollen stream 

 brought; down the superabundant loose material from the 

 kames and moraines of the glaciated area and deposited it in 

 the valley below. The material was so abundant that doubt- 

 less the whole channel was silted up so that the bed of the 

 river was considerably above that it now occupies. At Tren- 

 ton it flowed over, and through < an extensive delta of coarse 

 gravel forty feet above its present level ; and, above Trenton, 

 Over an accumulation of gravel from fifteen to twenty feet 

 above the present high-water mark. This period was marked 

 by the presence of the mastodon and other extinct animals 

 with palaeolithic man in the neighborhood of Trenton.* 

 < ,3. During the Terrace epoch the river worked its way 

 down, through the delta gravel at Trenton, and has since 

 eroded its present channel which is about two miles wide at 

 that point. Higher up, where the current is swift, the lateral 

 erosion in recent times has been small. 



4. To determine approximately the date of the earliest 

 evidence of man's appearance at Trenton we have as data : 

 (1.) The amount of erosion in the gravel at Trenton. (2) The 



* It should have been mentioned earlier that Professor Cook found in this 

 gravel, fourteen feet below the surface, the tusk of a mastodon, and that near 

 the same place, at a depth of sixteen feet from the surface, Dr. Abbott took 

 from it a portion of a human under-jaw, -also from another place a human tooth, 

 and from still another a " very thick and in several respects singular human 

 cranium." Interesting as these are, however, they are too fragmentary to add 

 materially to our information derived from the implements. See " Annual Geo- 

 logical Report for New Jersey for 1878," p. 24; "Report of Peabody Museum 

 for 1886," p. 408. 



