MAN AND THE GLACIAL PERIOD. 633 



extensive valley oi the river near which they lie, except per- 

 haps the fragments of some rather rare hypogene rocks.* 



It is now settled that the rocks from which these beds 

 were derived are all in place in the upper Delaware Yalley.f 



The distinction between the river-gravel and that which 

 overlies the larger part of southern New Jersey is marked in 

 several ways. The Trenton gravel is much coarser than the 

 general deposit, it is also largely composed of fresher looking 

 and softei pebbles, showing that it has been subject to much 

 less abrasion than the other, and that it is of more recent 

 age ; it is alsc limited to the river-valley, and finally is not 

 overlaid by the Philadelphia brick-clay which, so far as it 

 extends, rests unconformably upon the general deposit of 

 gravel. The general deposit of gravel in this region is com- 

 posed almost exclusively of small, well-rounded pebbles of 

 quartz and of hard limestone which " are not fresh looking, 

 but are eaten and weather-worn by age." 



a 





b 



C 



b a 





^jy/f?- ;V;.:.;-"- :;•::■': 7T. ■■ . ."<■ ^rV/ V- •',." .i-7-V ■"'.-' 



"■"-.• '-■'-/•• 



: '"-"- ■'■'-- ■; '3-Z; :■■»'*'■--'' 



■&M?g*A - -* "^ -*•. V 



Fig. 128.— Section across the Delaware River at Trenton, New Jersey, a, a, Philadelphia 

 red gravel and brick clay (McGee's Columbia deposit) ; b, b, Trenton gravel, in which 

 the implements are found ; c, present flood-plain of the Delaware River. (After 

 Lewis.) (In Abbott's "Primitive Industry.") 



The elevation of this implement-bearing gravel at Tren- 

 ton is not far from forty feet above the present high- water 

 limit; and Trenton is now at the head of tide-water. These 

 gravels are continuous as a terrace all along up the river. As 

 one ascends the river, however, their height (at least below 

 the Water-Gap) is reduced to fifteen or tw r enty feet above 

 the present flood-plain. 



But most significant of all the facts indicated are the 

 character and position of the Philadelphia red gravel and 

 brick-clay. This also is confined to the river-valley and its 

 tributaries, and rests unconformably upon the older gravel 



* "Report of Peabody Museum," vol. ii, 1876-79, pp. 44-47. 

 t "New Jersey Report for 1877," p. 21; Lewis on "The Trenton 

 Gravel," p. 5. 



