RELICS OF MAN IN THE GLACIAL PERIOD. 261 



Delaware Eiver, and about 150 feet above it, being nearly 

 at the uppermost limit of the Columbia deposit in that 

 vicinity. 



The age of these deposits in which implements have 

 been found at Claymont and at Trenton will be referred 

 to again when we come to the specific discussion of the 

 date of the Glacial period. It is sufficient here to bring 

 before our minds clearly, first, the fact that this at Clay- 

 mont is connected with the river floods accompanying the 

 ice at its time of maximum extension, and when there was 

 a gradually increasing or differential depression of the 

 country to an unknown extent to the northward. 



Two radically different theories are presented to account 

 for the deposits variously known as the Columbia gravel 

 and the Philadelphia brick-clay. Mr. McGee, in the 

 monograph above referred to, supposes them to have been 

 deposited during a period of a general subsidence of the 

 coast-line ; so that they took place at about tide-level. Mr. 

 Upham, on the other hand, supposes them to have been 

 deposited during the period of general elevation to whose 

 influence he mainly attributes the Glacial period itself. 

 In his view much of the shallow sea-bottom adjoining the 

 present shore off from Delaware and Chesapeake Bays 

 was then a land surface, and the Hudson, the Delaware, 

 and the Susquehanna Rivers, coming down from the still 

 higher elevations of the north, flowed through extensive 

 plains so related to the northern areas of elevation that 

 deposition was occurring in their valleys, owing in part to 

 the flooded condition of the streams, in part to the differ- 

 ential elevation, and in part to the superabundance of silt 

 and other debris furnished by the melting ice-sheet in the 

 head-waters of these streams. 



The deposits of Trenton gravel occurred much later, 



at a time when the ice had melted far back towards the 



head-waters of the Delaware, and after the land had 



nearly resumed its present relations of level, if indeed 



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