636 THE ICE AGE IN NORTH AMERICA. 



After an exhaustive examination carried on for several 

 years in connection with the New Jersey Geological Survey, 

 Professor Salisbury reports finding deposits in that state 

 corresponding to those here spoken of as Philadelphia brick 

 clay and red gravel, which he describes under the local names 

 of Bridgeton and Pensauken, reaching a height of 200 feet 

 in the case of the former and 150 of the latter. But he is 

 inclined to refer these "in large part to subaerial (fluvial 

 and pluvial origin"). On this theory the deposits were 

 chiefly brought into place by the Delaware and its tributaries 

 during the close of the first glacial period, when the land was 

 nearly at its present level, and the streams were overloaded 

 with glacial debris. This accumulated as a broad delta over 

 lowlands which were subsequently so much eroded that only 

 the present remnants are left. 



But the extent over which the deposit is spread, as well 

 as its character militates strongly against this theory. For 

 the deposit extends for many miles northeast of the Delaware 

 at Trenton, above where the unaided current of the glacial 

 stream would carry the material, while several miles south- 

 east of Trenton, bowlders, three and four feet in diameter, 

 are found at an elevation of 200 feet, the bowlders being 

 identical in material with others bearing glacial scratches 

 found in the valley of the Delaware, twenty or thirty miles above 

 Trenton. It hardly is possible that the whole area south of 

 Trenton to the limit of these bowlders was covered with 

 Bridgeton gravel to this height, Besides, the distribution 

 of the bowlders in the brick clays of Philadelphia was evidently 

 by stranded icebergs. In this clay bowlders two and three 

 feet in diameter are distributed in a manner that would be 

 impossible in any other way. A depression of 200 feet in the 

 lower valley of the Delaware, therefore, cannot easily be 

 dispensed with. Similar facts lead to the same conclusion 

 respecting the depression at the mouth of the Susquehanna 

 at the head of Chesapeake Bay, near Havre de Grace. 



