634 THE ICE AGE IN NORTH AMERICA. 



formations, rising to a height of one hundred and fifty feet 

 above the river, and there ceasing. This elevation relative 

 to the river is maintained as far up as Easton, where the 

 bed of the river itself is one hundred and fifty-seven feet 

 above tide-level. Finally, the Philadelphia brick-clay con- 

 tains numerous bowlders of considerable size, derived from 

 the ledges of Medina sandstone and other rocks above. This 

 marks it as a deposit of the glacial flood some time during the 

 declining centuries of the great Ice age. 



The succession of events would seem to be as follows : 

 During the early part of the Glacial period the ice accumu- 

 lated in the upper portion of the valley of the Delaware to a 

 depth of many hundred feet. The area in the valley of the 

 Delaware covered by the ice is not far from six thousand 

 square miles. It is not improbable that the average depth 

 of the ice accumulated over the region was considerably more 

 than fifteen hundred feet, or a quarter of a mile, making 

 the total accumulation of ice more than fifteen hundred 

 cubic miles, with its southern border sixty miles above 

 Trenton. All this as it melted must find its outlet to the 

 sea through the Delaware River. It is evident at a glance 

 that during the decline of the Glacial period, when the pro- 

 cess of melting was proceeding with greatest rapidity, the 

 floods in the valley below must have been upon a scale of 

 surprising magnitude. 



And yet it is impossible that these glacial floods in the 

 Delaware should have been so enormous as to have filled the 

 valley below Trenton to the height of one hundred and fifty 

 feet, for this valley is nowhere less than five miles in width 

 and constantly enlarges toward the sea. If the water at 

 Trenton were raised one hundred and fifty feet, the slope to 

 the bay would be about two feet per mile. Now, a current 

 of five miles per hour, one hundred and fifty feet deep and 

 one mile wide, would discharge a cubic mile of water every 

 eight hours, or three cubic miles per day. (The mean rate 

 of the Ohio River, with an average descent of five inches to 

 the mile, is three miles per hour — that of the Mississippi 



