1881 .] 
141 
Wright. 
itself bears every mark of glaciation. This is two thousand one 
hundred feet above the sea and one thousand six hundred feet 
above the level of the Lehigh at Mauch Chunk, and one thousand 
eight hundred feet above the level of the Delaware at the Water 
Gap. 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 one thousand five hundred feet, or a quarter of a mile, making 
the total accumulation of ice not far from 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 process 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 Dela- 
ware 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 no where less than five miles in width and constantly 
enlarges towards the sea. If the water at Trenton were raised 
one hundred and fifty feet, the slope would be about two feet per 
mile to the bay. 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 very nearly the same.) . To supply such a volume of 
water as this, the whole accumulation of ice in the upper Dela- 
ware would suffice for only five hundred days, or for about sixteen 
months. And to furnish this amount of water there would need 
to be, during such floods, a daily accumulation by rains and the 
melting ice over the whole upper valley of the Delaware of about 
three feet of water, which of course is incredible, even if we 
suppose the floods confined to a single month of each successive 
year. Hence, without doubt, we may conclude that the deposi- 
tion of the boulder-bearing brick clay in the Delaware valley 
below Trenton implies a depression of that region to the extent 
of one hundred or more feet. 
