196 T. C. Chamherlin — Diversity of the Glacial Period. 



He now admits, what I have not understood him to do before, 

 that the terraces, so far as they are rock, are due to base-level- 

 ing, but that the glacial gravels deposited upon them are due 

 to floating ice. To the latter part of this view, there seem to 

 me to be two fatal objections. In the first place, I think it is 

 entirely foreign to observation, as well as to a priori considera- 

 tions, to suppose that floating ice would produce, in the aban- 

 doned bends of old rivers surrounded by hills, and upon rem- 

 nant shelves along the sides of the valley, fine stratified 

 gravels bearing every aspect of river deposits. Under the 

 conditions postulated a very different class of deposits should 

 be formed. Besides, the deposit should have been practically 

 uniform over the whole bottom of the supposed lake, and not 

 simply confined to the old terraces. In the second place, 

 these terraces rise in altitude toward the headwaters of the 

 river until some of them are considerably higher than the sup- 

 posed ice dam, notably those at Warren, Tidioute and Oil 

 City, which are respectively 1395, 1390 and 1270 feet above 

 tide, while the height assigned for the dam is 1000 or 1100 

 feet. (See Bulletin 58, U. S. Geol. Surv., p. 27.) Aside 

 from these definite and tangible factors, the whole aspect, 

 association and relationship of these formations make them 

 river deposits and exclude them from the distinctive lacustrine 

 class. 



In admitting to his hypothesis the factor of base-leveling, 

 Prof. Wright has relinquished the last remnant of occasion 

 for postulating his ice dam, for all of the phenomena ap- 

 pealed to are the natural incidents of base-leveling. Stretches 

 of slack water, which are inevitable when a river reaches the 

 base-level stage and begins to buildup its bed, furnish deposits 

 indistinguishable from those of lakes ; indeed lakes form in 

 the abandoned channels of the stream. So also base-leveling 

 and the resilience from it involve the transfer of the position 

 of low divides which lie near the upper limit of the base plain, 

 the transfer being from the side of the shorter or easier course 

 toward the longer or more obstructed course. The trans- 

 ferred divide is carried into the bottoms of the stream whose 

 territory is invaded, so that its former flood plain deposits be- 

 come the new divide ; and hence the phenomena of slack 

 water deposits on some of the present divides become, under 

 the hypothesis of base-leveling, precisely what is to be ex- 

 pected. These may occur at altitudes greater than the base 

 plain' of the main river. These and coordinate methods of 

 action cover all the phenomena for the explanation of which 

 the supposed ice dam was brought into requisition. 



Being left thus without a raison d'etre, being robbed of all 

 support from the Beech Flat silts, and being entirely without 



