638 THE ICE AGE IN NORTH AMERICA. 



ice-sheet terminated, to Trenton, is two hundred and thirty- 

 two feet, or at the rate of nearly four feet per mile. 



Now, the transportation of gravel by a river is dependent 

 both upon the amount of material accessible to the running 

 stream and upon the rapidity of the current. Toward the 

 close of the Glacial period the pebbles accessible to the stream 

 were superabundant, having been deposited in excessive 

 amount by the melting of the glacier in the lower latitudes. 

 The water-worn pebbles at Trenton were probably largely 

 derived from this source. Even a glacial torrent may have 

 more loose material than it can manage, and so may silt up 

 its bed with gravel. Hence it is not necessary to suppose 

 the river at this point to have been of sufficient volume to 

 till the whole valley with water to the height of the terraces, 

 fifteen or twenty feet. The river may have flowed upon a 

 more elevated gravel bottom in a shallower current than the 

 terrace would seem to imply. 



When, now, the current, passing down this declivity of 

 four feet to the mile, reached the level of the sea at Trenton, 

 its transporting power would be greatly diminished, and thus 

 we should have an accumulation of gravel at the head of 

 tide-water, without bringing into the problem the supposition 

 of any very extraordinary increase in the volume of the 

 river. The transporting capacity of a stream of w T ater is esti- 

 mated to vary as the sixth power of the velocity ; i. e., if a 

 current is checked so that it moves at only half its former 

 rate, its transporting capacity is diminished to one sixty- 

 fourth.* It is easy to see that the sudden enlargement of 

 the valley just above Trenton, as well as the occurrence there 

 of tide-water, would diminish the rapidity of the river, and 

 hence cause an extraordinary deposition of gravel when 

 the moraines above were fresh and when ice-tields still lin- 

 gered in the southern valleys of the Catskills. The process 

 of deposition must have been so rapid that it might well 

 have taken place not long before the withdrawal of the con- 



*See Le Conte's " Elements of Geology," pp. 18-20. 



