RELICS OF MAN IN THE GLACIAL PERIOD. 257 



Washington, the surface of the Columbia deposit is 150 

 feet above tide, and that the deposit itself contains many 

 boulders, some of which are as much as two or three feet in 

 diameter. These are mingled with the gravel in such a 

 way as to show that they must have been brought down by 

 floating ice from the head- waters of the Potomac when the 

 winters were much more severe than now. That this de- 

 posit is properly the work of the river is shown by the en- 

 tire absence of marine shells. 



According to Mr. McGee, also, there is a gradual de- 

 crease in the height of these delta terraces of the Columbia 

 period as they recede from the glacial boundary — that at 

 the mouth of the Susquehanna being 245 feet, that of the 

 Potomac 140 feet, that on the Rappahannock 125, that on 

 the James 100, and that on the Roanoke 75 ; while the 

 size of the transported boulders along the streams also 

 gradually diminishes in the same order. During the 

 Columbia period the Susquehanna River transported 

 boulders fifty times the size now transported, while the 

 Potomac transported them only up to twenty times, the 

 Rappahannock only ten times, the James only five, and 

 the Roanoke only two or three times the size of those now 

 transported. This progressive diminution, both in the 

 extent of the deposit and in the coarseness of the material 

 deposited by these rivers at about the time of the maxi- 

 mum portion of the Glacial period, is what would naturally 

 be expected under the conditions supposed to exist in con- 

 nection with the great Ice age, and is an important con- 

 firmation of the glacial theory 



That the period of subsidence and more intense glacial 

 conditions during which the Columbia deposits took place, 

 preceded, by a long interval, the deposition of the gravel 

 terraces at Trenton, N. J., and the analogous deposits in 

 the Mississippi Valley where palaeolithic implements have 

 been found, is evident enough. The Trenton gravel was 

 deposited in a recess in the Columbia deposit which had 



