HISTORY OF DERRYFIELD. 33 



In Wright's "Ice Age in North America" the author says: 

 " In the deltas of rivers the sifting power of water may be ob- 

 served. Where a mountain stream first debouches upon a plain 

 the force of its current is such as to move large pebbles, or bowl- 

 ders even two or three feet in diameter. As the current is 

 checked the particles moved by it become smaller and smaller 

 until only the finest sediment is transported * * * and this 

 is deposited as a thin film over the previous coarse deposit. 

 Upon the repetition of the flood another layer of coarser mate- 

 rial is spread over the surface, and so, in successive stages, is 

 built up a series of stratified deposits. Water moving with vari- 

 ous degrees of velocity is the most perfect sieve imaginable." 



The author reaches many conclusions, specially applicable to 

 the restricted field of our inquiry, which we have only space to 

 epitomize : When a glacier dissolves, the torrents of water aris- 

 ing tear down and distribute as sediment to distant valleys the 

 material accumulated by the slow movement of centuries ; that 

 the transportation by water from the front of glaciers is certain- 

 ly of immense extent ; that the glacial debris still remaining is 

 but an insignificant remnant of the total amount transported, 

 and that sub-glacial streams must have sent their turbid currents 

 down through every New England outlet. 



Prof. Shaler estimates the total amount of drift in New Eng- 

 land and its neighboring terminal moraines at 750 cubic miles, 

 or more than the mass of the White Mountains. If evenly dis- 

 tributed this would make a layer of about sixty-five feet. 



Prof. Wright says that New England is gridironed by a system 

 of gravel-ridges deposited by glacial streams, and that in these 

 and in the terminal moraines we may study the skeleton of the 

 continental ice-sheet as intelligently as the anatomist can study 

 the skeleton of a dissected animal. 



The same authority says : "The scenes to have been wit- 

 nessed during the advance of the ice-sheet are as nothing com- 

 pared with those which must have occurred during its retreat." 

 " During the last stages of the great ice-age, through the months 



