MODIFIED DRIFT IN NEW HAMPSHIRE, II 



areas on each side may be nearly destitute of surface deposits, showing 

 only naked, striated ledges. The peculiar distribution of the till, the dis- 

 persal of boulders, the course of striae, and other topics connected with 

 the unmodified glacial drift, will form the subject of the next chapter of 

 this report. Having taken this brief view of the glacial period, we are 

 now prepared to understand the origin of the modified drift. 



The Ciiamplain Period. 



The departure of the ice-sheet was attended with a comparatively 

 rapid deposition of the abundant materials which it contained. It is 

 probable that its final melting took place mostly upon the surface, so 

 that at the last great amounts of detritus were exposed to the washing 

 of its innumerable streams. The finer portions of these materials would 

 be commonly carried away; and the strong current of the rivers which 

 would be formed near the terminal front of the ice-sheet could transport 

 coarse gravel, or even boulders of considerable size. When the glacial 

 river entered the open valley from which the ice had retreated, or in the 

 lower part of its channel while still walled on both sides by ice, its 

 current was slackened by the less rapid descent, causing the deposi- 

 tion, first, of its coarsest gravel, and afterwards, in succession, of its finer 

 gravel, sand, and fine silt or clay. The valleys were thus filled with ex- 

 tensive and thick deposits of modified drift, which increased in depth in 

 the same way that additions are now made to the bottom-land or interval 

 of our large rivers by the annual floods of spring. The portion of the 

 material contained in the ice-sheet which escaped this erosion of its 

 streams formed the upper till. The abundant deposition of drift, both 

 stratified and unstratified, during the final melting of the ice-sheet, has 

 been brought into due prominence by Prof. James D. Dana,* who de- 

 nominates this the Chainplain period, deriving the name from marine 

 beds of this era, which occur on the borders of Lake Champlain. 



The retreat of the ice-sheet was towards the north-west and north ; and 

 wherever the natural drainage was in the same direction, it would be for a 

 time obstructed by the ice, forming lakes in which the deposition of mod- 

 ified drift would be much different from that which took place where the 



* American Journal of Science, Third Series, vol. v, p. 198, and various papers in vol. x. 



