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 striz, 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. 
Tue CHAMPLAIN 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 Champlain 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 Fournal of Science, Third Series, vol. v, p. 198, and various papers in vol. «. 
