1877.] "Surfads Geology of the Merrimack Valley. 587 
posits of stratified clay or sand are found between the lower and 
upper till. In these places the contour of the land seems to have 
prevented free drainage from the foot of the melting ice-sheet. 
The water then melted large open spaces beneath the ice near its 
margin, in which these beds of stratified drift were deposited. 
The overlying till was contained in the ice-sheet, and fell upon 
the surface when its melting was completed. 
The distribution of the till in this valley is quite irregular. 
Sometimes no considerable accumulations of it are seen for sev- 
eral miles, and the ledges lie at or near the surface. Else- 
where the till occurs in large amount, covering the ledges which 
are scarcely exposed over some whole townships near the coast. 
Wherever it is found plentifully it is to a large extent massed in 
the peculiar lenticular hills, which, except a thin layer on the sur- 
face, are entirely composed of lower till; but we cannot explain 
how the ice acted to accumulate its ground-moraine over some 
sections in these regular hills, while over other large areas, appar- 
ently not otherwise different, they are wholly wanting. 
The departure of the ice-sheet was attended with a compara- 
tively rapid deposition of the abundant materials which it con- 
tained. 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 
surface of the ice-sheet became molded by this process of de- 
struction into great basins and valleys, and the avenues by which 
its melting waters escaped came gradually to coincide with the 
depressions of our present surface. When the glacial river en- 
tered the open area 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 deposition 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 extensive and thick deposits of mod- 
ified drift, which increased in depth in the same way that ad- 
ditions are now made to the bottom-lands or intervals 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 this final melting of 
the ice-sheet has been brought into its due prominence by Prof. 
James D. Dana, who denominates this the Champlain period, de- 
‘ving the name from marine beds of this era which occur on the 
borders of Lake Champlain. 
