MODIFIED DRIFT OF THE LAKE DISTRICT. 137 
and sand. They appear to lie between the two members of the coarse 
glacial drift, which we have denominated upper and lower till. In other 
portions of the state these are distinct from each other, and in a few 
instances they have been found to be separated by a thin layer of gravel 
or sand; but generally they are divided at a definite line, with no inter- 
vening stratum of modified drift, This section in Ashland shows that 
between the lower and upper till a depth of ten feet of stony stratified 
clay was deposited; and this seems to have taken place beneath the edge 
of the ice-sheet shortly before the completion of its melting, which con- 
tributed the three feet of upper till lying on the surface. 
The ice-sheet probably remained in a high mountain-like mass over 
these lakes after it had disappeared on each side from the basin of Ossi- 
pee lake and from the lower part of the Pemigewasset valley. As the 
melting continued, the drainage over this area was frequently obstructed 
because the ice-sheet retreated from the lines of water-shed towards the 
middle of these hydrographic basins. The water seems then to have 
melted large open spaces beneath the ice near its margin, in which beds 
of clay and sand were deposited. This would occur at the various heights 
and in the situations where these beds are found, and the till which over- 
lies them is shown by its material to be that which was contained in the 
ice-sheet and fell upon the surface when its melting was completed. We 
thus see how these deposits came to be spread over the slopes of the 
hills, thinly covered by large boulders and till. The frequent accumula- 
tion of such deposits in other parts of the state was prevented by un- 
obstructed drainage from the melting ice. This modified drift overlaid 
by till does not therefore appear to bear testimony to a warm inter-glacial 
period, or even to any retreat and subsequent advance of the ice. 
The course of the rivers which flowed from the melting ice-sheet over 
this area can still be pointed out. The extensive deposits of modified 
drift in New Durham and Alton mark a long continued outflow to the 
Cochecho valley. When the terminal front of the ice had retreated to a 
point a short distance north-west from Alton village, it seems to have re- 
mained nearly stationary during the deposition of the plain on which this 
village is built. At the same time the kame which lies between this 
point and Alton Bay was formed in an ice-walled channel. During the 
recent or terrace period portions of these deposits have been excavated 
VOL, Il. 18 
