Upham.] 
232 
[Feb. 18, 
Concord. The modified drift enclosing Lake Walden is thus in- 
dented by a hollow a half mile long from east to west and a fourth 
of a mile wide, which now sinks about 150 feet and originally may 
have been considerably deeper. Obviously the strong currents 
bringing the gravel must have filled the basin if it had been empty 
when the plain was deposited. 
Lake Cochituate, from which water is supplied to Boston, lies 
in Natick, Wayland, and Framingham, about sixteen miles west 
from the state house. It is three and a half miles long from north 
to south, and about a third of a mile wide in each of its three di- 
visions, which are united by straits. At the high water level, 13 
feet above the bottom of the water works conduit, lake Cochitu- 
ate has an area of 801 acres, and is 134 feet above tide marsh 
level of the sea, or 144 feet above mean low fide, which is the 
Boston directrix or levelling base. Below this high stage, the 
depth of the northern division of the lake is 64 feet ; of the cen- 
tral division, 50 feet; and of the southern division, 72 feet. 
Gravel and sand deposits, partly in knolls and ridges but mainly 
in low plains, almost wholly surround this lake, terminating at its 
shores in steep banks. Again we must conclude that the deep 
lake basin could not have been empty when the modified drift 
was spread upon each side. 
In Maine the remarkable kames, osars, and plains of gravel and 
sand described by Prof. George H. Stone, 1 enclose many ponds 
and lakes of this class. The largest and most noteworthy ex- 
ample is Sebago lake, which is 263 feet above the sea, being 
about thirty feet above the highest level that was held by the sea 
on the Maine coast during the Champlain epoch. The maximum 
depth of this lake is reported to be 400 feet. Professor Stone 
finds that plains of modified drift hold it at least 100 feet higher 
than it would be without their barrier dam around its most south- 
ern bay ; and he believes that the lake basin was occupied by ice 
having a thickness of not less than 400 to 600 feet at the time of 
accumulation of the adjacent gravel and sand beds. 
In New Hampshire these basins are frequent, representative 
examples being Silver lake, or Six Mile pond, in Madison ; Upper 
Beech pond, in Wolfeborough ; Willand and Barbadoes ponds, 
1 Proceedings of this Society, vol. xx, 1880, pp. 430-469. Proc. A. A. A. S., vol. xxix 
1880, pp. 510-519. Am. Jour. Sci., Ill, vol, xl, 1890, pp. 122-144. 
