278 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
“ While on the west the ice had retreated as far north as Billerica, 
it still occupied Boston Bay and a large part of the Boston 
Basin” (a, 265). (See also Crosby and Grabau, 129). 
If his view is correct, the ice-front, just before Lake Charles dropped 
down to Lake Shawmut, must have extended from near Riverside due 
north for about fifteen miles to Billerica. Although this extreme ir- 
regularity in ice-front is not impossible, one might rather expect that 
the ice would have melted back from eastern Massachusetts with its front 
more nearly east and west. In other words, one would naturally look 
for evidence that at the time the last 160-foot sand plains were formed 
in Lake Charles in Newton, the ice-front to the west stood somewhere 
in the Sudbury valley between Framingham and Concord; and that 
when further melting let Lake Charles down from 160 feet to the 60-foot 
level of Lake Shawmut, the level of Lake Sudbury fell from 160 feet to 
the level of the lowest pass on its rim,—the Morseville pass, at about 
145 feet. In this case, the sand plains of the Sudbury valley would not 
mark a single water-plane, but two water-planes, the first at 160 feet 
the confluent stage, in the southern part of Lake Sudbury, and the 
second at 145 feet, the stage when Lake Sudbury was tributary to the 
lower Lake Shawmut by way of the Morseville pass. 
From the preceding paragraphs it appears that the levels of sand 
plains in Lake Sudbury, according to the simple scheme of horizontal 
steps which Grabau used for Lake Bouvé, Crosby for Lake Nashua, and 
Clapp for Lake Charles, would show these features :— 
(a) No sand plains could occur above 160 feet. 
(>) Either the plains between Framingham and Concord would all 
mark a level of about 160 feet, or the plains in the southern part would 
be 160 feet and those in the northern part 145 feet, with possibly double- 
lobed plains in the zone of change from the higher to the lower level. 
Concerning the use of the various passes through the divide, as outlets 
of Lake Sudbury, this would be true :— 
(c) The Cherry Brook and South Lincoln passes, being above 160 
feet (as well as above the more southerly Morseville pass), could never 
have served as outlets for Lake Sudbury. 
(7) The Morseville pass alone could have acted asa spill-way, and 
that only in case Lake Charles fell to Lake Shawmut while the ice-front 
lay somewhere between Framingham and Concord. 
