88 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
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part of the Cherry Brook pass before the lowest part of the pass was 
free of ice. The lobes that seem to mark this range from 164 to 175 
feet. (d) The Cherry Brook water-plane, which is marked by nearly 
all the deltas between Wayland and Concord, ranging from 160 to 200 
feet. (e) The Hobbs Brook water-plane, marked by the lower deltas of 
Concord, with lobes in the vicinity of 150 feet. 
tecognizing that a grouping into seven-feet-per-mile water-planes may 
be incorrect, for reasons which will be brought out later, let us never- 
theless trace the probable history of Lake Sudbury on the basis of a 
postglacial tilt of seven feet per mile towards the south. For this pur- 
pose the groups of the system of slanting steps may be considered in 
order of their age. 
Probable History of Lake Sudbury. 
THE Stace oF CONFLUENCE. — The stage of confluent lakes which 
saw the birth of Lake Sudbury seems to be marked by deltas which 
occupy the southern part of the Sudbury basin as far north as the Sud- 
bury Aqueduct south of Wayland. They apparently form a continuous 
belt from the Charles River, near Lake Waban} northwest across the 
divide to the Sudbury basin at Cochituate village. They also occur on 
the divide just south of Natick. Their distribution in the two basins 
in such a way as to actually form the present water-parting at Natick 
and Cochituate, together with the fact that the deltas are considerably 
higher than the Morseville pass, point to a state of confluence between 
the two lakes while the ice in the Charles valley was retreating towards 
the present position of the Boston and Albany Railroad and its front in 
the Sudbury valley was receding towards Wayland. 
The levels of most of the plains of this stage have been determined 
only approximately. It has been hard to secure convenient bench-marks 
from which to measure some of them; consequently aneroid readings 
and map contours have been relied on for levels. Moreover, the plains 
in the southwestern corner of the basin, around South Framingham and 
southwest of Lake Cochituate, have not been mapped. 
THe Morsrvitte Srace.— The drop from Lake Charles to Lake 
Shawmut, when the ice had withdrawn to the position of the Boston 
and Albany Railroad, must have caused a simultaneous drop in Lake 
Sudbury from the confluent level down to the level of the Morseville 
pass. The lowering of level seems to have come when the ice-front in 
the Sudbury basin was near Wayland, and while the deltas at the Sud- 
