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GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 275 
The divide that separates the Sudbury basin from that of the Charles 
has several gaps which must have been occupied either by outlets of 
Lake Sudbury or by arms of confluence with the contemporary Lake 
Charles. Near the southern end of the Sudbury basin in particular, 
there seems to have been, for a while at least, a confluence of the two 
lakes; for the present divide is itself made up of sand deltas and asso- 
ciated gravel deposits which appear to carry the water-plane of Lake 
Charles over into Lake Sudbury. This is seen on the map in the line of 
sand plains which extend from the big bend of the Charles near South 
Natick northwest through Morse’s, Jenning’s, Mud, and Pickerel Ponds 
to Cochituate. The pass at Morseville, south of Lake Cochituate, 
where the present divide is still lower (about 145 feet above sea-level), 
may also have been a point of confluence between Lake Charles and 
Lake Sudbury. 
North of Cochituate, along the eastern rim of the basin, there are only 
two considerable sags or cols in the divide, — one east of Wayland, near 
the head of Cherry Brook, and one about a mile south of Lincoln station. 
Both these passes, however, are above the level of the Cochituate and 
Morseville passes, and so could hardly have served as outlets for Lake 
Sudbury unless they were originally lower, and have been subsequently 
raised by tilting. 
Another pass occurs just outside the limits of Lake Sudbury as it is 
here defined, east of the point of junction of the Assabet and Sudbury 
(where the Concord River begins), at the head of Hobbs Brook in Lin- 
coln. This is also a relatively high-level pass, about 160-170 feet, 
judging from the contours. The importance of these higher cols 
will be shown later. 
Arrangement of Sand Plains in Adjoining Districts. 
The sand plains of Lake Charles in Wellesley, Needham, and the 
southern part of Newton, mark a water-plane, the altitude of which is 
about 150-160 feet. This has been reported and discussed by Crosby 
and Clapp, who took their levels from the contour maps of the U. S. 
G. S. Boston and Framingham sheets. The recently revised map of the 
Boston area has made it possible to get these levels more accurately 
than before. A table follows which gives the altitudes in the district 
just mentioned, as they are shown by the newly drawn contours of the 
Boston map and maps of the Metropolitan Park Commission ; also a few 
altitudes determined much more accurately by a hand-level survey, 
