GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 277 
North of Waltham the country east of the Sudbury basin is mostly 
high ground, nowhere low enough to meet the 60-foot water-plane which 
would be expected to mark the maximum height of temporary Lake | 
Shawmut there. Naturally, therefore, there are no sand plains in that 
part of the district, between Lincoln and Lexington. The gravels far- 
ther east, in the Mystic River valley, from Arlington to Woburn, lie 
wholly below 60 feet. 
Expectation regarding the Lake Sudbury Water-planes. 
The work of Crosby, Grabau, and Clapp, on the temporary lakes of 
this region, pointed to the likelihood that the sand plains of the Sudbury 
basin would be found to be arranged according to the horizontal step 
scheme, i.e. either at a single altitude, marking a single water-plane, or 
in groups of decreasing altitude from south to north, marking succes- 
sively lower and lower water-planes. 
Clapp recognized from the contour map that the sand plains of both 
the Sudbury and the Charles basins, since they are over 150 feet, rise 
above the level of the Cochituate pass (145 feet), the lowest pass in the 
divide between the two basins ; and the two lakes, Sudbury and Charles, 
must then have had a common level higher than the Cochituate pass, 
which formed a narrow arm of confluence. The line of sand plains from 
the Charles at Wellesley northwest to the Sudbury basin at Natick, 
was mentioned particularly by Clapp as representing an arm of conflu- 
ence between Lake Charles and Lake Sudbury, during a stage when the 
water-level was about 160 feet. Clapp as of the deltas built in the 
confluent lakes in these words : — 
“The plains at this stage of the confluent lakes [Sudbury, Charles, and 
Neponset] are by far the best developed of any stage. The broad de- 
posits of Medfield, Millis, and Medway, as well as those in Wellesley, 
Needham, and West Roxbury were formed at this time. In Newton, 
they are developed as far north as the Boston and Albany Railroad ; 
. West of Needham and Sherborn the plains extend through Welles- 
ley, Natick, and Framingham, across the Cochituate water-parting to 
the valley of the Sudbury, where an extensive series of the same general 
elevation is found, extending even down the valley of the Concord River 
into Bedford and Billerica” (a, 265). 
According to this, Clapp recognized the 160-foot level as one that 
was maintained yery long in Lake Sudbury; or, to use his words 
again : — 
