The Charles River in Massachusetts.—Clapp. 231 
large area to the west. At this time the ice front in the Nashua 
valley probably stood in the vicinity of Clinton, and the outlet 
of lake Nashua was through the pass at South Clinton into the 
valley of the Assabet river and thence to lake Sudbury. Thus 
the drainage of the entire area included within the watersheds 
of lakes Nashua, Sudbury, Charles and Neponset was at this 
stage tributary to lake Bouve. 
The plains of this stage of the combined lakes are by far the 
best developed of any stage. In Newton they are developed as 
far north as the Boston and Albany railroad. Extending 
westward from Brighton along the line of the railroad into 
Weston is a well developed ice-contact slope which marks the 
northern limit of the lake at this stage. Northwest of Needham 
and Sherborn the plains extend through Wellesley, Natick and 
Framingham, across the Cochituate water-parting to the val- 
ley of the Sudbury, where an extensive series of the same gen- 
eral elevation is found, extending even dow the valley of the 
Concord river into Bedford and Billerica. Between Brighton 
and Hyde Park there is a satisfactory eastern land barrier for 
the lake (pl. 16), but the absence of such a barrier in the north- 
ern part of Newton, and again in Dedham, together with the 
characteristic ice-margins found at those places, indicates that 
while on the west the ice had retreated as far north as Bil- 
lerica, it still occupied Boston bay and a large part of the 
Boston basin. 
When the region directly north of the Blue hills was finally 
uncovered, the passes in Milton and Quincy were opened and 
the lake fell to still lower levels. To these lower stages of the 
combined lakes, entirely within the Boston basin, professor 
Crosby has given the name Lake Shawmut. 
During the time when the ice was retreating from the re- 
gion and the glacial lakes were falling successively to lower 
and lower levels, new streams and drainage systems were grad- 
ually developing upon the land surface recently uncovered. The 
extensive drift deposits had so changed the character of the 
topography, however, that the streams rarely resumed their 
former courses, the principle factor in reversing the drainage 
in the upper and middle Charles being the formation of the 
apron plains which blocked the southward-sloping valleys, 
and thus formed a new water-parting north of which all drain- 
