The Charles River in Massachusetts.—Clap p. 227 
is the most direct route to the sea for the Merrimac, and there 
is no necessity for supposing that at any time during the Ter- 
tiary period it took a different course. Another gap of less im- 
portance, lies between Bear and Doublet hills west of Waltham. 
This pass is in line with a marked depression extending west- 
ward from Sudbury. Several miles south of this is the gap al- 
ready mentioned in the vicinity of Cochituate. This depression 
is continued eastward through the present valley of the Charles 
between Needham and Dedham, and thence along Mother 
brook to the Neponset river. Westward it is continued be- 
tween Nobscot and Green hills to the valley of the Assabet. 
A fourth line of valley extends east from the vicinity of Sher- 
born through Millis, Walpole and Norwood, and along the 
Neponset river to the sea. The high ridge extending north 
from Bellingham, and the passes in the adjacent southern 
watershed have been already noted. As the streams which 
occupied these passes are shown to have held the same locations 
at the beginning of the Pleistocene period, ‘they, like the Merri- 
mac, evidently did not change their courses greatly during 
Tertiary time. 
The development of the stream occupying the Cochituate 
gap was more complex. This river, coming from an area of 
hard crystalline rocks, crossed the softer formations in Welles. 
ley and Needham, then flowed over the crystalline area east 
of Needham, reaching the sedimentaries again in Hyde Park 
(plate 13). It is possible that this stream may be the lower por- 
tion of the Wachusett river* discovered by professor Crosby, 
who has traced its course as far as the Assabet. The discovery 
of an ancient river flowing from the Assabet to the sea makes 
it a reasonable supposition that the two were synchronous and 
hence probably the same. This stream, coming from a long 
distance, and having many tributaries, must have carved out 
a broad valley, and appropriated the drainage of a consider- 
able area. The upper portion of the Sudbury and its trib- 
utaries belonged to this system. The valley of the Sudbury 
north of Cochituate is certainly very old, and was probably 
formed at that time. Being a tributary to the powerful trunk 
stream, it could erode its valley rapidly, cutting below the 
level of the Waltham gap, and beheading the stream occupying 
*Geoloxical History of the Nashua, valley, 1. c., page 312. 
