RELATIONS OF GRAVEL DEPOSITS 213 



valleys of Noanet and Mine Brooks suggest that this is another 

 instance of a glacial lake having both inflowing and outflowing 

 streams. 



The occurrence of so numerous and widely scattered sandplains 

 having a uniform elevation, which, notwithstanding their wide distri- 

 bution, were evidently formed in merely local lakelets, often resting 

 in part on the ice itself, is best explained by the assumption that the 

 ice was in such a state of decay as to allow a connection between the 

 individual lakelets sufficient to maintain the water at a common 

 level. Such being the case, the general level of the waters at the 

 different stages must have been determined by one of two causes; 

 either, first, by deposition in an arm of the sea, or, second, as was 

 more probable, by topographic features, probably passes between 

 the highlands on the east side of the lower Neponset valley, through 

 which the water escaped to Lake Bouve,' and thence to the sea. 

 The probable explanation for the lack of plains between the 270- and 

 1 70- foot levels is the absence of any pass intermediate between these 

 elevations on the northern side of the Blue Hill range. In the southern 

 portion of Lakes Charles and Neponset, where passes of other levels 

 occur, intermediate plains also abound. The relations of ice, land, 

 and water during the 170-150 foot stage are shown in Fig. 2. 



This stage of the lakelets existed during the entire time while 

 the outlets remained at this elevation. When the decay of the ice 

 had advanced sufficiently to open a 120-foot outlet north of the Blue 

 Hills, the ponded waters fell to that level. There is no indication 

 that during this time there was any considerable disappearance of 

 the ice in the upper and central portions of the Charles River basin, 

 for in these portions no plains are known below 160 feet. The absence 

 of synchronous plains in the Lake Shawmut area indicates that this 

 was still for the most part ice. By far the greater portion of the 120- 

 foot plains is situated in the town of Dedham, their location being 

 probably due to their favorable position at the head of the Charles 

 River valley glacier. Another view of their origin is that they repre- 

 sent merely a local stage, while the greater portion of the waters stood 

 at a lower level. 



^ A. W. Grabau, "Lake Bouve," Occ. Papers, Boston Society of Natural History, 

 Vol. IV, part III, pp. 564-600. 



