RELATIONS OF GRAVEL DEPOSITS 199 



ber of well-defined lake outlets are known, each of which, as a rule, 

 corresponds in level with the upper surfaces of a particular series of 

 plains. Moreover, a study of the individual plains shows that a num- 

 ber of them often reach approximately to a compion elevation, and, 

 with few exceptions, they may be classified into one of several groups 

 distinguished by their different levels. The elevations of the most 

 abundant of these — not including those near sea-level — have been 

 found to be approximately 150, 170, 200, and 270 feet in elevation, 

 although local groups and isolated plains occasionally occur at inter- 

 mediate levels. A fact thought to be of considerable significance is 

 that plains having a common elevation are distributed over very 

 wide areas. Deltas of the 170-foot series not only are known north 

 and south of the Dover highlands and in the adjacent Neponset 

 valley, but are very abundant across the divide of the Sudbury River 

 in the vicinity of Cochituate, and even far down the valleys of the 

 Sudbury and Concord Rivers. The widespread distribution of 

 plains of the same general elevation at first appears to indicate the 

 presence in the region of a large open body of water, which with suc- 

 cessive retreats of the ice-margin, opening outlets for the lake at con- 

 secutive lower levels, allowed the water to fall repeatedly to the next 

 stage below. Recent studies by Mr. M. L. Fuller in the Neponset 

 basin indicate, however, that in that region there was no such simple 

 sequence of events; and the present writer, working independently, 

 finds evidence in Lake Charles indicating that here too the open lake 

 hypothesis can be accepted only in part. 



DESCRIPTION OF THE REGION. 



The location of the main region under consideration in the pres- 

 sent paper is shown in Fig. i. In a general way, it includes that 

 portion of the Charles River basin lying south of the Boston & 

 Albany Railroad in Newton and north of the Dedham and Dover 

 highlands. More definitely, it is that part of the drainage basin of 

 the present Charles River lying within that of the preglacial Charles. 

 The portions of the valley lying north of the Boston & Albany Rail- 

 road, and those east of the Brookline and West Roxbury highlands, 

 are not included, being within the limits of what has been designated 

 Lake Shawmut — a glacial lake corresponding with later stages of the 

 ice-retreat. 



