RELATIONS OF GRAVEL DEPOSITS 207 



that these three plains are of the same height, and were controlled 

 by the level of a single body of water. They are all bounded on the 

 west by steep and irregular ice-contacts, and the West Roxbury and 

 Dedham Island plains rest visibly against preglacial knobs of land. 

 With the Cow Island plain, on the contrary, no till nor rock can be 

 found associated, and several wells drilled at the pumping station 

 on the southeast side of Cow Island prove that the drift is here very 

 thick. In one instance 90 feet of gravel was penetrated before 

 reaching bed-rock. 



Conclusions and discussion. — From the foregoing description sev- 

 eral characteristics common to the deposits may be formulated: 



1. The plains are nearly always marginal — deposited about knobs 

 of rock or till. Conversely, most of the preglacial prominences ris- 

 ing above the valley floor have glacial deposits associated with them. 

 This is probably partly due to the natural sinking of the decaying ice 

 masses into the valleys, and in part to the principle that the heat 

 reflected from rock masses accelerates the melting of the ice imme- 

 diately surrounding them, forming hundreds of small marginal lake- 

 lets, throughout the decaying zone of the ice-sheet. 



2. The plains without exception have a definite ice-contact slope, 

 which in several instances entirely surrounds them, except on the 

 sides which are bounded by higher land. This feature indicates 

 that some of the lakelets became entirely filled by gravel, forcing 

 further deposits to accumulate in quantity along the channels of 

 glacial streams or in superglacial lakelets. 



3. Where typically developed, the deltas are associated with 

 feeding eskers on the north, and with effluent eskers on the south. 

 Such a relation can only mean that the glacial lakes, like bodies of 

 water on the present land surface, had both inflowing and outflow- 

 ing streams, the latter carrying off the surplus water and providing 

 a transporting and depositing agent for the excess of gravel. This 

 is in accordance with what has been observed on the Malaspina 

 glacier.' The controversy in regard to the superglacial or subglacial 

 origin of eskers will not be entered here, but the writer believes there 

 is evidence that eskers of both classes exist in the region, and that 



'I. C. Russell, "Expedition to Mt. St. Elias, Alaska," National Geographical 

 Magazine, Vol. Ill, pp. 106-8. 



