694 WO OD J VOR TH A ND MA RBUT 



yard.^ There appears to be a causal relation between the local 

 conditions of this position and the deposition of the bowlders 

 from the front of the ice-sheet. A brief analysis of the con- 

 ditions which may be assumed to arise in such a position is 

 appropriate in this place. 



The stand of the ice front at the time of formation of the 

 bowlder belt may be assumed to indicate that for the time being 

 the rate of forward movement of the margin of the ice was 

 equaled by the rate of melting back of the front. There are 

 several reasons for believing that the ice was moving forward at 

 this time. As will be presently explained in detail, the attitudes 

 of some of tlie bowlders in the moraine at Exeter suggest the 

 application of force in this manner. Had the ice been stagnant, 

 the bowlders distributed in and under it or upon it, would have 

 come to rest as a sheet of discrete bowlders instead of being 

 brought up to a given line and there deposited. 



The moraine was formed on that side of the valley which 

 receives the larger amount of insolation. It is to be inferred 

 from this that the same ice front lying upon the southern side 

 of the valley with a less insolation to be reflected against the ice 

 or received upon it would not have melted it back at a rate 

 equal to the forward motion of the front ; that the ice would, 

 therefore, have moved over the crest to the next southern slope, 

 where the insolation rate would again equal the forward move- 

 ment and the ice be brought to a stand. On southern slopes 

 the forward movement of the margin of the ice would be 

 accelerated by gravity, on northward slopes retarded. But the 

 tendency of a northward slope to retard would probably in time 

 be overcome by the push of the ice from behind, while the 

 acceleration on a southward slope would give an actually increased 

 movement. With a balanced ice front on the northern side of 

 the valley, it is therefore probable that the line of halt along the 

 north side of Queen's River Valley indicates a slight advance 

 of the ice front from the country on the north and not an 



'See forthcoming atlas folio report on surface geology of Martha's Vineyard, by 

 J. B. Woodworth. 



