63 GEOLOGY OF THE NARRAGANSETT BASIN. 



moraines consist in part of ridges composed of glacial waste, in charac- 

 teristic, irregular, shoved attitudes, each ridge with a more or less distinct 

 sand plain or frontal apron on the side which lay away from the ice, and 

 in part of bowldery tracts where the glacier did not build a distinct or 

 frontal wall or where it may have overridden or broken down the barrier 

 after its construction. It should be noted that these moraines, unlike 

 those on Long Island and Marthas Vineyard, are not placed at right 

 angles to the general trend of the ice movement, but on the west side 

 of Narragansett Bay are turned into a nearly north-and-south position. 

 Beyond the head of the bay they again turn more nearly to the east- 

 and-west direction. On the eastern side of the basin the ridges are not 

 traceable with sufficient distinctness for mapping, but the outlines of the 

 ice front at the time of the formation of this moraine are well enough 

 shown to make it plain that there was a deep indentation at that point, so 

 that while the ice overlapped the present shore on either side of the basin 

 the front did not extend much south of Taunton. Such lobations of the 

 margin, as they have been termed by Chamberlin, were very common 

 along the front of the great ice field. They are due to irregularities of 

 the surface over which the ice moved or to other local conditions. In other 

 instances the presence of a deep, broad valley, such as is afforded by the 

 channels of the Narragansett Bay, led to the projection of the ice at its 

 mouth beyond the main line of the front. In this instance the retreat of 

 the front was probably due to great volume of the subglacial streams which 

 flowed from the areas to the northward. Their effect on the ice at the 

 margin would be to melt it away at the base, making the formation of ice- 

 bergs an easy matter. As the region was evidently depressed below its 

 present level at the time the ice was most advanced — lowered, it may be, to 

 some hundreds of feet below its present altitude — the undermining action 

 of the waters would naturally tend to detach bergs. 



It is evident that the ice front in the region between the northern part 

 of the basin and the sea was subjected to many alternations of advance and 

 retreat which are not registered in any distinct moraines. About twenty 

 years ago, when the Old Colony Railroad Company was widening its road- 

 bed, the new-made sections distinctly showed that there had been a score 

 of these oscillations in the line from North Easton to Somerset, each marked 

 by the disruption and erosion of the deposits which had previously been 



