70 GEOLOGY OF THE NARRAGANSETT BASIN. 



It is to be noted that the till materials of this basin contain a much 

 smaller amount of clay than is the case with the like deposits farther inland. 

 This is probably due to the fact that in this marginal district the materials 

 composing the till consist largely of esker and other washed gravels that 

 had already lost their clay element. In the irregular movements of the ice 

 and of the subglacial streams a large part of this clayless matter is brought 

 again into the ice, and in the end finds its way to the surface in the form of 

 till. The result is that in this marginal portion of the glacial field in New 

 England there is often little difference in the materials which go to make 

 up till or kame deposits, the clay element having been in both cases washed 

 away. The effect of this is to make the value of the fields for tillage 

 much less uniform than is the case where the till deserves its ancient name 

 of bowlder clay. 



The origin of the glacial detritus of the Narragansett Basin has not 

 yet been fully traced. From the studies which have been made it is suffi- 

 ciently evident that the carriage in the case of the materials contained in the 

 till has in general been for no great distance. Although, as will hereafter be 

 noted, there is at least one case where the transportation has extended very 

 far, the evidence shows that at least four-fifths of the till debris has been 

 carried not more than 5 or 6 miles. This determination is easily made on 

 the northern border of the basin, where the line between the pre-Cambrian 

 and igneous and the Carboniferous rocks can be traced with approximate 

 accuracy. The materials which have been conveyed in the subglacial 

 streams, here as elsewhere, have been subjected to much greater transpor- 

 tation. The exact extent of this has not been determined, but it has 

 probably amounted to many times the distance of the carriage of the 

 materials which form the till 



A large part of the waste which enters into the composition of the drift 

 of this district has come from the disintegration of the conglomerates of 

 the Carboniferous section. This is shown by the fact that a considerable 

 portion of the drift pebbles retain the distinct form which was given to them 

 by the stresses to which they were subjected in the beds in which they had 

 lain so long, and also by the fact that these pebbles are often composed of 

 the fossiliferous quartzite which yielded so much to the debris in the Car- 

 boniferous time, but does not now exist in the original bedded form in any 

 part of the district. The fact that the glacial deposit in the older moraines 



