GLACIAL PHENOMENA. 67 



built into the Carboniferous strata. The absence of large bowlders in the 

 Carboniferous is paralleled by what we find in the modern washed drift; 

 there, as might be expected, the larger fragments have been kept out of the 

 beds by the sorting process which takes place in water transportation. 



The facts disclosed by a study of the conglomerates of this basin lead 

 to the conclusion that the pebbles were probably formed by glacial action, 

 and that the fragments were brought to their present position by torrents 

 which swept into the basin from the highlands that bordered it. Their 

 transportation to their present sites, as well as their distribution into beds, 

 may have been due to waves and shallow-water currents acting during a 

 period of increasing* depression of the land. In no way save by glacial 

 work does it seem to me possible to account for the rapid formation 

 of the great mass of pebbly detritus which is contained in these beds. It 

 therefore may fairly be held that the Carboniferous period, in this district 

 at least, was one of extensive and long-continued glacial action, and that 

 the greater part of the section exhibited in the basin is made up of rocks 

 which owe their more important features to the action of glaciation. From 

 the Carboniferous to the Pleistocene this area affords no evidence of ice 

 action. 



LAST GLACIAL PERIOD. 



The last Grlacial period has left upon this district marks of its action 

 which are as indubitable as any that are found in the region to the north- 

 ward. In the time of the greatest southward extension the front of the ice 

 evidently lay considerably to the south of the whole southern shore of Mas- 

 sachusetts, Rhode Island, and Connecticut. This is shown by the presence 

 of extensive moraines on the Elizabeth Islands, Marthas Vineyard, and Nan- 

 tucket, as well as on Long Island, New York. A careful inspection of the 

 marine soundings off this shore has failed to reveal any indications of a 

 submerged moraine marking the extreme line to which the ice sheet 

 attained. Moreover, the evidence gained by the study of the front over 

 the land from the coast of New Jersey to the far West goes to show that 

 the extreme extension of the ice was of a very temporary character, the 

 considerable halts having taken place in the stages of retreat which probably 

 began very shortly after the farthest southward advance had taken place. 



The stages of retreat of the glacier in and near the Narragansett Basin 

 are fairly well marked by the occurrence of frontal moraines. These 



