74 GEOLOGY OF THE NARRAGANSETT BASIN. 



It is to be noted that the general form of the basin is such that the ice 

 during the period when its front lay beyond the present shore line of the 

 continent, as it probably did for the greater part of the time in which it 

 occupied this part of the country, was led somewhat to concentrate its 

 flow in the relatively narrow space occupied by the seaward part of the 

 basin. This concentration must have increased the speed of the move- 

 ment and thereby the erosive effect of the moving ice. I have elsewhere 1 

 endeavored to show, by clearer examples than are afforded by this field, 

 that the effect of such an increase of speed, due to the crowding of ice into 

 a relatively narrow way, is to intensify the erosive work which the ice per- 

 forms. It is also clear that the subglacial streams which discharged into this 

 bay were very large. Such streams, so long as they flow beneath the ice, 

 probably have a far greater cutting power than open-air rivers, for the rea- 

 son that thev move with an energy in some measure intensified by the 

 height of the column of ice whence they are derived. As the sheet may 

 well have had a depth of some thousand feet, the impulse can be accounted 

 as great. These subglacial streams were competent to urge forward over 

 level ground the bowlders, often several feet in diameter, which we now 

 find embedded in the eskers — masses which the most vigorous mountain 

 torrent would hardly be able to move. We may therefore reckon the sub- 

 glacial streams as powerful agents of erosion, quite competent to deepen 

 channels such as the preglacial rivers may have formed or to cut new ways 

 if the conditions compelled them to flow in other courses. 



It must be said that the form of Narragansett Bay is not that of a 

 characteristic fiord, such as in the regions farther to the northward clearly 

 attest the competency of glacial ice to excavate such basins. There is no 

 trace of the sill or rock barrier across the mouth of the bay, separating it 

 from the sea, such as marks the normal Scandinavian fiords. We may, 

 however, hold that while this Narragansett system of depressions is clearly, 

 as regards its general outlines, the product of erosion work done before the 

 ice time, it owes much of its form to glacial processes. 



Before closing this brief account of the glacial phenomena of the Nar- 

 ragansett district which demand notice in this memoir, we may refer to the 

 general form of the surface of the basin with reference to the possible effect 



'The geology of the island of Mount Desert, Maine, by N. S. Shaler: Eighth Ann. Eept. U. S. Geol. 

 Survey, Part II, 1889, pp. 1007, et seq. 



