60 J. D. Dana on the origin of some of the 



where the rock has been recently exposed, for half a mile west. The 

 ledges that have been long bare have lost their scratches by weather- 

 ing ; on this account, and owing also to the covering of soil over other 

 parts, observations have not yet been extended farther west. The 

 following is offered in explanation of this southwestern throw of 

 the under portion of the glacier. 



It has been stated on page 45 that the New Haven region, be- 

 tween the summits of the ridges confining it on the east and west, 

 has a width of seven miles to the north, and narrows to four at the 

 south. While the mass of the glacier was continuing its southward 

 movement, the portion below filling this depression would have had 

 to accommodate itself some way to the narrowing limits. This ac- 

 commodation might have taken place, through an increasing depth 

 of the depression southward. But if this was insufficient to meet the 

 whole, there would have been a tendency to a thickening upward of 

 the glacier and relief would have been obtained from the accumula- 

 ting pressure by a lateral escape of the ice. 



There was evidently no yielding or escape on the east or Quinni- 

 piac side, the side of the broadest and deepest valley, and therefore 

 of deepest or of thickest ice ; for the ploughings of the glacier which 

 are exhibited along that side on a grand scale over the East Haven 

 sandstone, have the usual southward (S. 13° W.) course. Hence the 

 escape, if any where, must have been on the west side; and here it is 

 that we find these S. 33° W. scratches. The place is southwest of 

 where the Quinuipiac valley opens on the New Haven plain, and con- 

 sequently it is situated just where such an effect from the expansion 

 and pushing action of this part of the glacier would be produced. 

 Now to the vjest of the region of these scratches within three-fourths 

 of a mile, there is the rather broad valley of Cove river, which ex- 

 tends southward and reaches the Sound two and a half miles below ; 

 it is pai'allel nearly with the New Haven region, but has a much 

 steeper slope, the descent to the salt water flats being at the average 

 rate of about 125 feet in a mile. This slope of the valley would have 

 given the ice that tille 1 it (the under portion of the glacier, if not the 

 whole above) relatively a rapid movement. The overflow from the 

 New Haven depression caused by the conditions stated would there- 

 fore have naturally taken a course into this valley. The direction of 

 the scratches, S. 33° W. accords well with this view. 



Making of Lake-basins. — The lifting of the lower or abrading sur- 

 face of the glacier by hard rocks, which has been shown to have re- 

 sulted in the production of the norfch-and-south ridges, and which ap- 



