400 FT. //. Hobhs — Instance of the Action of the 



agencies of subserial and of ice erosion, we may believe that it 

 would have presented an irregular surface not unlike that of a 

 mosaic from which a local area of the back had become dis- 

 placed and the overlying blocks allowed to slide down by 

 small amounts while still restrained by their friction upon their 

 neighbors. The effect of subserial erosion has been to etch 

 out the marginal areas of soft sandstone and leave the basalt 

 prisms of the central area in strong relief like the image of a 

 cameo. The basalt itself discloses no marks of the subserial 

 erosion, for the reasons : first, that it is intensely resistant ; and, 

 second, that its area is so small (six miles in length by two 

 miles in greatest breadth) that no streams of any power have 

 been developed upon it. It is not, however, to be assumed 

 that no considerable degradational action has occurred within 

 the area of the basalt masses of the valley, for the three upper 

 members of the Newark series found in the Connecticut Val- 

 ley area, which begins less than a score of miles to the east, are 

 missing from the Pomperaug Yalley series, and were doubtless 

 removed by subserial erosion, while large thicknesses of the 

 surrounding schists were being carried away. 



Fig. 1. Schematic profile of basalt ridges of the Pomperaug Yalley. 

 Black, basalt ; black spotted with white, amvgdaloidal basalt ; white, shale : 

 white with black circles, conglomerate (where stippled, baked zone of con- 

 tact) ; stippled area, drift and alluvium. 



The work of the ice within the valley is revealed in the pro- 

 files of the basalt ridges. These ridges have generally fault 

 scarps on their western and northern sides (which face in the 

 direction from which the ice moved) and gentle slopes to the 

 eastward and southward, conforming to the dip of the beds and 

 flows of which they are composed. In these general outlines 

 the action of the ice is not disclosed, but the caps of all the 

 ridges seem to have been removed by an appreciable fraction 

 of their height. This is brought out in the schematic figure 1 

 and in the author's report above cited. ^ That this degrada- 

 tional action by the ice is localized largely at the crests of 

 ridges is also shown by the texture of the rock found at the 

 crests when compared with that upon the fianks of the southern 

 ridges. Dense and massive at the crest in correspondence 

 with the lower beds of the flow, it is amvgdaloidal and vesicu- 

 lar upon the southeastern flanks, where it doubtless represents 

 upper layers of the flow. 



* Loc. cit. , pi. V, A. 



