6 
area of rocky woodland along the boundary between Cohasset 
and Hingham. The moderate disparity of these figures does 
not contradict the previous statement concerning the general 
uniformity of the rocky elevations; for the main point is that 
while there are hundreds of rocky hills and ridges or ledges rising 
10 to 50 feet or more above the ground between them, there are 
none rising to commanding heights, that is, there are no rock 
elevations decidedly overlooking the surrounding country, as 
do the drift hills or drumlins. It is of course this absence of 
true rock hills that proves the ancient peneplain ; for the present 
interstream surfaces, where not encumbered by drift, are essen- 
tially ledgy plateaus. The absence of crags and pinacles of rock 
is casily accounted for by the severe glaciation which this region 
has suffered ;. but we cannot thus explain the fact that there are 
no dominant rock hills of rounded or glaciated outline, like 
the Blue Hills. That glacial erosion was not equal to the com- 
plete obliteration of such reliefs is proved by the survival, even 
when quite isolated, of the numerous hills and ledges which do 
not rise above the surface of the dissected peneplain. Briefly 
stated, then, the rock hills of this district have all been carved out 
of the preglacial peneplain ; and the general equality of relief thus 
determined must continue until an increased elevation of the 
land, giving renewed energy to the ordinary agents of erosion, 
permits a more general and unequal effacement of the original 
interstream surfaces. 
That the depressions or valleys which now interrupt the репе- 
plain are not due chiefly to glacial, but to preglacial aqueous, 
erosion is sufficiently obvious, in many cases, from their direc- 
tions; and it is also seen in the fact that the rock hills and 
ledges of Nantasket and Cohasset, through the influence of the 
faults, dikes and joints by which they are bounded, are, in the 
main, approximately rectangular in plan, with, frequently, very 
steep or even precipitous slopes, and showing only secondarily, 
and not as their primary form, the roche moutonée outline due 
to glaciation. Hence, cliffs, sometimes rising abruptly from 
the water or the salt marshes, and straight, narrow defiles are 
