THE SURFACE GEOLOGY OF NANTASKET AND 
COHASSET. 
As a starting point for the surface geology of this district, 
including both its glacial and post-glacial history, we must 
aceept the preglacial peneplain, the evidence for which has 
been presented in sufficient detail in the general description of 
the topography (pages 5 to 7). The Mesozoic and Cenozoic 
eras are, apparently, unrecorded here, except in the erosion of 
the hard rocks. But, while it is to these long geological 
cycles that we must refer the removal of a great thickness of 
stratified rocks from this area, and the wearing down to the 
base-level of the hard granitic rocks, lavas, and conglomerates, 
the rugged contours presented today by the preglacial pene- 
plain undoubtedly date chiefly from the marked clevation of 
the land which, it is probable, ushered in the great ice-age. 
On account of the intense hardness of the rocks, these deeply 
incised lines were only partially effaced by the powerful abra- 
sive action of the ice-sheet ; but, as we have already seen, they 
are to a large extent obscured or concealed by the non-lithified 
deposits of glacial and post-glacial origin. 
The principal events recorded in the surface geology, since 
the development of the preglacial peneplain, are: (1) A con- 
siderable elevation of the land at the beginning of the glacial 
epoch. The elevation was, probably, a principal cause of the 
subsequent glaciation, and is proved by the deeply eroded 
character of the peneplain. (2) Long-continued glaciation, 
during which the divided peneplain was strongly eroded and 
the ground-moraine or bowlder-clay accumulated irregularly 
upon it, chiefly, at the last, in the form of drumlins. (8) 
A marked depression of the land, accompanied by the final 
melting and retreat of the ice-sheet, and the accumulation, 
either in the sea or in temporary lakes and streams, of the ex- 
