144 
by which they were gathered up and incorporated in the till; 
and he has shown that some of the species, at least, like the 
round clam, are now found only in more southern waters, 
indicating that the sea in this vicinity just before the glacial 
epoch was warmer than at the present time. The shells are 
also of special interest as proving not only that Boston Harbor 
was in existence in preglacial times, for otherwise we could not 
know but that this basin was above sea-level before as well as 
during its occupation by the ice-sheet ; but they also show that 
the harbor has now approximately its preglacial outlines and 
hence that the sea has regained, on this coast, very nearly its 
preglacial level. 
The peneplain, as already stated, and especially when we 
make an allowance for the glacial erosion upon its surface, 
proves that this coast is now one hundred feet or more above 
its preglacial level. jut this greater elevation is off-set, so 
far as the harbor is concerned, by the excessive erosion which, 
being a valley in comparatively soft rocks, it suffered in glacial 
and especially in immediately preglacial times. 
Although the fragments of shells are unquestionably the most 
interesting feature of the till орвегуе( іп Nantasket and Cohasset, 
other components of more normal character—the ordinary 
stones and bowlders—demand brief mention. No particularly 
striking instances of glacial transportation have been noted. 
The till of the Nantasket Peninsula, as may be most readily 
seen in the marine sections of the drumlins, consists very 
largely of fragments of slate, which is undoubtedly the under- 
lying rock not only of this peninsula but of the main part of 
the harbor to the northward. Bowlders of conglomerate are 
not wholly wanting north of Atlantic Hill, but they are so few 
and small as to prove that it сап occur only very sparingly, if 
at all, in the harbor area or beneath the extensive drift-deposits 
north of the harbor. In fact, pretty much all of the conglom- 
erate in the till north of the Nantasket ledges might be referred 
with considerable probability to the known exposures of that 
4| 
