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mat of peat-forming vegetation then covers the surface, and the 
marsh is complete. In other parts of the Boston Basin, how- 
ever, and generally wherever excavations have been made in 
the salt marshes, the evidence is conelusive that a slow sub- 
sidence has permitted the formation of several successive beds of 
peat, separated by beds of clay. 
Although, as the beaches, marshes, and marine cliffs so 
nt 
level for a very long time, the rocks of our coast are so intensely 
plainly indicate, the sea has undoübtedly maintained its prese 
hard and resistant that there are very few points where they 
show any appreciable amount of marine erosion. All along 
the Nantasket and Cohasset shore it is perfectly obvious that, 
save where the rocks are very finely jointed, or a dike has 
yielded to the ceaseless pounding of the waves, the ledges are 
still essentially intact, showing still the roche moutonnée forms 
impressed upon them by the ice-sheet, even the glacial striae 
being, in some cases, well preserved for several feet below the 
high-tide level. 
Above the present level of the shore evidences of marine 
erosion on the hard rocks are, so far as I have observed, wholly 
wanting. If the sea has stood at higher levels in post-glacial 
times, the evidence must be sought in the erosion, not of the 
hard rocks — granite, felsite, melaphyr, etc., but of the 
non-lithified or drift deposits. Fortunately, the drumlins, 
which are such a prominent feature of Boston Harbor, present 
in their firm but yielding material and regular outlines condi- 
tions exceedingly favorable for making and preserving a record 
of even a very brief occupation of a higher level by the sea. 
Any one who notes the extensive erosion of the drumlins by 
the sea at its present level and the comparative stability of the 
erosion scarps, cannot doubt that if similar features — terraces 
and scarps — had ever been developed on the drumlins at higher 
levels, some indications of them would still be traceable. A 
general study of the drumlins of the Boston Basin has satisfied 
me that undoubted horizontal erosion-marks are a common 
