160 
consequent upon, a marked depression of the land. This 
depression was, apparently, more than sufficient to restore the 
preglacial relations of the land and sea; or at least evidence is 
not wanting that the shore-line was, for a brief period, somewhat 
above its present level. The re-elevation of the land here 
referred to amounted to 520 feet in the vicinity of Montreal, 
225 feet on the coast of Maine, but only to 20 or possibly 30 
feet in the Boston Basin; and the evidences for both the eleva- 
tion and the slow subsidenee which has since been in progress 
are much less perfectly developed in the Nantasket area than in 
other parts of the Dasin. 
Partly by subglacial and superglacial streams, but mainly 
by the great torrents and the temporary lakes resulting from the 
final melting of the ice-sheet, the till or bowlder clay was very 
largely modified, — that is, washed, assorted, and stratified in 
the sand plains or deltas, gravel ridges or eskers and kames, and 
clay beds. The explanation of modified drift in general requires 
a constant blending or intermingling of glacial, fluvial, and 
lacustrine conditions ; and probably, also, in the Boston Basin, 
for the clay beds if not the lower deposits of sand, we may pos- 
tulate marine conditions. 
Among the various forms of modified drift the eskers or 
gravel ridges appear usually to be the oldest; and being the 
product, chiefly, of superglacial and subglacial streams, they 
evidently record the actual presence of the ice-sheet as a fairly 
continuous body of ice and not as the detached masses of ice 
indieated by the kettles and other features of the sand plains. 
If any important or esker-forming glacial streams crossed the 
Nantasket and Cohasset area, their accumulations have been 
effaced by subsequent erosion or deposition ; for no distinct 
eskers have been observed. The sand plains, also, are rather 
scantily and imperfectly developed. 
Sand plains, in general, we must regard in part as repre- 
senting the flood-plains of great rivers flowing from the melting 
ice-sheet and in part as deltas formed by these rivers in temporary 
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