Annual Meeting.] 
16 
[May 7, 
but being there upon the upthrow side of the great marginal faults, 
they have been completly swept away by erosion. 
Quiet erosion, accomplished chiefly by chemical agencies, and 
finally reducing this region nearly if not quite to a base-level plain, 
is then, so far as we know, the whole story of the Boston Basin 
during the vast interval of time separating this period of great dis- 
turbance from the marked elevation of the continent which gradu- 
ally ushered in the glacial epoch. Mechanical erosion was then in 
the ascendant ; but it does not appear probable that the ice-sheet 
modified the topography so much by the erosion of the hard rocks 
as by the accumulations of drift which it left upon the surface in 
the forms of drumlins, sand plains, clay beds and kames. The 
high land north of the basin was, on account of- its elevation, an 
area, mainly, of glacial erosion ; while the detritus scraped from it 
into the basin made this mainly an area of deposition, as the drum- 
lins testify. During the period of maximum glaciation the ice-sheet 
extended far to the south and east of Boston, to Cape Cod and 
Nantucket and possibly much farther. A subsidence of the land 
finally brought back a milder climate and the margin of the ice-sheet 
retreated northward. It did not halt long enough in this vicinity 
to form any distinct terminal moraines ; and the ground moraine 
or till was left chiefly in the form of drumlins. Partly by subgla- 
cial streams, but mainly by the great torrents and the temporary 
lakes resulting from the final melting of the ice, the drift was very 
largely modified, that is washed, assorted and stratified in the sand 
plains, gravel ridges or kames and clay beds, which now so gener- 
ally occupy or fill the old valleys and form deltas where the valleys 
emerge from the high land bordering the basin. In the distribution 
of the drift, and especially of the modified drift, we have an ade- 
quate explanation of: (1) the ponds, lakes and swamps of this 
region, as well as the dry kettles or depressions of the sand plains ; 
and (2) the diverted and circuitous drainage and the resulting 
waterfalls. The elevation of the land during the advent of the ice 
age enabled the streams to deepen their channels to such an ex- 
tent that although subsequent subsidence has not, apparently, re- 
duced the land to its preglacial level, the rocky beds of the larger 
streams are, near their mouths, from 100 to 200 feet below the level 
of the sea. The fiord character of the shores in high latitudes is 
usually to be explained in this way. Since the close of the glacial 
epoch the streams of the Boston Basin, with their greatly reduced 
