The Boston Basin contains nearly 200 drumlins, and for many of these we now have data about the formations upon which they rest. The relation of these drumlins to the other glacial deposits and to bedrock gives information about the age of the various formations and evidence as to the number of glaciations that have affected this region.
The topography of the Boston Basin is now much less rugged than it was just before the last glaciation, owing largely to the fact that the deep valleys in the rock surface are now partly filled with glacial deposits. Despite the lowland character of the basin, the differences of elevation of bedrock are more than 400 feet, the higher parts being generally conglomerate and the lower parts slate. Several deep valleys in the rock surface, with their bottoms about 200 feet below sea level, cross the area.1 The lower part of . . .