GLACIAL DRIFT. 289 
rock occur near their base, and evidently form a pedestal capped by a 
rounded mass of till fifty to one hundred feet in depth. 
These hills of glacial drift may be recognized by their smoothed sur- 
faces, overspread, indeed, with the large boulders which are common in 
the upper till, but moulded in gracefully curved outlines unbroken by 
jutting ledges, which give to all our other hills a more or less angular 
and abruptly undulating surface. Our elevations of rock are commonly 
in irregular groups or ridges, with outlying spurs, and, except in the 
south-east part of the state, they are far more massive and prominent 
than the lenticular hills. These accumulations of till are further distin- 
guished by their fertile soil, well adapted for pasturage or cultivation, 
which frequently makes them the most valuable land in the districts 
where they occur. On this account, they have almost invariably been 
cleared, while the more rugged, ledgy hills remain covered with woods, 
During exploration for mapping the lenticular hills, notes were also 
taken of sections in the glacial drift, shown by excavations for building 
and repairing roads, or for wells. These observations are presented in 
the following table, which shows depths of upper till varying from one 
to seventeen feet, resting, in all but five instances, directly upon the lower 
till, their separation being a definite line. It will be seen that thin lay- 
ers of sand are occasionally found in both these deposits, appearing to 
be most frequent in the lower till, where they are sometimes inclined or 
nearly vertical. In most cases where thick beds of gravel, sand, or clay 
occur in the glacial drift, their position is between the upper and lower 
till. A few examples appear in the annexed table, and others are de- 
scribed on pages 108, 131, 137, 159, and 163 of this volume. It will be 
seen, also, from this table, that thick stratified deposits are sometimes 
found in or beneath the lower till. Wells in this compact boulder-clay, 
which is usually impervious to water, often encounter springs issuing 
from these beds or from thin seams or layers of sand, which therefore 
must extend a considerable distance. In most of the sections noted, the 
base of the boulder-clay was not reached. Nearly half of these sections 
are upon lenticular hills or slopes, which are thus shown to consist of a 
thin stratum of upper till at the surface, while the larger central portion 
is a massive accumulation of ground-moraine or lower till. 
