Upham.] 4: f^ov. i6, 
The till foriiiing the drumlins invariably exhibits the chanictef- 
istic features of subglacial till or ground moraine, excepting its 
superficial portion which was englacial and superglacial when the 
ice-sheet melted away.^ Many boulders, which are commonly 
strewn plentifully on the surface of tlie drumlins, appear to have 
fallen upon them from the receding ice-sheet, together with a 
stratum of the till that varies usually from one foot to a few feet 
in depth near Boston, but is sometimes from 10 to 15 feet thick 
on the tops and ilanks of drumlins in New Hampshire. This 
upper part of the till is comjmratively soft and easy to dig, Avhile 
its main portion below is so cojupact that it must be jiicked and 
is far more expensive in excavating. The probable cause of the 
contrast in hardness was the pressure of the ice-sheet upon the 
lower till during its accumulation, while the upper till was con- 
tained in the ice and dropped loosely at its melting. Occasion- 
ally a thin layer of sand or gravel lies between the englacial and 
subglacial till, as on the top of the drumlin named Convent Hill 
in Somerville, wdiere the ui)per 3 feet of the till, forming the sur- 
face, are underlain along an observed distance of several rods by a 
bed of sand from 1 to 3 feet thick. 
Subglacial till is further distinguished from that which was 
finally dropped from the departing ice by its smaller rock frag- 
ments, Avhich are mostly less than two feet in diameter, and some- 
times consist only of pebbles, cobbles, and small boulders not 
exceeding half this size, though often it also contains large bould- 
ers ; by the glacially worn faces of many of these stones, wliich 
are frequently marked with striae ; and by traces of a peculiarly 
bedded structure, in parallelism with the surface. The last 
feature is especially characteristic of the till in drumlins, except- 
ing its upper few feet. Although boulders, gravel, sand, and 
clay are thoi-oughly commingled, the deposit is imperfectly lami- 
nated and tends to separate and crumble into thin flakes. This is 
frequently noticeable in a fresh excavation, but is most distinctly 
seen after a few weeks of exposure. It shows that the ice in its 
passage added new material to the surface of the ground moraine, 
which generally lay undisturbed beneath. 
To a depth that commonly \aries from 5 to 10 feet on low or 
' Inequality of distribution of the englacial drift. Bulletin G. S. A., vol. 3, 1891, 
p. 134-148. Criteria of englacial and subglacial drift. Auier. geol., vol. 8, Dec. 1891, 
p. 376-385. 
