3r)») The American Geologist. Decomber, i892 
]ji'vt'iTtt has traced a still laiiicr niiuihiT through Illhiois, liuli- 
aiia. and Ohio. With the iiicicascd thickness and steeper gradient 
of the outer belt of the ice-sheet while the recession of its 
boundary was slackened, wholly stoj)i)ed, or changed to a re-ad- 
vance, due mainly to v(mt al)undant snowfalls, much drift which 
had biH'U formerly exposed on the ice surface w(^uld become again 
englacial, so that a stratum of drift several feet thick might be 
enclosed in thc^ ice at an altitude increasing inward from less than 
50 feet to 500 feet or more. 
Jcf Ci(i-rciifs nuisi<!ii<i fh<' EiKjlKiiiil St nil mil in fa J hii ml ins. — Tiie 
up{)er currentof the thickened ice al)ove tlie englacial Ix'dof drift 
would move faster . than that drift, which in like manner would 
outstrip the lower cui'rent of the ice in contact with the ground. 
Close to the glacial boundary, whethei- it halted and even re-ad- 
N'anced or merely its retreat was mucli slackened but did not en- 
tirely cease, which last seems probably to liave been the case 
with the drumlin areas of Massachusetts and New Hampshire, 
the u[)i)er })art of the ice must have descended over the lower 
part. 'I'his differential and sliearing movement, as 1 think, 
gathered the stratum of englacial drift into the great lenticular 
masses or sometimes longer ridges of the drumlins, thinly under- 
lain by ice and overridden by the upper ice flowing downward to 
the boundai'v and l)ringing with it the formerly higher i)art of the 
drift stratum to be added to these growing drift accumulations. 
The courses of the glacial currents and their convergences to the 
places occupied by the drundins were apparently not determined 
so mucli l)y the topography of the underlying land as by the con- 
tour of the ice surface, which under its ablation had become 
sculptured into valleys, hills, ridges, and peaks, the isolation of 
the elevations by deep intervening hollows being doubtless most 
conspicuous near the ice-margin. 
Tn New England, on account of the absence or extreme rare- 
ness of :uiy beds of modified drift which give evidence of having 
l)een covered by a re-advance of the ice, the till of the drumlins, 
acc<n"ding to this view, appears to have been collected in its pres- 
ent masses in the basal part of the ice-sheet, while a moderate 
thickness, proltably seldom more than 50 or 100 feet, of ice la}' 
beneath. Over the drumlins a somewhat greater thickness, per- 
liaps varying from 200 to 500 or 1,000 feet, of ice formed largely 
fi'oni the snowfalls of recent years or the immediately preceding 
