GLACIAL EROSION. 37 
[Bird Mountain in the northern part of the Taconic Range have trav- 
Iled from 47 to 58 miles S. 5° W., S. 12° W., S. 25° W.,« and Mr. 
[toloffit recently found one on the top of Grass Mountain in Arlington 
Ibout 38 miles S. 10° W. from its source. The presence on the 
■Taconic Range in northwest Massachusetts of many quartzite 
|)owlders from the Green Mountain Range confirms evidence from 
lome of the stria? that there was also a southerly motion to the ice 
Iheet. PL VIII, B, shows a broad glacial furrow with a S. 10° E. 
lourse in Pittsford, Vt, a half mile west of Otter Creek ; and PL VIII, 
1, shows a quartzite bowlder lying upon marble at the eastern foot 
l)f the Taconic Range in the same town, which originated in the 
preen Mountain Range, and must therefore have traveled in a south- 
erly direction. Portions of the schist masses show rounding north 
knd northwest shoulders, and there are also southerly facing east- 
ivest dill's, as Bij*d Mountain and Rounds Rock on Grey lock. 
The beautifully preserved glacial stria? along Lake Champlain and 
In the New York and Vermont slate belt show what must have once 
characterized the entire region, and what, but for the mantle of drift 
and vegetation, would still be visible. 
Glacial erosion, however, appears to have been of less importance 
than pre-Glacial stream erosion, though perhaps more of a factor 
than was post-Glacial. Probably the ice sheet, aside from its gen- 
eral erosive effect, aided materially in the sculpturing of those valleys, 
fca vines, cuts, ridges, and spurs which have northwest-southeast or 
north-south axes; but in view of the vast amount of time which 
lelapsed between the Green Mountain elevation and the advent of the 
glacier compared with eA T en the most liberal estimates for that which 
covered the period of glaciation, it is likely that the principal factor 
in the sculpture of the region was pre-Glacial stream erosion. 
This is the mo're certain, as the preservation of the striae on the 
schist and quartzite surfaces away from the ravines is so perfect 
that it is evident that at such points post-Glacial erosion was practi- 
cally nothing, while the protection afforded to the limestone valleys 
by Glacial and post-Glacial deposits has effectually preserved large 
areas of them to this day. And, although, as has been pointed out, 
a considerable part of the work of the post-Glacial streams has con- 
sisted in undoing the work of the Glacial epoch, the time these 
streams have had to work is relatively so short that their effect can 
not be very considerable. 
One effect of glacial erosion which should not be overlooked is 
" rock shattering.' 1 b Though doubtless supplemented later by frost, 
"A Study of Bird Mountain, cit., pp. 20, 21. On page 20, third line from bottom, 
" west " was erroneously written for east. 
b Geikie, Tames, Earth Sculpture, or the Origin of Land Forms, London, 1898 ; Chapter 
X, Land Forms modified by Glacial Action, pp. 170-185. 
