BOWLDER-PAVEMENT AT WILSON, N. Y. Mh) 
of striation were observed in the cases of ten bowlders in situ, 
and found to be substantially parallel. Nine ranged between Si 
45° W. and S. 50° W., and the tenth was S.55° W. The accom- 
panying illustrations (Pl. XIV, Figs. 1, 2, and 3) show the upper 
faces of two of the bowlders and the side of one. It seems clear, 
that after the deposition of the lower till it was over-ridden by a 
glacier moving toward the southwest. This is the general direc- 
tion of striation on the bed rock of the region, but no observa- 
tion was made in the immediate vicinity of Wilson. 
All the bowlders strongly glaciated on their upper surfaces 
were found to have one diameter less than the others, and to lie 
in such position that the least diameter was vertical. So far as 
observed, bowlders without pronounced differences in their sev- 
eral diameters were not more strongly glaciated on the upper 
side than on other sides, although lying at the same level as the 
others. 
To account for these peculiarities, as well as for the accumu- 
lation of bowlders at the summit of the lower till, the following 
explanation is offered: The glacier which deposited the upper 
till slowly eroded the lower till as it moved over it. When this 
erosion began to uncover a bowlder, differential pressures 
resulted. In Fig. 1 the horizontal line represents the upper sur- 
6 a@ 6 
q 
Fic. 1. Diagram illustrating theory of bowlder arrangement. 
face of the till undergoing erosion, and 1 the discovered bowlder, 
projecting above the till at z. If the glacier were stationary it 
would mold itself plastically about this protuberance and press 
equally on a and 04, but as it is in motion and has great viscos- 
ity, the pressure is greater at @ than at 66, and the differential 
pressure is not merely momentary, but continuous. It has the 
effect of a simple pressure on the bowlder at a, forcing it down 
into its plastic matrix, and as the erosion of the till continues, 
