78 THE JOURNAL OF GEOLOGY. 
shores of the lake and tends to advance upon them. Some part 
of the strain may be relieved by the arching of the ice, but this 
could not do away with shoreward pressure. Under favorable 
conditions, the shoreward movement may be considerable. When 
the shoreward movement of the ice begins, its margin may be 
frozen to stones of greater or less size. As it crowds upon the 
shore, the stones frozen to its lower surface will grind the surface 
over which it moves. If this surface be of material capable of 
receiving striz, as limestone, it may be striated as the bowlder- 
bearing ice is shoved over it. Taken singly, the strie thus pro- 
duced may closely resemble those found in association with the 
drift. At the same time, the stones which marked the bedrock 
would themselves be worn after the fashion of the stones of the 
dritt. ~ Ihe process of ice movement here described imayebe 
repeated many times in the course of a single season. 
At the close of winter, fed by the swollen currents of streams 
flooded by melting snows, the water in the lakes rises. The 
melting ice is broken into blocks. In the flooded condition of 
the lakes, these blocks are carried above and beyond the usual > 
shore lines. Waves may drive them still further. Where the 
slopes of the shore are gentle, and the waves high, the ice may 
be driven many feet above the high-water shore line, and many 
rods beyond it. In this movement, those blocks of ice which 
were originally frozen to the shore sometimes carry stony mate- 
rials in their bases, and, thus armed, wear the surface over which 
they are shoved. The ice is sometimes broken up and driven 
on shore during the winter as well as in the spring. Once on 
the shore, the ice blocks may be frozen to the land surface, to be 
again moved inland by the waves and crowding ice blocks of 
succeeding storms and floods. While it seems altogether pos- 
sible that striae may be produced both on the bedrock and on 
loose stones by the shoreward movement of lake ice, good exam- 
ples of strize demonstrably thus produced, and comparable to 
those affecting the surface of the rock beneath the drift, are not 
known. The vertical range of such striae would be very limited 
at best, unless the level of the lake changed. 
