80 O. D. VON ENGELN 
rock material, being of comparatively slight resistance to grinding 
(mostly shales and thin bedded sandstones, commonly argillaceous), 
is reduced to rock flour almost immediately, while the resistant 
quartzitic and igneous material from distant sources survives. An 
alternative interpretation is that the local material of little resistant 
nature is not much subject to plucking, is eroded only by grinding, 
hence yields few sizable fragments. The same conclusion is sug- 
gested by the fact that the large-sized surface and near-surface 
erratics of local origin in the glacial deposits of the area about 
Fic. 5.—Stranded icebergs, showing some with included débris. West side of 
Yakutat Bay, Alaska. 
Ithaca, New York, are in very high percentage fragments of the 
Tully limestone, which outcrops in relatively massive layers, 2 
to 10 feet thick, about 4 miles to the north of Ithaca and at other 
points more remote. It is also of interest to note that these large 
Tully erratics are in almost every case very conspicuously smoothed 
by ice wear and have usually the appearance of having lost a con- 
siderable part of their mass by such grinding, though it is of course 
commonly rather difficult to estimate how large the plucked block 
was originally. That the single large, local sandstone fragment 
found in the clay deposit was also very notably ground off is evi- 
