TRANSPORTATION OF DEBRIS BY ICEBERGS 77 
inclusions within the particular section of the clay that was 
examined. Possibly adjacent areas might have much greater 
masses, but the probability (as indicated by inspection of other 
near-by excavations) is that the average number would be lower, 
though occasional very large bowlders might make the mass as 
great, or greater, per average cubic foot of clay. It should be 
noted that away from the spots where the clustered bowlders 
occurred the clay was almost absolutely free from sand or grit. 
Two distinctive charactersitics are immediately apparent on 
inspection of the material: (1) Many of the specimens (26 per cent 
of their number) show signs of glacial grinding, have striations, are 
“soled,” or rudely faceted. In Fig. 2 a number of these are indi- 
cated by the letter A. (2) Much of the material (55 per cent by 
number, 67 per cent by weight) is from quite distant outcrops, 
that is, of rock material not available at the surface for a distance 
of 50 miles or more to the north even if the bottom of the lake is 
taken into consideration. Twenty-two of the 125 bowlders come 
from so distant a source as the Adirondacks or perhaps from 
Canada. ‘There are 3 granites, 1 syenite, 12 gneissic, and 6 schis- 
tose specimens irrespective of size. Very prominent in the foreign 
material are Medina sandstones and Potsdam sandstones and con- 
glomerates; of these three varieties there are 46 specimens. With 
one exception the large fragments, in general, are of the more 
resistant rock kinds from distant sources. The exception is a 
notably large piece of local sandstone derivable from bedrock out- 
crops extending from the area of the clay deposit to 10 or more miles 
to the north. This large local specimen is very conspicuously 
ground off on one side. 
Source of the bowlders.—The only feasible explanation of the 
occurrence of these large rock fragments interbedded with the fine 
clay is that they are iceberg droppings. Icebergs, calved from the 
relatively distant glacier front, floated over the areas on which the 
clay was depositing and, on sufficient melting, dropped their rock 
load into the fine clay sediment, later deposits of which buried them 
completely. The bergs do not seem to have been grounded, for 
there is no apparent disturbance of the clay layering, though the 
clay material is so fine that in its oozy, under-water condition it 
