TRANSPORTATION OF DEBRIS BY ICEBERGS 79 
rocky débris and may deposit this at considerable distances from the 
calving end of the glacier. The fact that so large a percentage of 
the bowlders found in the clay deposit showed evidence of wear 
indicates that the material was largely derived from the bottom ice 
of the glacier. In fact, in collecting specimens of glacially striated 
pebbles, it is the common practice in this locality to resort to a 
deposit of glacial-lake clay for the material, as a large proportion of 
the bowlders found in such deposits exhibited these markings 
remarkably well preserved. Assuming that each of the bowlder 
Fic. 4.—‘‘ Nests” due to rocking of stranded icebergs. Pebbles and bowlders 
on the surface of the sand and in the pits were included in the ice and deposited on its 
partial or complete melting. West side of Yakutat Bay, Alaska. 
“‘nockets”’ in the clay is the result of the melting down of a single 
berg (and this seems very clearly to have been true of the material 
included in the smaller pile of Fig. 2 at least), it follows that the 
bottom ice must have been quite thickly shod with rock fragments. 
In other words, the material in transport at the bottom of the ice 
in any one cross-section must have been of considerable mass, and 
this material could have been acquired only by actual ice erosion 
of the bedrock over which it passed. Such being the case, the 
striking absence of local rock material in these iceberg deposits 
acquires a particular significance. It would appear that the local 
