SEDIMENTATION IN MACKENZIE RIVER BASIN 353 
The plowing and gouging action of ice is nearly everywhere in 
evidence along the Mackenzie. At the head of the river, in the 
shallow eastern channel, one can see through the clear water numer- 
ous deep grooves made by ice cakes or bowlders pushed by ice in 
the bowlder clay of the bottom. In the gravel or silts of low islands 
the broad grooves made by ice-shoved bowlders or ice blocks can 
often be traced for a considerable distance (Fig. 7). In some local- 
ities the plowing and scooping action of the ice carries large quan- 
tities of mud from the bottom to the banks of the river (Fig. 8). 
Fic. 7.—Trail left by ice-shoved bowlder or ice cake. Note the cratic course 
unlike that left by drifted tree roots, Mackenzie River. 
The upstream ends of some of the low islands are built up in this 
way several feet higher than the rest of the island. In such cases 
the ice is likely to build a clay dike across the head of the island at 
right angles to the course of the river terminating at the top in a 
sharp ridge. The front of such a dike is frequently bowlder-paved 
and thus becomes almost as resistant to river erosion as a hard rock 
cliff (Fig. 9). 
A feature of the ice work along the banks of the Mackenzie is 
the distribution of great numbers of a small bivalve, Sphaerium 
vermontanum, over the higher levels of the bank, much higher than 
the ordinary stages of the river in summer could carry them. I 
