350 E, M. KINDLE 
Good examples of rock basins which represent apparently one 
phase of the work done by river ice occur on the limestone island 
opposite old Fort Wrigley. A considerable area of limestone beds 
lying approximately horizontal on the northeastern side of the 
island is covered by the river during the spring break-up, but 
exposed through the summer. On this area a group of four rock 
basins has been developed in the limestone. The rim of the largest 
of these rises from 5 to 10 feet above the bottom. This basin has 
a maximum length of 65 feet and a width of 30 feet (Fig. 11). 
Another of these basins has a diameter of 10 feet and a depth of 5 
Fic. 11.—A rock basin in limestone between low and high water, Old Fort Wrigley, 
Mackenzie River. 
feet. The basins appear to be the product of the plucking action 
of the river ice which covers them during the late winter stages of 
the river. 
LAKE FILLING 
Two large lakes, Athabasca and Great Slave, lie in the path of 
the Mackenzie-Athabasca drainage system. Great Slave Lake, 
much the larger of the two, has a length of about 290 miles from 
east to west. A recent survey of part of the north shore by A. E. 
Cameron, of the Canadian Geological Survey, changes the rank 
in size of Great Slave Lake from fifth to fourth among the great 
lakes of the continent. As pointed out by McConnell,* it seems 
originally to have had ‘‘the form of a great cross with one arm 
1 Geol. Surv. of Canada, Ann. Rept., TV, 1888-89 (1890), p. 65 D. 
