SEDIMENTATION IN MACKENZIE RIVER BASIN | 349 
Detour cut banks of this yellowish sand and gravel 40 feet high are 
exposed. 
The outermost island at the west end of the Grand Detour 
illustrates the lateral migration of islands which is sometimes 
observed. The western border is a recent sandbar formed during 
the present year. Inside this border is a crescent of willows one 
year old, while the third zone is a very narrow belt of willows nearly 
mature. The eastern and oldest part of the island is covered with 
poplars. This east side, however, is a cut bank and is being 
removed apparently at the same rate at which the western shore 
is being built up. On the north half of the Grand Detour the 
cutting is all on the outside bank, but this is not true of the south 
half, where islands in the channel deflect the current to the inside 
of the bow, where it now appears to be doing the maximum amount 
of cutting in beds of buff sand toward the upper end of the bow. 
Near Fort Smith the sandbanks of the river reach their maxi- 
mum height. At the steamer landing at Fort Smith the top of 
the bluff is 125 feet above the level of the late-summer stage of the 
water. The base of the section is a bed of thinly laminated gray 
clay 6 feet thick with numerous concretions which have the appear- 
ance of being built up of a series of disks each smaller than the pre- 
ceding. Above this laminated clay the beds appear to be composed 
entirely of sand. 
For 16 miles above Fort Smith the Slave River flows over a 
series of granite ledges and between numerous low, rounded, granite 
islands which interrupt navigation. Above this series of rapids 
most of the islands are composed either partially or exclusively 
of granite or limestone instead of silts as in the lower Slave. Above 
the Stony Islands, where the granite islands rise to a maximum of 
- more than too feet above the river, numerous low, granite bosses 
also rise at intervals a few feet above the water surface, in many 
cases only a foot or two above low-water stage. Some of them are 
only to to 20 feet in length. Groups of half a dozen or more of 
these small granite islands are seen in a distance of 200 yards up 
and down stream. ‘These granite knobs are the nuclei of the many 
long alluvial islands seen in this part of the river. Frequently a 
pile of driftwood caps the top of one of the low granite knobs 
