SEDIMENTATION IN MACKENZIE RIVER BASIN 351 
feet, when willows become common. Ten miles above the river 
mouth the banks rise to 5 or 6 feet, and large willows and alders 
are common, but no spruce trees or other evergreens are seen. The 
banks of alluvium increase gradually in height till at the old Fort 
Forks at the head of the delta they rise 10 or 12 feet above low water. 
Numerous good examples occur in the lower part of the Atha- 
basca, in the cut banks, of tree stumps which have been buried 
where they grew under from 2 to 6 feet of alluvium by the shifting 
of the course of the river. They are illustrated by one of E. T. 
Seaton’s figures.t. O’Neill? has described similar examples of allu- 
vium-buried forest beds in the delta of the Mackenzie where the 
buried stumps are much larger in girth than any other trees now 
growing in the delta. The testimony of the land surveyors who 
have run their lines across an extensive region between the lower 
Peace and Athabasca rivers indicates that with the exception of 
two or three localities bedrock outcrops are wanting over a vast 
area between these two rivers, an area which is doubtless underlain 
throughout by fluvial and lacustrine deposits. 
About 15 miles above the Old Fort forks the river cuts into a 
sand bluff 75 or 80 feet high. The heavy load of sand acquired by 
the river at this and other points higher up results in extensive 
sandbars which are spread over the middle of the river and interfere 
with steamer navigation at low water. About 3 miles below Point 
Brule the extensive alluvial and lacustrine deposits are terminated 
on the east side of the river by land rising 200 feet or more above it. 
CONTRASTING FEATURES OF THE MACKENZIE RIVER 
The bowlder pavements are among the most striking features 
of the Mackenzie River. These marvelous pavements, resembling 
cobblestone roadways, often stretch along both banks of the great 
river without interruption for miles (Fig. 6). They frequently 
extend up the concave banks from below low water to a height of 
25 feet or more above it. The shores of many of the islands as well 
as the banks of the river are paved with bowlders. On the Slave, 
lower Peace, and lower Athabasca rivers the pavements are entirely 
' The Arctic Prairies, p. 197. Charles Scribner’s Sons (1911). 
2 Canada Geol. Surv., Sum. Rept. (1915), p- 230. 
