352 E, M. KINDLE 
absent. The channels of these streams are cut in lake and river 
silts which contain no bowlders, while the channel of the upper 
Mackenzie is cut for the most part in glacial till containing an 
abundance of bowlders, which through the grinding and sliding of 
the ice during the spring break-up are pressed deeply into the clay 
and built into pavements. 
The contrast between the bowlder- caved banks of the Mackenzie 
and the bowlder-free banks of the rivers just mentioned is related 
to another feature in which the Mackenzie contrasts sharply with 
these rivers in the bowlder-free silts. The latter meander widely, 
while the former pursues a fairly direct course, its bends showing 
none of the characteristics of typical meandering streams. 
Fic. 6.—Bowlder pavement on island opposite Old Fort Wrigley, Mackensie 
River. 
The Grand Detour on the lower Slave is an example of the 
meanders of this stream and the lower Peace. At the Grand 
Detour the Slave swings abruptly to the westward in a great loop 
of about 20 miles. The distance across the base of this meander is 
only one mile. The relatively direct course of the Mackenzie as 
compared with the meandering lower Slave and Peace rivers can 
be explained, in part at least, by the protection which the bowlder 
pavements afford against lateral cutting. These pavements furnish 
protection against erosion of the banks as effective as artificial 
riprap, and thus prevent the excessive cutting at the bends which 
in many streams leads to the formation of loops and oxbows. 
