SEDIMENTATION IN MACKENZIE RIVER BASIN 355 
tion of this phenomenon as observed by him at the mouth of the 
Liard: 
Huge cakes of ice under the enormous pressure were constantly raising 
themselves on end and falling, and the whole mass, urged forward by the 
terrible energy of the piled-up waters behind, was battering a way across the 
Mackenzie. The ice of the latter, fully five feet thick and firm and solid as in 
midwinter, was cut through like cardboard, and in a few moments two lanes 
were formed across its entire width, while a third was open for some distance 
below, before the force of the rush was exhausted and the movement ceased. 
In the afternoon the crashing of trees in a channel behind the island, con- 
cealed from view by the intervening forest, was distinctly heard and showed that 
a temporary vent had been found there, and in front of the fort intermittent 
fountains played at intervals from holes and crevices in the ice. At midnight 
Fic. 1o.—A remnant of a mass of river-shoved ice and the bowlder pavement which 
such ice levels and smoothes, Mackenzie River 15 miles above Fort Wrigley. 
the dam at the mouth of the Liard gave way and the massive crystal structure 
was hurled by the liquid energy behind it against the firm ice in front with 
such force that the whole sheet, for some miles below the fort, was crushed 
into fragments by the impetuosity of the assault. 
At the Ramparts ice jams are reported to have sometimes raised 
the river nearly too feet. . 
About 15 miles above Fort Wrigley, I observed the remnants 
of one of these ice jams—an accumulation of great ice blocks 
which had remained unmelted as late as July 22 (Fig. 10). At 
this date the principal mass of ice blocks had a thickness of 25 feet. 
They were covered with a thin veneer of dark mud, and hundreds of 
specimens of Sphaerium vermontanum were scattered over their 
surface. 
