34 THE ROYAL SOCIETY OF CANADA 
While this process of deepening the valleys was in progress detrital 
material was being constantly brought into them by wash from their 
sides, and by smaller streams from the ridges between them, and, as 
the rocks from which this material was derived were gold-bearing, the 
detritus contained a small quantity of gold. Thus gold and particles 
or masses of rock were fed gently into the main streams. 
When the Yukon river had eroded its valley down to base level, 
the smaller inflowing streams were no longer obliged to continue to 
deepen their respective valleys to keep pace with it, but were able to 
cut them down to grade, and then to widen and form flood plains in 
them, thus changing the V-shaped valleys into U-shaped ones, floored 
by alluvial plains through which the rivers and brooks meandered from 
side to side. 
At the same time the rock on the sides of the valleys was weathered 
and broken up by atmospheric agencies and the decomposed rock was 
carried down into the low lands. These processes continued until a 
general mature form of topography was the result. 
Towards or at the close of the White Channel Period or Second 
Cycle of Erosion, the Glacial Epoch began and great fields of ice accu- 
mulated on the Coast Range of Mountains to the south-west and on the 
Rocky mountains to the north-east. From these mountains, glaciers 
or ice-fields flowed inwards over the Plateau country, rounding the tops 
of the hills and scouring the bottoms and sides of the valleys, but these 
ice-fields never met over the Klondike district. In the valley of the 
Yukon the glaciers from the Coast mountains extended only as far north 
as the mouth of the Nordenskiold river, while those of the Rocky Moun- 
tains reached only to the valley of Flat Creek. All the intervening 
country remained free from ice, except possibly for the presence of 
small local glaciers at the heads of some of the higher valleys. 
During this epoch of glaciation the Klondike district was again 
raised for at least a few hundred feet, and a “Third Cycle of Erosion” 
began. | 
The increase of grade seems to have been first felt by the Klondike 
river, and to have added greatly to its transporting power. In con- 
sequence it was able to bring down and spread over the bottom of its wide 
valley a mixed deposit of gravel, derived doubtless from the decomposed 
rock and older gravel banks higher up its valley. After it had carried 
down this load of gravel which had previously been rendered available for 
it by atmospheric, and perhaps also -by glacial agencies, both it and the 
Yukon river began to quickly deepen their valleys and at the same time 
the small streams, such as Bonanza and Hunker creeks which flow 
directly into these larger valleys, also began to cut gorges in the bottoms 
of the floors of their old sloping valleys. 
