40 THE ROYAL SOCIETY OF CANADA 



a Tew gravel-covered terraces were formed on the sides of these narrow 

 valleys. At last all these valleys were cut down by the streams 

 flowing in them to even grades, without falls or heavy rapids, and beds 

 of gravel were spread pretty regularly over the bottom lands. Other 

 streams, such as Goldrun Creek, accomplished but a very slight 

 amount of erosive work during this Erosion Cycle, and in some cases 

 they merely excavated shallow channels in the pre-existing beds 

 of White Channel gravels. In such cases two gravels of different ages 

 are superimposed on each other. 



All these gravel beds of both the Second and Third Cycles, 

 which now cover the bottoms and terraces of the valleys, were 

 formed in the usual way. The streams were not sufîficiently large 

 and rapid to carry away all the material fed into them from both 

 sides, and consequently they distributed it over the floors of the val- 

 leys drained by them. Thus at the end of the Third Cycle, they had 

 built up a deposit from five to thirty feet in thickness of stratified 

 gravels over the bottoms of many of the valleys. 



Up to that time the conditions of erosion and sedimentation had 

 been fairly normal. The streams had wound between gravel banks 

 similar to those that we see on many of the streams in this country 

 at the present day, the sides of the vallej^s had not been locked and 

 barred from the influence of erosive agencies by frost, and the gravels 

 of the alluvial bottoms had not been prevented from being distributed 

 and redistributed by the same restraining agency. The climate had 

 been temperate, or at all events not arctic, and large numbers of 

 animals, such as Bison, Mammoth, Elk, Moose, Horse &c. had 

 roamed over the country. 



Suddenly, a new set of climatic conditions began to prevail. 

 The Glacial Period began, and, while the vast sheets of ice which 

 covered so large a portion of Canada during that Period never extended 

 over the Klondike district, the cold undoubtedly became very intense, 

 and as a consequence the ground became permanently frozen. With 

 the freezing of the soil and of the underlying rock the processes of 

 oxidation and disintegration of this rock were no longer possible, and 

 the small tributary brooks which fl.owed over the frozen land into the 

 main streams were no longer able to collect and wash down sand and 

 gravel from it. The supply of sand and gravel having been thus cut 

 ofi^, it could no longer be distributed by the main streams over the 

 alluvial flats as it had been distributed before, but nevertheless the 

 sand and gravel flats themselves were not worn away by the streams 

 as they would have been under normal conditions, for they were 

 cemented into very resistant masses by a matrix of ice. 



