530 O. H. HERSHEY TERTIARY AND QUATERNARY GEOLOGY 



extent to a point several miles above Kellogg. Much of the fine gravel 

 and sand in the old channel is probably a delta built into the lake by 

 the river. Finally, the period of volcanic activity came to an end, the 

 lake was filled by sediment, and the river meandered on a broad valley 

 floor, which is now represented by the few remnants of the 600-foot ter- 

 race. Hence the age of this terrace may be nearly the same as that of 

 the termination of the lava flooding. 



■ Early Glaciation in northern Idaho 



I consider the granite boulders of the 200-foot terrace evidence of 

 glaciation somewhere not far distant from the Coeur d'Alene Yalley at 

 the time of their distribution in the valle}^, but the question remains as 

 to whether they were carried by a glacier or by ice floating in a lake. 

 They came from some region where granite and other igneous rocks 

 form a much larger percentage of the total area of the rock formations 

 at the surface than in the Court d'Alene region. The gneisses that re- 

 semble Archean rocks may be from contact metamorphic zones in the 

 Belt formations, but no gneisses are known from the Coeur d'Alene 

 Mountains. 



The strongest evidence of direct glacial action in the valley lies in 

 the considerable masses of angular and subangular local debris with 

 which the boulders are associated in places. They resemble lateral 

 moraines, but it is difficult to conceive of a shallow narrow tongue of ice 

 extending 20 miles up the valley and transporting boulders to its end. 

 The strongest evidence against actual glaciation I believe to lie in the 

 absence of basalt in the material above Lane. A glacier coming up the 

 valley could not have failed to tear some of the basalt from its bed and 

 distribute it as far as the granite boulders were carried. Hence, I am 

 inclined most strongly at present to favor the hypothesis of a lake and 

 floating ice as 'the mode of transportation of the boulders. It implies 

 that some great glacier coming down from high mountains in the north 

 obstructed the outlet of Coeur d'x\lene Valley and formed an extra- 

 glacial lake, probably much larger than the present Coeur d'Alene Lake. 

 Boulder-bearing icebergs broke from the front of the glacier and were 

 driven up the lake by the prevailing westerly winds to melt and drop 

 their burden of glacial debris on the lake floor. The boulders are known 

 to have, roughly, an altitudinal limit of distribution, and a more thor- 

 ough search might develop a regular line as their upper limit. The ice 

 barrier may have been no more distant than a few miles below Lane, but 

 is likely to have been somewhere in the region of the north end of Coeur 



