536 O. H. HEUSHEY TERTiAHY AND QUATERNARY GEOLOGY 



narrower valleys in the Coeur d^Alene Mountains (giving the latter the 

 unique distinction among western mountains of having been nearly as 

 rugged in middle Miocene time as today), with a great low plain on the 

 west of the mountains. This plain was flooded with basalt lava, which 

 gradually rose against the western slope of the Idaho mountains, and 

 in the middle Miocene period so obstructed the Coeur d'Alene Valley 

 that a lake was formed beyond the lava barrier and the deep old valley 

 filled with sediment to a depth of 500 feet. At the close of the Pliocene 

 period the Idaho mountains were uplifted, the lava plain tilted south- 

 westerly, and the streams began to cut new canyons. When the Coeur 

 d'Alene Eiver had trenched its new valley to a depth of about 400 feet, 

 a glacier advanced across the valley somewhere below Lane and formed 

 a lake of very short duration^, in which floated icebergs laden with glacial 

 debris. At this time probably between two-thirds and three-fourths of 

 the Pleistocene period had passed. When the ice retreated, the river 

 cut its valley 100 to 350 feet deeper, locally forming river terraces. 

 Near the end of the Pleistocene period alpine glaciers formed in the 

 Bitter Eoot Mountains of Montana and Idaho and ran short distances 

 down the valleys. Valley trains of over-wash gravels led down the main 

 valleys, such as the Saint Eegis in Montana and the South Fork of the 

 Coeur d^Alene in Idaho. Then the ice largely melted away from the 

 high mountains and the valley trains were nearly removed by erosion. 

 Finally, the glaciers advanced again, though to a less distance than 

 formerly. Their valley trains are largely buried under the Modem 

 alluvium except that of the Spokane Valley. This obstructed the Coeur 

 d^Alene Valley and formed the present Coeur d'Alene Lake. The lake 

 was originally a little higher than at present, as evidenced by the bed of 

 sand in the city of Coeur d^Alene. At the opening of the Eecent period 

 the glaciers disappeared and the rivers began to build up the Modern 

 floodplains, including the delta of the Coeur d'Alene Eiver. 



