394 Adventures in Scenery 



the slate which forms the higher walls of the gorge. The depth 

 to which the river canyons have been cut in this Sierra slope 

 indicates the amount of erosion that has occurred since the 

 range was uplifted. These canyons are topographically what the 

 geologist calls young features of the landscape. When they 

 have become "old" the ridges between them will have been 

 worn down to low rounded divides, and the streams, instead 

 of rushing through steep rock canyons, will roll leisurely in 

 meandering channels through broad green meadows. 



Old Emigrant Trail in Notch in 

 Lava-Capped Ridge 



Blue Canyon (station) is near the crest of one of the flat- 

 topped lava-capped ridges which are characteristic of this part 

 of the mid-Sierra slope. The deep canyon of Blue Creek joins 

 American River near by. Emigrant Gap is a grass-covered 

 notch in the slates and schists of the Calaveras formation, 

 through which a branch of the old California emigant trail 

 led down into the valley of Bear Creek. It is said wagon-wheel 

 marks of the old trail are still visible. 



Here the character of the surface soil changes as the crest 

 of the Sierra Range is approached. High on the crest the soil 

 has been largely removed by the ice of glaciers, and the surface 

 of the rocks in many places has been scoured and smoothed 

 by the moving glacier ice. Nearing the summit the rocks are 

 principally granite (technically granodiorite) and lavas. Vol- 

 canic rocks generally cap the ridges, the lava rocks being hard 

 and resistant to erosion. The canyons have been generally cut 

 through into granite or into the sedimentary rocks which were 

 invaded in the upheaval of the granite. Most of the sedi- 

 mentary formations are of the Calaveras formation (Carbon- 

 iferous) and the Mariposa slate (of Jurassic age) . 



South Fork Yuba River Scoured by 



Glacier Ice 

 Near Cisco north (left) of the valley of the south fork of 



