196 GUIDEBOOK OF TTTE WESTERN UNITED STATES. 



of ice on reservoirs built along the Sierra streams near the railroad. 

 From this town onward many ice plants and storage houses will be 

 observed, as ice cutting is the principal industry of many of the 



small places along the route. Iceland; a small station just beyond 

 Floriston, has a name suggested by this industry. 



Boca is an ice station and seems to consist principally of a pic- 

 turosque little hotel and a store. It is the starting point of the 



Boca & Loyalton Railroad, primarily a lumber road, 

 ^^^^- running north to Loyalton (26 miles) and thence to 



Elevation 5,534 feet. Portola (45 milcs), wherc it connects with the Western 



Omaha 1,565 miles. -r^ - ^ -r> <i mi t„ j. 4- 



racmc Kailway, ihe canyon opens somewhat at 

 Boca, and to the rear maybe seen the high continuous crest of the 

 Carson Range, just passed- West of the Carson Range and between 

 it and the main summit of the Sierra there is a broad and relatively 

 depressed area, the southern part of wliich is occupied by Lake 

 Tahoe and the northern part by Sierra Valley. A belt of relatively 

 low though mountainous country connects the basin of Lake Tahoe 

 with Sierra Valley. This depressed belt, like the mountain scarps, is 

 of structural origin. The area corresponds to a block bounded by 

 faults, that has sunk or has been less uplifted than the adjacent ranges. 

 During the uplift of the Carson Range the upper portion of Truckee 

 River was occasionally dammed to form a lake, but in the main the 

 river kept its course by cutting down its channel across the hard 

 rock as the mountains rose. West of Boca terraces built at former 

 higher levels of the stream channel are represented by benclihke 

 renmants along the sides of the valley, but the unmistakable 

 evidence of the damming of Truckee River is found in certain dis- 

 tinctly and evenly bedded or laminated deposits of clay, sand, and 

 gravel, wliich are interpreted as laid down under standing w^ater. 



V glance at the geologic map will show that these deposits spread 



over an extensive area west of the Carson Range. It is supposed 

 that after the close of the andesite eruptions there followed a long 

 period of erosion, during which Truckee Canyon was cut to very 

 nearly its present depth. Then came a basalt eruption, covering 

 large parts of the valley and damming the river afresh. The resulting 



Pleistocene lake probably persisted during a large part of the 

 glacial^ period, gradually diminishing in size as Truckee River cut 

 down its outlet. Its beach gravels are found all around this upper 

 Truckee basin. 



Low terraces overflowed by basalt may be seen along the river, at 

 one place (milepost 214) sho^ving a good illustration of columnar 

 joint structure, which is a characteristic shrinkage phenomenon fre- 

 quently exhibited by such lava flows. 



