GEOLOGICAL FEATURES OF THE TRUCKEE REGION 



667 



faulting was accompanied by tilting, one slope of the range being the more or 

 less eroded fault scarp, the other the tilted older surface, which often shows 

 but slight relief. The faulting is determined by topographic form, by the rela- 

 tion of older structures to range front, and by the attitude and distribution of 

 the lalve beds, ash beds, or lavas, as the case may be.* 



The areas of uplift (or downthrow), and therefore the fault scarps, generally 

 strike north-south or at angles not far removed from that direction. High values 

 are represented by the fault on west side of Spanish Springs valley, which 

 strikes north 28 degrees east, and the Peavine fault, north 45 degrees west. 

 Where short independent ranges are produced, as best developed in the country 

 north of the Truckee meadows, they are generally not bounded on the north or 

 south by faults, but the block is flexed so that its surface of deformation rises 

 in a curve from the valley on the north and descends unbroken underneath the 

 valley to the south, like the axis of an anticlinal fold.t A longitudinal curvi- 

 ture of the depressed portions is probably the rule, but rarely observable. An 

 exceptional opportunity for observation is afforded by the depression between 

 the north end of the Carson Range block and the Peavine Mountain block. 

 The Carson range is a large block with high scarp on the east side, and rises to 

 10,800 feet in mount Rose. Going nortli it rapidly drops to the Truckee river 

 (4,500 to 5,000 feet). On the river slope the lowering is about 4,000 feet in 

 4 miles. Peavine mountain is also a fault block with a scarp on its north- 

 east side and tilted gently to the southwest. It rises to the height of 8,270 

 feet, and drops off to the Truckee river to the south at the rate of about 1,000 

 feet to the mile. The lower part of the depression between the two blocks is 

 occupied by lake beds lying on the andesites and the whole folded into a syn- 

 cline, with axis east and west, the Truckee River valley lying along this axis. 

 In other words, the surface of deformation of the Carson block drops to the 

 north in a curve somewhat sharper than usual and terminates the range; it 

 then rises again from the valley and becomes the curved surface of deforma- 

 tion of Peavine mountain. 



The principal fault scarps recognized, together with the present elevation of 

 range summit above surface of valley at the base of the scarp, follow : 



Range. 



Sierra west of Verdi 



State Line range 



Peavine mountain 



Small ranges northeast of Peavine 



Range west of Spanish Springs valley 



Carson range 



Virginia range — 



By Truckee meadows 



By lower Truckee valley 



Scarp facing. 



East 



West 



Northeast 



East 



South of east. 

 East 



West 



East 



General strike. 



North-south 



North-south 



North 45 degrees west.. 

 /North 5 degrees east) 

 1 North 12 degrees east j 



North 28 degrees east... 



North-south 



North-south 



North 5 degrees west.... 



Feet. 



3,400 

 2,900 

 3,300 



2,000 



1,500 

 5,500 



2,500 

 4,000 



It may be noted that the large blocks have faults closely approximating or 

 practically running north-south. The fault differing most from this direction 

 is the shortest one given in the table. 



Secondary faults within the general range blocks are not uncommon, giving 



* Methods ah-eady described in detail in Bull. Geol. Soc. Am., vol. 15, 1904. pp. 289-346. 

 t This was also pointed out for the Humboldt mountains, loc. cit., p; 338. 



