MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



495 



The following kinds of physiographic evidence have been used: the 

 front of the range is linear and cuts indiscriminately across the rock struc- 

 ture; the range rises abruptly from a waste-filled valley; many steep, 

 narrow V-shaped ravines cleave the mountain block and open abruptly 

 onto the gravel fans of the valleys, and triangular facets are aligned 

 along the mountain front. Major valleys cutting through the ranges are 

 generally absent; mature topography or thin caps of volcanic rocks mark 

 summits or back slopes of the ranges; landslides are common along the 

 range fronts; hanging valleys are present on some range fronts; and the 

 lowest point in the adjoining valley is close to the scarp along the range 

 front. 



Rlackwelder ( 1928 ) has reviewed these and other proposed criteria 

 and has pointed out that several of them are equally applicable to ex- 

 humed or "fault-line" scarps. He regards the following features as positive 

 evidence of true fault scarps: (1) lack of correlation between rock re- 

 sistance and surface form; (2) rift features; (3) alluvial deposits on the 

 down thrown block thickest near the fault line; (4) lake or sink close to 

 ;he scarp base; (5) alluvial fans abnormally small; (6) frequent severe 

 nirthquakes; (7) displacement of an older topographic surface; (8) dis- 

 location of Recent or late Pleistocene formations; (9) basal scarplets; (10) 

 varped terraces in the canyons; and (11) the fault plane identified as 

 orming part of the scarp face. Nolan ( 1943) comments that some of these 

 ^atures are of relatively little value because of their infrequent oc- 

 currence (item 10, for example) or because of the absence of adequate 

 Information (item 3); and others, such as item 6, are of questionable 

 jlependability. Other observers would probably regard additional features 

 s equally valid evidence. 



When critically used there is little doubt that physiographic evidence 

 lone is adequate and diagnostic. In many places, however, use of evi- 

 ence of this type has resulted in a failure to distinguish between fault 

 carps and fault-line scarps; and there has even been a tendency to con- 

 sider that any elevated block with an approximate linear trend is neces- 

 iirily a fault block. 



| Stratigraphic evidence of faulting along the borders of ranges is gen- 

 ;ally difficult to find because valley fill commonly conceals the down- 



thrown block. Stratigraphic proof of faulting has been found in the 

 Humboldt Lake and adjoining ranges, Nevada (Louderback, 1904); the 

 Lake Tahoe region, California-Nevada (Reid, 1911); the Oquirrh Range, 

 Utah (Gilluly, 1928b); the Warner Range, California (Russell, 1928); the 

 Wasatch Range, Utah (Eardley, 1934); the Deep Creek Range, Utah 

 ( Nolan, 1935 ) ; the Roulder Dam region, Nevada ( Longwell, 1936 ) ; and 

 the Comstock Lode, Nevada (Gianella, 1936). In other places faulting 

 along the range front has been inferred from the presence of parallel 

 step faults within the range (Fuller and Waters, 1929). 



In a few places, no evidence of faulting at the contact between the 

 rocks that form the ridges and the Tertiary sedimentary beds that under- 

 lie the valleys is apparent. Ferguson and Cathcart (1924), however, have 

 interpreted similar occurrences in central Nevada as the result of sedi- 

 mentation on the downthrown block, which overlapped the outcrop of 

 the fault. 



Actual exposures of faults bordering the ranges have been made ac- 

 cessible by artificial excavations, but in a few places they have been 

 revealed by erosion. The W'asatch fault has been located by Pack (1926) 

 and Eardley (1934), several faults along the west edge of the Oquirrh 

 Range have been located by Gilluly (1928b), several Pliocene faults 

 in soudiern Nevada have been located by Longwell (1936), and addi- 

 tional faults in central Nevada have been located by Ferguson. In the 

 region studied by Longwell a considerable vertical extent of the fault 

 was revealed, and here at least the dip of the fault steepened upward; at 

 the other localities fairly steep valleyward dips prevail, ranging from 

 50 to 72 degrees. 



Small scarps formed by recent faulting, called piedmont scarps by Gil- 

 bert (1928) or fan scarps by Longwell (1930), correlate closely with 

 the scarps bordering many of the basin ranges. This was first pointed out 

 by Russell ( 1884 ) , and since that time these recent scarps have been com- 

 monly considered to indicate the presence of persistent faults. Many of 

 them have been recognized throughout the Great Basin, those in the La- 

 hontan and Bonneville basins by Russell (1885) and Gilbert ( 1S90, 

 1928a); those along the Sierra Nevada by Hobbs (1910), Lawson (1912 V 

 and Knopf (1918); those in central Nevada by Jones (1915). Page 



