FAULTS OF GREAT BASIN RANGES 95 



plateau, about 1,600 feet high, "outcrops are largely concealed by landslides. 

 The slides have formed many undrained basins on the face of the steep slope 

 and also irregular lines simulating terraces. A very large portion of the de- 

 tails in this, as in many other fault-scarps formed of broken strata of Columbia 

 lava, are thus accounted for. Reference to similar features in other uplifts 

 will occur again and again as we proceed. The reason why landslides have 

 been so numerous in the steep slope of Columbia lava ... is because the 

 strata are in layers separated in some instances by thin sheets of lapilli or of 

 clay and sand, which weather out and allow large slices of the dense rock 

 resting on them to break off." The Simcoe and Topinish landslides are de- 

 scribed on later pages. 3 



The alternation of strong and weak beds described by Russell doubtless fa- 

 cilitates the dislodgment of slides ; but they might be precipitated from massive 

 rocks if the initial scarp were steep and high enough. The main point brought 

 out by his observations is that landslides characterize young block mountains. 



My own experience in the Great Basin is brief, but a single excursion in the 

 summer of 1904 sufficed to discover a very large landslide at the western base 

 of the Canyon Range in west-central Utah. The slide stands about 500 feet 

 high for some two miles along the mid-front of the range and spreads about 

 two and a half miles forward; but it is of so remote a date that its surface 

 is maturely dissected by branching valleys with graded slopes and it retains 

 no trace of the hill-and-hollow forms such as young slides commonly possess. 4 

 A smaller slide was seen in the north-south Wasatch Range where it consists 

 of a monoclinal mass of strong and weak strata dipping to the southeast ; the 

 thickest body of weak strata, along which an oblique depression is worn across 

 the range, has "spilled" westward from the range base, as if because of a 

 recent uplift. 3 



An earlier experience in the West tempts me to think that landslides may be 

 more common along the base of the Basin ranges than the absence of mention 

 of them in the brief descriptions of most of the ranges would imply. In the 

 summer of 1900, when an excursion led me across the Colorado River at Lees 

 Ferry, some 50 miles northeast of the Grand Canyon, a large number of huge 

 landslides were noted along the base of the Echo cliffs and the Paria plateau 

 for some 20 miles on each side of the river, evidently because of the removal 

 of the weak blue clays from beneath the heavy Vermilion sandstones in the 

 current cycle of erosion ; 6 and yet, although this district had been visited by 

 Powell, Gilbert, and Dutton, the landslides are not mentioned by them, perhaps 

 because the slides are subordinate to the larger phenomena that engaged the 

 attention of those explorers ; but some of the slides sprawl forward for the 

 better part of a mile from their source, and they are easily seen and recognized 

 at a distance of 10 miles or more from the summit of the Kaibab plateau on 

 the west. 



8 A geological reconnaissance in central Washington. U. S. Geol. Survey Bull. 108. 

 1893, pp. 40, 43, 47, 48. 



4 The Wasatch, Canyon, and House ranges, Utah. Bull. Mus. Comp. Zool., xlix, 1905. 

 1). 30. 



5 Ibid., p. 21. 



6 An excursion to the Grand Canyon of the Colorado. Bull. Mus. Comp. Zool., xxxviii, 

 1001, pp. 123-125. 



