300 GROVE KARL GILBERT— DAVIS tM ™ I,1S [ft™xt 



associated with the valley block as to receive the same deposit, and have since been lifted with 

 reference to the valley block. ... It is noteworthy that all four of the fault-block spurs are 

 accompanied by thermal springs, the waters rising along their outer bases. I know of but two 

 localities of thermal springs on the line of the frontal fault of the range." 



PHYSIOGRAPHIC EVIDENCE OF FAULTING 



Scarps in the piedmont alluvium near the mountain base are given first place among physio- 

 graphic features indicative of faulting. A review of them is thus closed: "The preceding 

 statement of facts and considerations bearing on the origin of the piedmont scarps has 

 a regrettable prolixity because it is the record of an endeavor to free my own mind from an 

 initial bias in favor of one hypothesis by a thorough examination of its rival," namely, local slips 

 in the alluvium without movement on the mountain fault blocks; and the result of this examina- 

 tion is the discovery of a series of minor features by which the two kinds of displacements may 

 be distinguished. " The scarps created by such surface movements can not be confused by the 

 physiographer with the piedmont scarps" due to true f aid ting. 



The features of the range escarpment itself, especially the ends of many lateral spurs or 

 "ribs" into which the escarpment has been divided by the erosion of ravines and canyons, are 

 next considered. " There are a few angles in the outline [base line], but in the main it swings in 

 free curves. Along this line end the ribs of the range. The base line, or rock base, of each rib 

 end runs somewhat directly from the mouth of a canyon to the mouth of the next canyon. It 

 has indeed some curvature, but the curvature is part of the general curvature of the range base, 

 and has little or no relation to the character or size of the rib. It is in as many places concave 

 toward the valley [piedmont or intermont lowland] as convex. To express this character briefly, 

 we may say that the rib ends are aligned. ... A row ot rib ends . . . seems to be part of a 

 once continuous plane or gently flexed surface, the outer surface of a great original rock body 

 out of which the canyons have been carved. ... A range front characterized by the terminal 

 facets of truncate ribs is appropriately said to be facetted." Ravines and canyons are found to 

 be truncated at the mountain base as definitely as the mountain ribs; their sides vary with the 

 nature of the inclosing rocks, but their beds are narrow, and whatever the nature of the rocks, 

 "the narrowness of the bed continues to the rock base and there ends abruptly. . . . The 

 canyons hold their characters to the plane of the facets and there lose them. In the same sense 

 in which the ribs are tnmcate the canyons are truncate." 



Truncation of structures is next taken up. The strike of the range strata is occasionally 

 parallel to the range base, but along "not less than seven-eighths of the range front, the strike 

 meets the rock base at a notable angle. The greatest fold of the range, a syncline bringing a 

 great series of Paleozoic and Mesozoic formations in succession to the range base, runs athwart 

 the range in the latitude of Salt Lake City. . . . The obverse of these facts is that the line of 

 the range base crosses the strike lines of the structure at all angles. The range front transects 

 the range structure. The structures that are exhibited in the range front are truncate, just as 

 are the features of relief." Then comes a beautifully deliberate interpretation of truncation 

 such as could have been written only by a geologist of matured wisdom: 



Men are prone to take the familiar as a matter of course and to ask the meaning of the strange only: and 

 by our manifestation of this trait we gelogists are assured of our humanity. To one who lived only in a glaciated 

 region, the forms of landscape peculiar to ice erosion suggest no mystery, and it is only through acquaintance 

 with the dominant topography of aqueous origin that he is able to recognize the strongly marked characteristics 

 of the work of ice. The illustration is drawn from personal experience, but its moral finds its support in the 

 circumstance that those who have undervalued the power and accomplishment of glaciers have been dwellers 

 in lands once overrun by Pleistocene ice sheets. It has occurred to me, as I wrote of the phenomena of trunca- 

 tion, and especially the truncation of canyons and of structures, that such phenomena may be so familiar as 

 features of mountain fronts that not all students of orogeny feel the need of their explanation, and I shall there- 

 fore preface a consideration of the meaning of truncation by a brief account of a group of escarpments whose 

 characters stand in contrast. 



The escarpments chosen for this lesson are those of the great stairway, "described by Powell 

 in one of the classics of physiography," by which ascent is made from the plateau in which the 

 Colorado Canyon is cut northward to the high plateaus of Utah. 



