INFLUENCE OF JOINTS ON EROSION. 1(1!! 



ravines, where the structure was not clearly visible, the peculiar topo- 

 graphic conditions indicated a similar origin. These faults are instructive for 

 the reason that they illustrate the manner in which a series of displaced 

 blocks sometimes present a nearly uniform dip, so as to appear as a single 

 monoclinal when the exposures are not sufficient to show the true structure. 



Joint-valleys. — The rocks in the precipitous bluffs of the Yukon exhibit a 

 pronounced jointed structure in many localities. As in other regions, the joints 

 occur in systems which cross each other at various angles. Their influence 

 on topography is sometimes plainly traceable not only in the pyramidal form 

 of rocky pinnacles, but in the contour of the valleys which separate them. 

 In a number of instances two main systems of joints exist which at their in- 

 tersection with a horizontal plane form parallel lines, but are so inclined as 

 to meet below the surface at an angle of twenty or thirty degrees. When 

 this occurs the prism of rock bounded by the joint planes and the surface of 

 the land has sometimes been eroded out, leaving a sharply defined V-shaped 

 valley of low grade. When so situated as to open in the tops of high bluffs 

 along the Yukon, these valleys discharge their water in cascades into the 

 river below. 



The origin of certain low-grade lateral valleys iu the glaciated portion of 

 the High Sierra of California, which open high up in the bluffs bordering 

 larger valleys and discharge their waters in cascades, has never been satis- 

 factorily explained. The fact that similar valleys in a non-glaciated region 

 have resulted from the weathering of jointed rocks may help to account for 

 these peculiar topographic forms. Should the joint-valleys along the Yukon 

 be occupied by local glaciers their forms would be modified principally by 

 a broadening of their bottoms, and they would resemble still more closely 

 the smaller of the high lateral valleys of glaciated mountains. 



Bluffs on the Upper Yukon.— The most remarkable bluff on the Yukon is 

 about twenty-five to thirty miles west of the international boundary, on the 

 left bank of the stream. This is a sheer precipice of contorted slate, about 

 600 feet high and more than a mile in length. The beds are seldom more 

 than a few inches thick, and composed of black, somewhat metamorphosed 

 slates, separated by yellowish-white layers. The strata are much contorted 

 and broken by small faults, along which a peculiar crumpling of the slate 

 has occurred. The general dip is toward the west. The cliffs terminate 

 abruptly at the east end, where they are cut off by a bold scarp trending at 

 right angles to the river. This scarp is mostly bare of vegetation, trends 

 N. 60° E., and slopes east at an angle of about 60°. It is really a fault- 

 face of so recent origin that it is not vet covered with vegetation. The steep 

 slope of the fault scarp has a pinkish color, seemingly due to debris of cer- 

 tain red rocks which, when undisturbed, occur above the contorted strata. 



The series of contorted slates forming the great bluff mentioned may be 



