134 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



is true that the Permian scarps of the northern Uinkaret are long, 

 bein"- from three to five hundred feet in relief, and that the Shinarump 

 can may be only fifty or a hundred feet thick. Nevertheless, the oc- 

 currence of considerable sheets of tains on the scarps in the valley east 

 of Workman spring serves to show first that the Shinarump capping 

 yields sufficient material to form a cloak; secondly, that the cloak will 

 quickly accumulate if erosion is retarded at the base of the slope; 

 and thirdly, that aridity of climate cannot account for the absence of 

 talus on the bare scarps a few miles further south. Hence it seems 

 that the bareness of these Permian scarps must be exi^lained by the 

 recent removal of a talus that once covered them. 



Perched Boulders. — There are certain details connected with the 

 problem of retreating escarpments that are finely illustrated southwest 

 of Lee's Ferry, where the full height of the Permian scarps with their 

 Shinarump capping is revealed. Here, as on the Uinkaret, the scai'ps 

 are bare ; the bareness is appropriate to the active recession of the 

 scarps already indicated by the landslides of this neighborhood. An- 

 other item was noted. We frequently saw large blocks of sandstone, ten 

 to thirtv feet in diameter, evidently derived from the cliff above, and now 

 perched on pedestals of weak shale a little distance forward from the 

 base of the scarp, as in Plate 2. The perched blocks correspond to 

 rock tables on glaciers. The pedestals are from three to fifteen feet 

 in height ; some of the blocks have lately fallen to one side, and the 

 pedestals that once supported them are not yet wholly worn away. The 

 top of the pedestals represents the height of the graded platform at 

 the base of the scarp when the block rolled down from above. Since 

 then the platform has been degraded by the height of the pedestals ; 

 at the same time, the scarp has been pushed back a decidedly greater 

 amount, and at a rapid rate compatible with the absence of a talus cloak 

 on its slope. 



Spurs and Ravines. — The minute morphology of the carved scarps 

 affords a pleasing study; it is a subject of subordinate importance, 

 truly, yet perhaps as deserving of careful attention as the morphology 

 ■ if the leaves of plants. "Where the mesa scarps have a straight front, 

 the ravines descend in relatively direct and parallel courses with few 

 branches. Each main ravine receives the waste from but a small length 

 of cliff front above. The spurs are correspondingly simple, each one de- 

 scending with little variety of form from top to bottom of the scarp, 

 although with increasing relief downwards. But where a curved re- 

 entrant occurs in a mesa front, the ravines converge more or less dis- 



