426 H. E. Gregory — Shinarump Conglomerate. 



scientific world ; for although more detailed work justifies less 

 sweeping statements than those used by Dutton, yet it is remark- 

 able that a bed of such variability as a conglomerate and limited 

 to a thickness of usually less than a hundred feet may be traced 

 over such a wide area (see map, fig. 1). Where I have exam- 

 ined the Shinarnmp conglomerate, on the San Juan, at Oljato, 

 Canyon de Chelly, Fort Defiance, Echo Cliffs, the petrified 

 forest, and elsewhere, its uniformity in composition and tex- 

 ture, in stratigraphic position, and in relation to the beds above 

 and below is striking ; and even at Wingate and in parts of the 

 Little Colorado valley where its typical characteristics are less 

 in evidence, differences are less noticeable than likenesses. 



Because of its siliceous character and its position between 

 formations, which in general offer relatively little resistance to 

 weathering, the Shinarnmp conglomerate assumes a topographic 

 prominence apparently out of accord with its thickness. Wher- 

 ever this bed attains its typical development it is a cliff or ridge- 

 maker. To the north of the Colorado Canyon it forms the 

 first step in the stairway which rises by giant treads from the 

 Carboniferous platform to the summit of the High Plateaus. 

 Where the strata are horizontal the Shinarump appears as a cap 

 covering the beautifully banded Permian (Moencopie) beds. 

 Where gently folded and exposed at the surface by the removal 

 of the overlying shales, the conglomerate forms domes or 

 cigar-shaped ridges as at Monument Pass and south of Toquer- 

 ville. In places where folds have been eroded as at Fort Defi- 

 ance, Oljato, and Bellevue, the conglomerate plays the role of 

 a protecting cover of a group of beds which form well-defined 

 cuestas. With steeper dip it becomes a ridge, as for example 

 near the San Juan river, where the upturned edge of this stra- 

 tum, dipping eastward at a high angle is cut for a distance of 

 twenty miles into jagged pinnacles. The walls of the cele- 

 brated Canyon de Chelly and of Canyon Bonito are preserved by 

 a mantle of Shinarump conglomerate, and its resistance to 

 erosion is so much greater than that of the rock immediately 

 beneath, that in several places the Shinarump projects as a 

 molding which renders the cliff scalable only along weathered 

 joints. " West of St. Michaels, northeast of Agathla, and west 

 of Willow Springs, as well as at points in Utah, the conglom- 

 erate forms a bare floor of grit many acres in extent; else- 

 where it is strewn with debris from numerous landslides. 



Structure and Composition. 



As is to be expected from the nature of the rock, the Shina- 

 rump conglomerate varies in thickness and structural character. 

 Powell remarks that " occasionally it is a little more than a 

 coarse sandstone" with a maximum thickness of one hundred 



