PEPORT ON THE GEOLOGY OF THE SIERRA ABAJO AND WEST 

 SAN MIGUEL MOUNTAINS. 



By W. H. Holmes. 



In the summer of 1876 I accompanied Mr. Wilson, director of the 

 primary triangulation, to the Sierra Abajo or Low Mountains of South- 

 east Utah. We approached them from the east, by way of the old 

 Spanish trail, which led us around the south base of the La Plata Moun- 

 tains and across the Great Sage Plain. It was not diiSicult to recognize 

 the various features of the country west of the south bend of the Eio 

 Dolores by the descriptions of Dr. Newberry, recently published. Leav- 

 ing the Dolores at the point touched by the trail, we passed up to the 

 south over the blufi's of the Dakota sandstones, and found smooth, 

 monotonous traveling over the almost unbroken plain. Considerable 

 areas of the fossil-bearing shales of the Middle Cretaceous were observed 

 in localities subject to the least amount of drainage, but the main floor 

 of the plain is composed of the Dakota sandstones. 



Ten miles out we came upon a shallow wash in which was a spring. 

 A small band of Utes was encamped here, and on the west bank of the 

 ravine extensive ruins were noticed. This is probably the Surouara of 

 Dr. Newberry. At night we made camp in a pretty little valley about 

 midway between the Dolores and the Sierra Abajo. At this place a 

 small spring issues from the lower slope of the Cretaceous bluff, and 

 affords water enough for a small party. On the following day we left 

 the tortuous trail, which seemed to be leading northward toward the 

 Sierra la Sal, and rode directly westward. Alter some 40 miles travel 

 we struck the caQon of the Montezuma near its head, and, crossing this, 

 encamped in the evening close under the east slope of the Sierra Abajo. 



In traveling 85 miles we had not changed altitude more than 500 feet, 

 and had not varied 200 feet above or below the geologic horizon that 

 separates the Middle and Lower Cretaceous strata. The floor of the 

 plain is everywhere of the Lower Cretaceous sandstones, which have 

 generally a gentle southern dip. Small areas of the shales remain on 

 the less eroded districts. In no case do the stream courses penetrate to 

 the Eed Beds. At the point of crossing, the Montezuma Canon is only 

 150 feet in depth, and the walls are composed at the top of about 40 feet 

 of massive, yellowish sandstones, beneath which are soft sandstones and 

 shales, mostly covered with debris. The altitude at the crossing is 6,200 

 feet above sea-level. From the caiion, which runs north and south, a 

 gentle slope of 9 or 10 miles leads up to the base of the steeper faces of 

 the mountains. The sketch (Plate XIII,) taken from the east bank of the 

 Montezuma Canon, will give a good idea of the general appearance of 

 the eastern face of the group. There is nothing striking in the outlines 

 of the mountains, and there is a total absence of the bold forms and 

 naked rock-masses that characterize thelarger groups to the east. 



We encamped at the base of the principal eastern spur, where we 

 were unfortunately detained for three days by a rain and snow storm. 

 On the fourth day, the 19th of September, we succeeded in making 



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