M. R. Thorpe— The Abajo Mountains, Utah. 379 



Art. XXVI. — Structural Features of the Abajo Moun- 

 tains, Utah; by Malcolm Rutherford Thorpe. 1 



The Sierra Abajo, or " Blue Mountains,' ' of San Juan 

 County, Utah, occupy a roughly circular area of about 150 

 square miles nearly in the center of the Canyon Lands 

 section of the Colorado Plateau physiographic province. 

 They rise abruptly 3,000 to 4,000 feet above the encircling 

 plain, presenting a prominent land mark from all direc- 

 tions. The Abajo Mountains comprise a group of lacco- 

 liths with associated sheets and dikes. The intrusive 

 bodies composed chiefly of hornblende-latite porphyry 

 show a remarkable similarity in mineral content and 

 megascopic appearance and undoubtedly came from a 

 common reservoir. 



Detailed Structure. 



The Abajo group will be divided into four sections, as 

 follows : 1. Shay Mountain, northwest section ; 2. Mt. 

 Linnagus, western section; 3. The southern area, includ- 

 ing Abajo and South Peaks, and the region east of 

 Johnson Creek, and 4. The northern area, comprising 

 Horsehead, North and Twin Peaks. 



1. Shay Mountain is essentially a dome of sedimen- 

 tary rock. At its crest is an outcrop about 110 square 

 yards in area of blue porphyry, identical in composition 

 with the rock forming the main mass of the Abajo Moun- 

 tains. This outcrop is probably a plug or short dike 

 which has broken through the roof of the laccolithic 

 chamber. The dimensions are approximately 50 feet 

 north and south and between 15 and 25 feet in an east- 

 west direction. Dakota sandstone is the highest forma- 

 tion now present in the area. Strata of McElmo and La 

 Plata are exposed on the flanks in steep-walled ravines 

 and canyons. The dip of the strata of the dome to the 

 east is 12°, to the south 5°, to the west 9°, while on the 

 north they dip 15° and then flatten out to 5°. It is 

 thought that the magma insinuated itself between strata 

 of the Chinle formation, but erosion has not cut deeply 

 enough to prove this. A Dakota-capped synclinal ridge, 



1 The detailed geologic work on which this paper is based was done during 

 the summer and fall of 1915 under the supervision of Professor Herbert E. 

 Gregory; the complete report has been submitted for publication to the 

 U. S. Geological Survey. This article is published with the permission of 

 the Director of the Survey. 



