OURAY UNCONFORMITY 456 



Hermosa Pennsylvanian strata, and the plane truncating them is at the 

 base of the Triassic conglomerate. This view also brings out the dis- 

 cordance between the San Juan tuffs and the sediments. The tuff line 

 descends from a level of 10,400 feet on the crest of the ridge south of 

 Cascade creek to somewhat below 9,100 feet, the elevation of its lowest 

 exposures at the back of the Amphitheater. The tuff thus cuts off the 

 Dakota, McElmo, La Plata, and Dolores formations and a considerable 

 part of the Hermosa. The irregular plane of pre- volcanic erosion causing 

 these relations dips in general easterly, away from the point of view. 



Plate 83 is a nearer view of the cliffs east of Ouray, where the uncon- 

 formity is so well exposed. By means of the diagrammatic explanation 

 the structure may be rendered clear. 



The cliff section of this view is chiefly composed of light reddish strata, 

 in varying tints and shades, up for 1,200 feet above Ouray ; yet the bright 

 red, almost a vermilion, so characteristic of the Red beds in general is 

 absent, and simply because the horizons to which that strong red color is 

 normal are not present. From the base of the lower fall of Cascade creek 

 to the point adjacent to the Amphitheater (shown in the oversheet) 

 where the Triassic conglomerate appears, the strata all belong to the 

 Hermosa Carboniferous. The more massive beds are pink grits and sand- 

 stones. Alternating with them occur gray limestones or calcareous shales 

 carrying the characteristic Pennsylvanian fauna. The red color of this 

 section is unusual in the Hermosa formation of the San Juan, and appar- 

 ently marks a transition to the still stronger color exhibited by the Maroon 

 formation of the Elk mountains. 



The plane of unconformity is easily reached by approaching from the 

 Amphitheater side, and, as there is commonly a ledge below it, one can 

 trace this horizon almost continuously across Cascade creek and as far as 

 the southern border of theBlowout recess. At the angle of the cliffs nearest 

 the Amphitheater, seen in plate 83, the unconformity appears between 

 a band of dark, reddish purple beds, consisting of thin limestones and 

 calcareous shales, highly fossiliferous, and the thin Triassic conglomerate. 

 The latter is in general of dark gray or reddish color, carries numerous 

 pebbles of limestone a small fraction of an inch in diameter, with vary- 

 ing amount of sand and gravel. Cross-bedding is prominent and the 

 stratum is thoroughly characteristic of the limestone conglomerates 

 appearing in the Dolores formation all through the San Juan. Small 

 fragments of bone were observed, but such remains seem less abundant 

 than elsewhere. 



The Carboniferous beds near the Amphitheater strike north 65 degrees 

 east and dip 12 degrees northwesterly. The Triassic strata have a strike 

 which is nearly the same and dip 5 or 6 degrees, also northwesterly. 



