FAWN CREEK VALLEY. 39 



soft and fine-grained, massive bed, whose outcrops often weather a light- 

 brown. The thickness is 50 feet, and it is overlain by carbonaceous shales 

 which are quite arenaceous at the base and become more argillaceous above. 

 These beds form the cliffs extending eastward around the sides of the 

 crescent-like amphitheater cut in the northwestern wall of the mountain. 

 The black shales extend eastward along the crests of the cliffs of the 

 crescent amphitheater for nearly a mile, when they break down and the 

 sandstone forms the summit of the cliff. In the northwestern slopes of 

 the mountain, sandstone is exposed in nearly all the lateral gullies and 

 stream channels, the upper sandstone bed of the Dakota being especially 

 prominent and forming a persistent ledge that extends around the north 

 spur of the mountain and constitutes the wall of the amphitheater cut in 

 its eastern face. Beneath the cliffs which form the wall of the northwestern 

 part of the mountain an andesite-porphyry sheet has furnished the material 

 for a great morainal accumulation of angular rocks, concealing all expos- 

 ures and rendering travel difficult. The persistent nature of these andesite- 

 porphyry sheets is shown by their occurrence in so many localities at 

 the same stratigraphic horizon. In the stream channel north of Little 

 Quadrant the sheet of andesite-porphyry occurring between the Dakota 

 limestone and the conglomerate is well exposed at 9,000 feet. In the 

 vicinity of the lakes at the head of the valley the porphyry forms low 

 rounded knolls, whose surfaces are scored and polished by glacial action. 

 The lower slopes east of Little Quadrant have been carefully examined, 

 but the morainal drift obscures all outcrops. 



FAWN CREEK VALLEY. 



The valley of Fawn Creek shows good exposures of the Carboniferous 

 rocks, overlain by the softer Mesozoic series. In a little gulch near the 

 forking of the creek, the Quadrant quartzite series is well exposed. On 

 the south fork of the stream, just above this junction, a green magnesian 

 bed, whose surface is red from the wash of the weathered outcrop, is also 

 exposed. The overlying bed is a dark purplish-red rock spotted with 

 o-reen, highly ferruginous and argillaceous, being a very impure dolomite. 

 This rock is overlain by an outcrop of brecciated limestone, which is 

 believed to represent the highest bed of the Madison limestone series. 

 This brecciated character and the granular weathered surface of these 



