846 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



TJNCOMPAHGRE KEGION. 



On one of the small branches of Henssen's Creek, a tributary of the 

 Lake Fork, we were camped for several days during 1874. Our location 

 there was about west longitude 107° 30', north latitude 38°. Heading at 

 a rhyolitic peak, southwest of the great Uncompahgre Peak, a swift lit- 

 tle mountain stream flowed through its narrow valley in an easterly 

 direction. Above the camp, massive basaltic rocks protruded through 

 the broken rhyolites, forming steep, almost inaccessible walls. Farther 

 down stream, the valley widened a little, bordered on its south side by 

 timbered hills, on the north by a long-extended, grassy slope. Several 

 thousand feet above this slope, black basalt presented vertical walls, 

 the crumbling masses of which rolled down into the valley below. 



Cut in the form of a horseshoe into the grassy slope was an extensive 

 excavation, filled with "monuments". In height they ranged from 2 to 

 30 feet, forming a most striking contrast with their surroundings. 

 Rising from a massive base, the conical columns supported heavy blocks 

 of black basalt. Grooved and corrugated surface, pyramidal lateral 

 points, and the almost white color of the monuments denoted them as 

 belonging to a curious type. Little rills and grooves covered the entire 

 exterior portion of the shaft, terminating often in small cave-like exca- 

 vations. Densely clustered together, the total isolation of this occur- 

 rence appeared as thoroughly characteristic. Black or red bowlders of 

 basalt strewn throughout the monuments relieved the color, and the 

 bright green of the hillside formed an admirable frame for the picture. 



An illustration given in the Annual Eeport for 1874 (fig. 1, page 195) 

 represents two of the monuments near the entrance of the horseshoe. 

 Imagining the entire space, about 150 yards long and 100 yards wide, 

 filled with forms of this kind, varying in height and essentially white 

 and black in color, we can construct for ourselves a picture of the scene. 

 Deep, narrow gullies are worn down through the edges of the horseshoe, 

 and dry runs separate the several most prominent monument groups. 



A trachytic tuff, that has been designated as Trachyte No. 1, composes 

 the columns. Local accumulations of this material occur throughout 

 the region, and generally give rise to the formation of more or less pic- 

 turesque products of erosion. Physically, this tuff is a loosely cemented 

 agglomeration of feldspathic and quartzitic constituents mainly, yield- 

 ing readily to fluviatile and pluvial erosion. Admixtures of caolinite 

 render it less liable to successful attacks by sand-blasts, but afford an 

 opportunity for the greatest possible effect that can be produced by 

 frost. The grooving and fluting, caused either by beating rain or by 

 slowly moving water, shows conclusively, by its arrangement, the thor- 

 ough preparation which the material has undergone. Blocks of black, 

 sometimes red, basalt form the protecting caps imposed upon the white 

 or light yellow, rarely pink, columns. Their origin must be looked for 

 at the steep faces of the plateau edge, high above their present level. 



