THE ORIGIN OF THE DEBRIS-COVERED MESAS OF 

 BOULDER, COLORADO 



At the base of the mountains south of Boulder, Colorado, is 

 found a series of table lands, or mesas, which rise 300 to 500 feet 

 above the bed of Boulder Creek, and which slope away from the. 

 mountains at an angle of about 3^°. Mesas of a similar nature 

 are found at numerous points along the mountain front, some 

 of them much more extensive than the ones of which I write. 

 This article is the result of a study of those included between 

 Boulder Creek and South Boulder Creek. Their location is 

 shown in Fig. I. A photograph (Fig. 2) shows their general 

 aspect and their relation to the foothills. Fig. 3 is an east- 

 west section of the mesa shown in the photograph, giving its struc- 

 ture. A sheet of unconsolidated fragmental material, 25 to 50 

 feet in thickness, rests upon the eroded surface of the upturned 

 Cretaceous formations, principally the Fort Pierre shale. The 

 Benton, Niobrara, and Dakota are inconspicuous at this point, 

 owing to the proximity of the Boulder arch described by Eldridge. 1 

 This covering of debris forms the protecting cap of the mesas. 

 It is composed almost wholly of sandstone and conglomerate 

 from the Red Beds of Permo-Trias age, which formerly covered 

 the mountain front and which still rise about 2000 feet above the 

 level of the plains. Their serrate peaks are shown along the lower 

 mountain front in Fig. 2. At this point the Red Beds dip at an 

 angle of 48 . The fragments of the detrital capping vary in size 

 from grains of sand to bowlders twenty feet in diameter. They 

 are notably angular, bearing little evidence of long continued 

 water action, and are very imperfectly sorted. Coarse and fine 

 materials are bedded together in the most intimate relations. 

 Originally they formed a continuous sheet of debris, the greater 

 part of which has been destroyed by subsequent erosion. 



I U. S. Geol. Surv., Mon. 27, p. 105. 



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