136 



-GEOLOGY OF THE DENVER BASIN. 



The North Bouider faults. — These appear in the low sandstone bluffs on the 

 north side of Boulder Creek, midway between the towns of Valmont 

 and Canfield. Erosion has brought the present surface of the country 

 within the limits of the Fox Hills and Laramie formations 

 and has cut in the bluffs of the channel the natural section 

 given above. The series of faults is in effect repetitive. 

 For tour successive blocks the measures are reproduced 

 within a few feet of the same stratigraphic and vertical 

 range, and for as many times the coal horizon has been 

 elevated and wholly or in part eroded from the region, 

 until at the present time the first workable beds in passing 

 eastward are found at the town of Canfield, on the western 

 edge of the Erie Basin, several miles east of the area here 

 described. 



The inclination of the fault planes is unobservable 

 for No. 1 and No. 4, the western and eastern, respectively; 

 for No. "_' the inclination is to the westward 70°; for No. 

 3, apparently eastward. 



In each of the faults the downthrow is on the west; 

 the blocks are tilted to the east with a dip of from 5° to 

 25°; the displacement at the fractures is approximately 

 180 feet for the western or No. 1 fault, 60 feet for the 

 second, 150 feet for the third, and 230 feet for the fourth 

 or eastern fault. The northern extent of the several faults 

 as platted on the map is entirely hypothetical, the fractures 

 being lost in the clays of the Laramie in the hill beyond; 

 their southern extent may carry them into some of the 

 breaks already described in the Held to the south of Boul- 

 der Creek, but it is impossible to so trace them. 

 Only the two central interfault blocks display any broad, general 

 structure beyond the usual crushing or folding in immediate proximity to 

 the fractures. The western of these, which forms a prominent part of the 

 White Rocks outcrop, presents an indistinct turtle-back dipping into the 



