THE BOULDER VALLEY REGIOS. 139 



is the triangular piece cut off, showing Laramie clays and ironstones and, 

 possibly, a portion of the coal measures, all greatly fractured; opposing 

 the piece, on the north, are the < !oal Measures, including their upper sand- 

 stones, also considerably fractured for some distance from the fault line. 



The Louisville fault. Till' single <'\]MISU1V of till- fault IS III tllC CUt l >f the 



Colorado Centi'al Railroad in the bluffs of Coal ('reck immediately south 

 nt Louisville. (Fig.9,p. 12.").) [ts course, N. tO to IT E., is not thai of 

 tin- bluffs, bu1 forms an acute angle with them of aboul 15 . To 1 1 1 * - 

 southwest it passes into the prairie and is obscured beneath surface deposits 

 or by reason of the similarity of beds mi either side; to the northeast it is 

 hist in the bottom lands of Coal ('reek, and mi trace is found iii the bluffs 

 beyond. 



At the railroad cut the fault is very pronounced, the downthrow 

 being to the southeast, with upper Laramie of doubtful horizon opposed 

 to sandstone B and a few feet of overlying beds on the northwest. The 

 fault plane has an inclination nt' aboul 65 to the southeast, with the beds 

 on both sides apparently bent down, though it is possible that those on 

 the southeast, which are badly shattered, have Keen bent up. 



The Rock Creek fault. — This IS probably a simple dislocation of about 75 



feet, with downthrow to the west, appearing in the bluffs east of Rock 



('reek opposite the point where it is crossed by the Colorado Central 

 Railroad. The plain- of the fault is not visible, but the evidence of 

 fracture lies in two distinct benches of the basal conglomerates of the 

 Arapahoe formation, horizontally disposed along the hill, and separated 

 on its slopes by a zone of clays and ironstones belonging to the upper 

 portion of the Laramie. Unconformity may be an alternative explana- 

 tion of the conditions. 



The Erie fault. Tllb t a 11 1 1 (Tosses |hc I'.oldder Valley Uailroad between 



tOO and 500 feet north of the Erie depot, having a trend about \. 64 E., 

 and extending from the lower Coal Creei fault across Coal ('reek for an 

 unknown distance to the southwest. Along the line of fracture east of 



Coal ('reek, the maximum throw is at lea.M 250 feet, the upper beds of the 



Fox Hills on the north being opposed to the coal measures of the Laramie 



on the south. The inclination of the fault plane is not shown at the 



