FAULT NEAB JiULLDKl;. 



115 



IIII. I Al LT Ol l III SOU! II BOl I I.I R I'l \K-. 



General description. '|'1|C \ ( -|- \ ( • 1 1; I |:|C| ( -n-t i( • l|nl|lll<- peak of Solltll I i< .Ill< 1 < T, 



which forms a prominenl landmark from tli<- plains, owes its twinning to a 

 strike fauh in the higher foothills of the Front Range overlooking the 

 prairie. Reduplications of this nature are nol infrequent in this range, 

 but through topographic position or extended erosion they are rarely 

 prominent. 



The linear extenl of the South Boulder Peaks fauh is between '■'> and 4 

 miles, the southern end being well defined, the northern passing into granite 

 and becoming difficult to trace. The break has the appearance of a normal 

 strike fault, the plane, where observed in Bear Canyon, inclined to tin- 

 west — against the dip of the bed — at an angle with the horizon of from 

 60 to 80 .' Notwithstanding its "normal" appearance, however, it will be 

 shown beyond that it is of the 

 "reverse" type, the deception 

 occurring through the position 

 of the beds and the topo- 

 graphic features of the region. 

 The downthrow is on the west 

 of the fauh plane, the dis- 

 placement varying from at 

 the ends to approximately 1,400 feet ;it the point when- the Profile II of 

 tin- general sections crosses it, ti> 3,250 feet, as observed in the gorge of 

 Bear Canyon, and to a figure perhaps somewhat in excess of thi> ;it other 

 points along it- line. 



The fault, in it- surface appearance, resembles an extensive renl in the 

 Red Beds, occupied by the granites of the Archean. The southern end is 

 well shown in the ridge just north of South Boulder Creek, which connects 

 tin- Dakota hogback with the main mass of the Trias. The first suggestion 

 of a possible fracture is here found in the tendency displayed by the 

 Dakota sandstone to the structure "en Echelon" so frequently met with 

 along the front of the Colorado Range. ( >n passing to the deeply eroded 



Fig. 6.— Thi -'.nil, Boulder Peaks twinned by fault. From U.S. Geo- 

 logical Surrey <>!' the Territories, by 1 . V. Hayden. 



In 6g. 7. Itodj the Hayden Survey, tl»- hade in given east. 



