THE BOULDEB VALLEY. REGION. ] _',' 



trough, and al one point, also, with the Louisville. The exposures in this 

 portion of the field, however, are do1 sufficient to permil a clear insighl into 

 the precise structural relations of the three subordinate basins. < In the 

 north the Mitchell Basin i- separated from the Jackson or Canfield-Erie 

 depression l>\ one of- two structures: by the Erie fault, which has a general 

 outhwesl trend from the lower Coal Creek fracture in the vicinity of 

 tin- old Boulder Valley mine, nearl) across the general depression; or by a 

 po sible anticline, which has ;i trend somewlial nearer north than the fault, 

 and by which, combined with erosion, the basal sandstones of the Laramie 

 are brought to the surface in a narrow bell between the two regions. In 

 the latter case, if is probable thai between Canfield and Erie the fold 

 disappears. The presence of the anticline ie suggested l>v heavy sands, 

 usually a product of disintegration <>f the sandstones at the base of the 

 Laramie 



In the Mill-lull trougli the base of the Laramie lies ;it a depth beneath 

 the surface of aboul 230 feel — this from measurements in the Mitchell shaft. 

 Within the area mined the strata show ;i slighf general dip southwestward, 

 hut there are many minor mils. The eastern rim of the basin is sharply 

 upturned; the coal outcropping beneath the wash of the valley midway 

 between the Mitchell shaft and < !oal < ireek, and the basal sandstones of the 

 Laramie showing at several points in the channel. The western rim is a 

 gentle rise to the western face of the low ridge between Coal Creek and 

 Sand Gulch, the coal measures occupying the crest of the ridge. Prom 

 the Lafayette trough to the northern end of the field there is, also, an appar- 

 ent rise in the strata, for in the Canfield-Erie depression the coal measures 

 lie at even a shallower depth, 100 feet, than in the Mitchell — this, apart 

 from the natural fall of the surface in the direction of the drainage. 



The < !anfield-Erie trough occupies the northern ••ml of the < !oal ' !r< i I. 

 syncline, It is coincident with the hitter in width, and also in outline 

 except "ii the south, where it is separated from the Mitchell area by one of 

 ili" structural alternatives already described. 



The several subdivisions of the Coal Creek syncline will be discussed 

 wiili especial reference to the coal, in Chapter VI of this report, on the 

 economic geology <>i the basin. 



