THE BOULDEB YALLKY REGION. 12] 



disturbed portion, but in the numerous interfault blocks within this, the 

 upper part of the formation alone reaches the surface. 



The Laramie, in the eastern part of the region, is fully developed, 

 except what may have Keen eroded prior to the deposition of the Arapahoe; 

 in other portions of the area much of it has been removed by the erosion 

 of recent times. 'The Arapahoe is represented by the lower 100 leer — the 

 basal conglomerate and sandstones, together with a small thickness of 

 the overlying clays. The Quaternary sands and gravels occur both 



along the river bottoms and upon the higher mesa lands. 



Surface distribution of the formations. — The areas of Lai'amie and Fox Hills strata 



in the disturbed portion rapidly succeed each other, owing to a succession 



of gentle rolls and faults, to the short, vertical range within which the Fox 



Hills sandstones and the basal sandstones and coal measures of the Laramie 



occur, and to the general level to which erosion has taken place. The 



slightest variation one way or the other in the amount of movement, dip, 



or erosion has 1 n sufficient to bring one or another of these Weds to 



the surface. This rapid change in the superficial relations of the formations 

 constitutes a most prominent feature of the region and has an important 

 bearing on the distribution of coal and artesian water. 



With the exception of two narrow belts of the lower part of the upper 

 Laramie measures, one in the Coal Creek Valley, the other diagonally 

 crossing the Davidson and Lake mesas, the surface of the region west and 

 north of Coal Creek is occupied by either one or another of the series of 

 beds near the line of the Laramie and Fox Hills — that is, by the upper 

 portion of the Fox Hills, or by the basal sandstones or coal measures of 

 the Laramie. South and east of Coal ('reek, except the small area of coal 

 measures in the vicinity of the Baker mine, near the confluence of Coal 

 and Rock creeks, and the narrow strip of Fox Hills and lower Laramie 

 leading along the former creek to the Erie mines, the upper division of the 

 Laramie prevails, capped by Arapahoe at the summit of the ridge between 

 Rock ami Little Dry clicks. The homogeneity of the Laramie clays, the 

 unconformity at their summit, and the gentle folding which the strata of 



this region have undergone, prevent for it recognition of definite horizons 

 or an estimate of the thickness of the underlying beds, and consequently 



