76 REPORT UNITED STATES GEOLOGICAL SURVEY. 



still rotaiuinj; its protecting: cap, near the Grand. As is the case with the 

 preceding Dakota Group, the Colorado outcrop follows the same course 

 indicated by the upheaval, or rather upturn, of the Hogback Ridge, and 

 owing to the stratigraphical structure confines itself to a narrow, long 

 strip. 



It was a matter of considerable difficulty to make any satisfactory 

 estimate of the thickness of the Colorado shales, owing to the want of 

 sufficiently fa;Vorable outcrops. By adding the observations made at a 

 number of localities, however, I have estimated it to about 900 to 1,100 

 feet, widening, perhaps, and contracting locally. Besides this exposure 

 of the shales, there is but one more in our district. This occurs on the 

 White River, a little east of Station 40, extending from there up-stream 

 about two miles beyond the junction of Douglas' Creek with the White. 

 At that place the shales are exposed in a depression named Raven's Park 

 by Dr. White. They have appeared in consequence of an exceedingly 

 interesting flexure of the strata, which has brought to view them as well 

 as the younger beds, elsewhere hidden from sight by the superincum- 

 bent strata of the Green River Group. In speaking of the Tertiary beds 

 the nature of the flexure involved will be discussed. Here it may suf- 

 fice to say, that the shales are brought to-day and occupy an almost 

 perfectly level valley for about ten miles along the river. Its maximum 

 width south of the White is two and a half miles. Farther north the 

 main depression is found, the one quoted being merely its southern 

 edge. Dr. White, in treating of his district, will speak of it at length. 



Fox Hills Group. — Of the three Cretaceous groups, this one certainly 

 occupies the most prominent, topographically speaking, position in our 

 district. In the preceding pages the Grand Hogback has freqently been 

 mentioned. It rises abruptly from the valleys formed by the Colorado 

 shales, shows a steep face toward the east, and falls off more or less 

 steeply westward. Starting along it from the White River southward, 

 we find that its relative elevation is about 1,000 feet; but after we have 

 crossed the divide between the White and Grand, this is increased to 

 1,600 feet. While north of the divide the ridge is more or less broken by 

 erosion, it presents an almost impenetrable wall farther south. Passages 

 are afforded only by a few openings that scarcely owe their existence 

 entirely to the action of water. Almost the entire Hogback, from the 

 White down to the Grand, is formed by members of the Fox Hills Group. 

 In its northern half, where it is lower, these beds form both the eastern 

 face and a large portion of the western slope, but south of the divide 

 they scarcely reach to the summit. We can assume for the Fox Hills 

 series an average thickness of about 1,500 feet. It begins with 

 dark, soft shales, which are followed by a characteristic white sand- 

 stone. This is covered by a succession of grey to yellowish-brown 

 shales, underlying a series of similarly colored sandstones and shales. 

 In the northern half of the Hogback the strata dip westward at an 

 angle of 15° to 18°, but as we proceed south we find that this is greatly 

 increased. At the passage of Rifle Creek through an opening in the 

 ridge, the strata stand perfectly on edge, but assume a westerly dip 

 again, which, in the overlying Tertiary beds, rapidly becomes very gen- 

 tle. Erosion has produced very striking effects on the eastern slope of the 

 ridge, carving the soft and yielding strata into singularly regular ridges 

 and symmetrical troughs. Cretaceous fossils scattered through the 

 strata composing this group, furnish evidence as to its relative position 

 and agt!, were those not already furnished by the position occupied. It 

 has been mentioned in speaking of the Colorado G."oup, that the sbales 

 of the latter dip under the Fox Hills beds at the eastern base of the ridge. 



