GENERAL RELATIONS OF THE ORDOVICIAN 545 



probably it is the most conspicuous sedimentary formation in these moun- 

 tains, for its hard, massive limestone outcrops in long high escarpments 

 surmounting the slopes of Deadwood rocks. To the north its thickness 

 averages about 300 feet, including an upper series of about 100 feet of 

 softer, thinner bedded limestone and a basal white sandstone, which have 

 been included in the formation mainly on account of their Ordovician age. 

 In the southern portion of the uplift the formation thins out and is 

 absent, but it reappears in the northern and western portions of the 

 Bridger range.* 



The principal exposures of the Bighorn limestone are in the lines of 

 cliffs which face inward on the higher slopes of the limestone front ridge 

 of the mountains, and it caps some of the highest divides in the Bald 

 Mountain region. It is also a prominent feature in the numerous deep 

 canyons leading out of the mountains, especially along Bighorn, Tongue, 

 and Little Bighorn rivers and Shell, Lodge Grass, Wolf, Goose, Eapid, 

 Paintrock, Tensleep, Canyon, Otter, Beartrap, and Crazy Woman creeks. 

 Along either side of the higher part of the uplift the outcrop of the forma- 

 tion usually is narrow, but in the Bald Mountain region, where the strata 

 lie more nearly level, some wider areas are exhibited. It caps the main 

 divide north of Bald mountain and occurs on either side of the upper 

 portion of Tongue Kiver valley. In the high plateau between Tongue 

 river and Shell creek it is largely covered by Madison limestone. In 

 Hunt mountain the formation presents to the west a high, straight escarp- 

 ment, which is visible from far out in the Bighorn basin. The formation 

 is cut out for short distances by the great faults at various points along 

 the uplift, so that it does not reach the surface. Its outcrop area is 

 shown in plate 73. 



CHARACTER 



The massive limestone which constitutes the greater part of the forma- 

 tion is a rock usually of light buff color, somewhat darker when weathered, 

 filled with a coarse mat or network of irregular, silicious masses, mostly 

 from one-half to 1 inch in diameter. On weathering this silicious ma- 

 terial stands out a half-inch or more on the rock surface as a ragged net- 

 work, the purer rock between having been dissolved. The nature of this 

 weathered surface is shown in figure 1, plate 76. This feature and the 

 very massive bedding are characteristic. It is owning to the softness of 

 the underlying Deadwood shales and the hard, massive nature of the Big- 

 horn limestone that the latter forms high cliffs with a talus of huge blocks 



* The first notice of Ordovician in the Bighorn mountains was in a paper by C. E. 

 Beecher, Am. Geologist, vol. xviii, 1896, p. 32. 



