CHAPTER V. 



DESCRIPTIVE GEOLOGY OF HUCKLEBERRY MOUNTAIN 

 AND BIG GAME RIDGE. 



By Arnold Hague. 



GENERAL FEATURES. 



The country described in this chapter embraces a mountainous area 

 irregular in outline and of great diversity of form. It consists mainly of 

 a series of ridges, trending northwesterly and southeasterly, composed for 

 the most part of Mesozoic rocks. Older sedimentary rocks are well exposed 

 in a number of localities, as well as areas of coarse breccia and broad 

 fields of rhyolite, but the region is essentially one formed of rocks of 

 Cretaceous age. 



The southern line of the area described here is sharply defined by the 

 forty-fourth parallel of latitude, coinciding with the southern boundary of 

 the Yellowstone Park forest reservation. 1 The broad valley of the Snake 

 separates it from the mountains and uplifted sedimentary rocks of the Teton 

 Range lying on the west side of the river. On the northwest and north the 

 rhyolites of the Park Plateau, reaching their southern limit, rest directly 

 against the upturned edges of the sedimentary beds. In much the same way 

 the western border of andesitic breccias of Two Ocean Plateau sharply 

 delimit this area of sedimentary rocks from the unbroken mass of basic 

 lavas which stretch far away eastward in broad plateaus and serrated 

 ridges. In striking contrast to these areas of breccias and rhyolites that 

 surround it, this region stands out strongly marked by its physical features. 

 In a certain way this group of sedimentary ridges rises as an island, or 

 rather as a projecting promontory, into a vast sea of lavas. The irregular 



1 See map of Yellowstone National Park, accompanying Part I, and Geologic Atlas U. S., folio 30, 1800. 



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