GEOLOGICAL SURVEY OF THE TERRITORIES. 33 



Eocky Mountain ^' divide^' tlie series of rocks exposed may be summed up 

 as Carboniferous, Red beds, Jurassic probably, some Cretaceous, with 

 patches here and there of Eocene, or Upper Cretaceous, containing im- 

 j)ressions of deciduous leaves. Igneous rocks have also been thrust up 

 through them all and spread over the summit. These have shared in 

 the later movement to such an extent as to incline at moderate angles. 



About two miles below the Junction Station, on the south side of Red 

 Rock Creek, thereis a great exposure of the Carboniferous conglomerates, 

 dipping 21^ a little west of south. The creek here passes through a 

 close monoclinal interval for half a mile, and then opens out into Rock 

 Creek Yalley, with two high ridges, with yellow and red beds (Jurassic) 

 at their base. Red Rock Creek forms one of the head branches of the 

 Jefferson Fork of the Missouri, and rises in the " divide.^' It receives 

 its name from the numerous exposures of the brick-red sandstones (Ju- 

 rassic) and Cretaceous clays along the banks. Along the streams are ter- 

 races more or less well defined, of various heights, showing the water-line. 

 About five miles north of the Junction we find the Pliocene beds, filling 

 up the valleys of the streams, sometimes reaching a thickness of several 

 hundred feet. The greater portion of this deposit is a light-gray marl, 

 with concretionary masses, and a sort of pudding-stone. In these con- 

 cretions are often inclosed masses of the basalt, which occur here and 

 there all over the country. While we have the evidence of a period of 

 effusion subsequent to the deposition of these lake-beds, from the fact 

 that the basalt lies over them, we see by these inclosed masses, frequently, 

 that there were other periods, either before or during the Pliocene. At one 

 locality I found in these lake-deposits the fossil remains of a species of 

 Anchitherum, and a land-snail, Helix. The inclination of these modern 

 beds is west 5^. In passing over the divide from Red Rock Creek to 

 Black-tailed Deer Creek, and from the highest point, 7,044 feet, we could 

 look back on a large extent of country drained by the different branches 

 of these streams. 



This broad valley, like most of those in the west, was formed by ero- 

 sion, and has been filled up with lakes, at the bottoms of which were de- 

 posited 500 to 1,000 feet of marls and sandy clays, during the later 

 Tertiary period. Here and there, these deposits have been stripped 

 away, showing remnants of old granite ridges, which either fill up the val- 

 leys, through the walls of which the streams make their way^ or they are 

 exposed as remnants of larger ridges, which extended originally across 

 the valley. Some of these modern beds have a light brick-red appear- 

 ance, somewhat resembling the Jurassic group. Reaching the drainage 

 of Black-tailed Deer Creek, we find an immense development of the 

 gneissic strata, inclining about west 30° to 45^, and extending about eight 

 miles. There are alternate beds of quartzites, true gneiss, mica schist, 

 the quartzites largely predominating. There are also thick seams oi 

 Avhite quartz. Large portions of the area occupied by the metamorphic 

 rocks are concealed by the outpouring of basalt. The metamorphic 

 beds are here separated from the Pliocene deposits by a deep ravine or 

 dry valley, the sides of the former having a regular slope, and indicate 

 a sort of sliore-line for this lake. Here and there we find curious local 

 anticlinals in the metamorphic strata, caused by the elevation during 

 the effusion of the basalt. On the west side of Wild Cat Canon, through 

 which the road x)asses to Black-tailed Deer Creek, the mountains rise 

 to a height of 8,500 feet, and over a large area are groups of the harder 

 feldspathic quartzites, which have resisted erosion, and now remain like 

 old ruins, and present a very picturesque appearance. These quartzites, 

 3 G s 



