CHAPTER III. 



SURPACE GEOLOGY — GUNNISON KIVEE, AND ITS TKIBUTARIES. 



Gunnison Eiver is the principal branch of the Grand, on the south 

 side. It rises on the western side of the Sawatch range, opposite the 

 Arkansas River, and on the southern side of the Elk Mountains, oppo- 

 site Roaring Fork. Its total course has a length of about two hundred 

 miles, the average rate of fall per mile being about thirty feet. In 

 Gunnison's and in Fremont's reports it is called the Grand. In the 

 West, however, it is now known as the Gunnison River, the name 

 Grand being given to the principal stream, as already mentioned in a 

 previous chapter. The principal branches of the Gunnison on the 

 south are Cochetopa Creek, Lake Creek, Cebolla Cr^ek, and Uncom- 

 pahgre River. Those on the north, in our district of this year, are Ohio 

 Creek, Smith's Fork, and the North Fork. The entire area drained by 

 all the branches on the north is about twenty-six hundred square miles. 

 From the mouth of Cochetopa Creek, the Gunnison flows a few degrees 

 south of west, to Lake Fork 5 here it changes and flows west, gradually 

 turning to the northwest until it is opposite station 80, a distance of 

 nearly thirty miles ; when it again turns and flows nearly due north to 

 the mouth of the North Fork; where it turns abruptly and flows west to 

 the head of what the Indians call Unaweep Caiion. * Its course thence 

 to its mouth is generally northwest. 



There are three large caiions and several small ones in the course of 

 the river, which will be described as we reach them in going down the 

 stream. The upper one is in granitic rocks, and was described last 

 year. The drainage of the streams uniting to form the Gunnison near 

 its head flows in two directions, viz, southeast or south-southeast, and 

 southwest. In this part of its course it is within last year's district, 

 and will be found described in the report for 1873. 



Our work last year extended as far west as Slate River, and. we com- 

 mence this year, therefore, with Ohio Creek, the next stream coming into 

 the Gunnison on the north side. 



In an air-line, from the head of Ohio Creek to its mouth the distance 

 is twenty-two miles. The actual length, however, is nearer thirty miles. 

 It has its origin in a group of isolated peaks that mark the termination 

 of the Elk Mountains to the westward. Its sources are opposite those 

 of Slate River on the north and east,- and those of Anthracite Creek, a 

 tribiitary of the North Fork of the Gunnison, on the northwest. It has 

 two forks which unite below a high sugar-loaf peak of x>orphyritic 

 trachyte, station 30. The western branches have their origin in a group 

 of mountains made up almost entirely of breccia, which in all probability 

 rests on sandstones of Cretaceous age. 



The most northern of these streams flows along the southern edge of 

 a short range of sharp peaks, whose slopes are destitute of timber, and 

 which form a serrated edge along the summit. This mass is composed 

 of porphyritic trachyte and forms a portion of the divide between 



* Pacific Railroad Eeport, vol. ii. 

 94 



