HAYDEX.] 



GEOLOGY SAWATCH RANGE. 49 



sas I described, in my third annual report, for 1869, a group of light- 

 colored marls, 800 to 1,200 feet in thickness, under the name of Arkan- 

 sas marls. I then regarded them as of Pliocene age, and noted their 

 inclination as 3° to 5°, which would imply that they were deposited be- 

 fore the great Sawatch range had reached its present height. It is plain 

 that at a period comparatively modern in date there was a lake here at 

 least forty or fifty miles in length, and from five to ten in breadth, 

 and that in the lower portions several hundred feet in thickness of fine 

 sediment were deposited in moderately quiet waters. The numerous lit- 

 tle streams that flow from the Sawatch range toward the Arkansas cut 

 deep channels through this modern deposit. The quantity of rounded 

 bowlders of various sizes in the vicinity of these streams is immense. Even 

 after the lake-waters had passed down the Lower Arkansas, through the 

 canon, there must have been tremendous glacial as well as aqueous forces 

 operating from the deep gorges in the mountains, transporting a vast 

 amount of drift material into the valley. Just how much of this broad 

 expansion is due to erosion it is difficult to determine, but I am inclined 

 to the belief that there was originally only the fracture of elevation, 

 and that the old lake-basin is mainly due to erosion. On neither side 

 of the valley do we see any of the older sedimentary rocks. From 

 Poncho Pass to the very source of the Arkansas, a distance of 80 miles, 

 no rocks but Igneous and Metamorphic can be seen upon the east side. 



The Sawatch range is one solid mass of granite, intersected to a 

 greater or less extent by dikes. If we follow any of the little streams 

 that flow from the range on the east side, up to the sources of Trout 

 Creek, for example, just before reaching the borders of the South Park, 

 we shall find the full series of the sedimentary rocks, from the Silurian, 

 resting on the granites, up to the Cretaceous, inclusive, at least, inclining 

 in an easterly direction. The tendency of the waters of the Arkansas 

 Eiver was to gravitate to the extreme eastern side of the valley, from 

 the slope given by the anticlinal character of the elevation. The abrupt- 

 ness of the east side of the granite rocks on the east side of the valley, 

 from one end to the other, shows the part which they sustained in the 

 uplift. From a point about two miles below the mouth of Pine Creek 

 the Arkansas Eiver flows through a very narrow, tortuous channel, with, 

 the granite rocks of a great variety of texture projecting over the base 

 in some instances, and rising in a precipitous wall a thousand feet high. 

 Below the mouth of Trout Creek the eastern portion of the anticlinal 

 becomes the Front range. I have dwelt so long on this great anticlinal 

 because it constitutes the key to the physical structure of a great area, 

 and also because it will throw much light on other portions of the Eocky 

 Mountain region. It seems to illustrate a statement that I have often 

 made, in previous reports, in regard to the simplicity of the structure of 

 the eastern portion of the Eocky Mountain region. 



In general terms, while the details are extremely complicated, we may 

 express the structure of a belt of country known as the Sawatch range, 

 eighty miles in length from north to south, and at least forty from east 

 to west, as a single wedge of granite, thrust upward, and the sediment- 

 ary rocks inclining from either side. The illustration of which the 

 Sawatch range is the central mass is probably on a 'grander scale than 

 any other in the West, but there are abundant examples of smaller size. 

 The Black Hills of Dakota, the Laramie range, Big Horn, "Wind Eiver, 

 and many others are of the same type. 



Our last movement, July 22, was along the divide from Weston's 

 Pass to the base of Buffalo Peak. We have described the splendid 

 view we obtained of the entire Upper Arkansas Valley, from the Ten- 

 4 G s 



