28 SURVEY OF COLORADO AND NEW MEXICO. 



ments of tlie red sandstone are found, but the rocks are mostly granitic. 

 Sometimes there is a valley scooped out between these benches and the 

 foot of the mountains ; and again, they ascend gently up to the base 

 and lap on to the flanks. Sometimes in the interval between these 

 benches there is a low, intermediate level or terrace about fifty feet 

 above the valley. The higher benches are about two hundred feet above 

 the bottom. It is to this peculiar configuration of the surface into bench 

 and terrace, that the wonderful beauty of this region is due. In the 

 distance southward can be seen a continuation of the ridges of tertiary 

 sandstone as they project above the surface far in the plains, five to 

 eight miles from the base of the mountains. There are some of these 

 sandstone ridges from one hundred to three hundred yards apart ; the 

 intervals level and completely grassed over, so that the laminated clays 

 or coal beds are entirely concealed from view. These ridges continue 

 to appear above the surface now and then, nearly to Denver, Where 

 they pass across the valleys of streams, or even dry branches, openings 

 are made of greater or less depth and width, which give the irregular 

 outlines to the sandstone ridges. 



Between St. Vrain Creek and Left-hand Creek there is a broad plateau, 

 about ten miles wide, which is as level to the eye as a tables top. It is 

 covered over with partially worn boulders. Near the base of the foot- 

 hills, behind this plateau, there is a most beautiful valley scooped out, 

 about two miles wide, which must have been the result of erosion in past 

 times, for there is very little water in it at present. 



Further southward those long narrow benches extend down into the 

 prairie from the foot-hills. As we come from the north to the south 

 side of the plateau, we can look across the valley of Left-hand Creek 

 to near Boulder Valley, at least ten miles, dotted over with farm-houses, 

 fenced fields, and irrigating ditches, upon one of the most pleasant 

 views in the agricultural districts of Colorado. These plateaus and 

 benches are underlaid by cretaceous clays, only here and there passing 

 up into the yellow sandstones of No. o, with Inoceramus and BaeuUtes. 

 The plateau on the north side of Left-hand Creek comes to the stream 

 very abruptly and seems to have presented a side front to the later 

 forces which transported the boulder drift from the mountains, the sides 

 being covered thickly with worn rocks of all sizes. This district is 

 very aptly called Boulder County, but the culmination of this boulder 

 drift is to be seen in the valley of Boulder Creek. 



From Left-hand Creek to Golden City the flanks of the mountains 

 seem to be formed of the transition sandstones, or cretaceous No. 1, 

 with all the older sedimentary rocks lying against the metamorphic 

 rocks in such a way as to render them very obscure and the scenery 

 quite remarkable. 



Indeed, south of St. Train Creek the change in the appearance of the 

 belt formed of the ridges or "hog-backs" is very marked. 



As I have before stated, I believe that the agencies which produced 

 the present configuration of the siu-face of the country are local and 

 came from the direction of the mountains ; and I have seen no evidence 

 that among the later geological events there was any drift agency uni- 

 versal in its character as that attributed to the drift action in Canada 

 and the Atlantic States. The forces may have acted synchronously 

 and all over the continent west of longitude 100°, from the Arctic Ocean 

 to the Isthmus of Darieu, but the mountain ranges were the central 

 axes from which the eroding agencies pioceeded. The agency wliich 

 produced the erosion and deposited the drift in the valley of a stream 

 originated m the mountain range at the source of that stream. I shall 

 refer to this subject from time to time, and it is one fraught with the 



