32 GEOLOGICAL SUKVEY OF THE TERKITORIES. 



tlieir base, and one can wind about among tliem like meandering the 

 streets of some old ruined city. The view from either end, that is from 

 the north end southward, or the south end northward, is very fine, but 

 presenting a confusion of unique forms, the varieties of color, red, mot- 

 tled, &c., adding much to the picturesqueness of the scene. The roof- 

 like sides of many of these broken ridges are peculiarly marked by the 

 vertical farrows down which the rains of heaven have fallen for ages. 

 The opportunity for the study of forms presented here would rouse the 

 enthusiasm of the artist to any extent. It was difficult to estimate the 

 thickness of the Eed group, but it appeared to be at least 1,500 or 2,000 

 feet. The ridges of i>Io. 1 Cretaceous outside formed a continuous wall 

 about 30° dip. The Jurassic as well as Cretaceous beds are well shown. 

 From the divide between the South Platte and the Arkansas there 

 is a marked change in the character of the sediments composing the 

 rocks above those, usually understood as Cretaceous. This divide rises 

 from 1,000 to 1,500 feet above the valley of the Platte, near Denver, 

 and also about the same elevation as at the north end of the valley 

 of the Arkansas, near Pueblo at the south. The strata are very 

 nearly or quite horizontal, except near the base of the mountains, 

 where they have, in many instances, been slightly disturbed. The full 

 series of beds from the Eed Triassic to the summit of No. 5, inclusive, 

 are quite well shown to be the main water-divide near the source 

 of Monument Creek, though obscured here and there by a modern de- 

 posit of what seems to have been swept from the mountains, and must 

 be of the Post-Pliocene age. This latter deposit is made up of the de- 

 composed feldspathic granites and schists of which the immediate foot- 

 hills of the mountains are composed. The red beds also have contrib- 

 uted their portions to this modern deposit. From the source of Plum 

 Creek to the Arkansas it seems to have filled up the irregularities of the 

 surface more or less, and it is only when it has been swept away that 

 the outcropping edges of the ridges are exposed. There is a certain de- 

 gree of obscurity about the geology of this district. The modern ap- 

 pearance of the group of coarse sandstones and conglomerates above the 

 true Cretaceous beds and their position with reference to the granites 

 induced the belief that they belonged to the Miocene period, and in my 

 third annual report of 1869 I gave them the name of the " Monument 

 Creek group." For a distance of from fifteen to twenty miles they ap- 

 pear to jut up against the base of the mountains with an inclination of 

 not more than 15"^ at the highest, and usually not more than 3° to 5° • 

 and there is also an apparent unconformability with the older formations. 

 For example, the red sandstones at the north end lie on the sides of the 

 granite foot-hills at an angle of 70° to 80°, and suddenly disappear under 

 the nearly horizontal beds of coarse quartzose-sandstone. It is out of 

 these sandstones that meteoric agencies have carved the remarkable 

 forms which have given names to the little streams and other localities 

 in this region. There is one locality in the valley of Monument Creek 

 called Monument Park, from the great number of columns which are, 

 standing thickly over the surface, each one surmounted with a cap of 

 harder material. The shaft of the column is usually thick at the base, 

 rising up 10 to 20 feet, tapering to the cap, composed of a- coarse ag- 

 gregate of quartz-grains, small i)ebbles, all water- worn, very loosely held 

 together with rather coarse sand cement. The cap is a deep rust color, 

 composed of sand cemented with oxide of iron, and by its greater 

 hardness has resisted more effectually the eroding agencies. I cannot 

 believe that all the effects which we now see were accomplished by the 

 ordinary atmospheric influences at present in operation in this region, 



