36 GEOLOGICAL SURVEY OF THE TERRITORIES. 

 MONUMENT CREEK GROUP. 



On the high divide between the drainage of the Arkansas and South 

 Platte Eivers, we find a somewhat singular formation, differing in some 

 respects from any other that we have met M'ith iu the West. The sedi- 

 ments were undoubtedly deposited in a rather modern fresh-water lake; 

 but ^vhether we can synchronize this grouj) with any of the other lake- 

 deposits in the West remains yet to be discovered. 



This group was named by me in 1869 the Monument Creek group, 

 from the fact that the atmospheric agents have carved out of some of 

 the beds a very peculiar kind of monument, or columns, which long ago 

 attracted the special attention of the traveler. These columns have 

 given name to a small stream, which rises in the divide, and flows south 

 into Fountain Creek, also to a very interesting locality now known as 

 Monument Park, in the valley of West Monument Creek, where these 

 singularly-shaped columns do most abound. These singular columns 

 have been frequently described in previous reports of the Survey ; but 

 the accompanying figures on Plate 3 presents them to the eye in their 

 varied forms far more clearly than any description in words. 



The boundaries of this basin have not yet been determined, but it is 

 believed that it does not occupy a very large area, probably confined to 

 the high ridge or divide which seems to give origin to so many small 

 streams, which, as seen on the map, flow south into the Arkansas River. 

 In this basin, Beaver, Kiowa, Bijou, Box Elder, and Cherry Creeks, 

 branches of the South Platte, take their rise ; while to the south, many 

 branches of the Fountain and Chico take their origin, and flow far south- 

 ward into the Arkansas. With our present knowledge, we may estimate 

 the area approximately as about forty miles from north' to south and fifty 

 from west to east, or about two thousind square miles. It is plain that 

 it originally extended over a much larger area ; the evidences.of denuda- 

 tion by which large portions have been removed being apparent all 

 around its borders. The basin itself lies in the Lignitic group. All 

 around its south, north, and east borders, we find the beds of this group 

 cropping out, while on the west side they^are exposed, when not con- 

 cealed by the nearly horizontal beds of the more modern group. Except 

 along the base of the mountains, it is not easy to detect any want of 

 conformability in the connection of the two groups ; and this relation is 

 obscure, when the older beds are lilted up at the base of the granite 

 hills, on account of the great amount of local drift, which seems to cover 

 everything to a considerable thickness. 



The texture of the rocks of this group is quite varied. The aggre- 

 gate thickness is probably about one thousand five hundred feet. The 

 lower portion is composed of rather massive beds of sandstone, varying 

 from a puddingstone to a fine-grained sandstone, usually of a light color, 

 sometimes yellow or iron-rust, with their intercalations of arenaceous 

 clay. In the distance, the whole group presents a chalky-white appear- 

 ance in many localities. At the immediate base of the mountain's, just 

 south of the small lake on the divide, the rocks are variegated sandstones, 

 brick-red, white, and yellow, varying in texture from a fine sandstone to 

 a puddingstone, with all the signs of deposition in moving waters, and 

 so closely resembling the older red sandstones, which we had usually 

 regarded as Triassic, that I had no small difficulty in determining their 

 exact position. Still farther north, on the divide proper, the beds jut 

 against the granites, inclining not more than 3°, and are made up of a 

 coarse aggregate of feldspar and quartz crystals, so that it resembles a 

 very coarse granite. It is plain that the sediments of this group were 



