40 SURVEY OF COLORADO AND NEW MEXICO. 



Te, wliich I shall call the Grallisteo sand group. To this group of 

 modern tertiary deposits I have given the provisional name of the 

 Monument Creek group, and they occupy a space of about forty miles 

 in width from east to west, and fifty miles in length north and south. 



Continuing our course southward, we find some cnrious mesas in the 

 valley of West Plum Creek. We ascended one lofty hutte, with a flat 

 table summit, situated Avest of the Plum Creek road. The top of this butte 

 is about one thousand feet above the road, and is capped with a rather 

 close-grained, cream-colored rock, which looks quite porphyritic, fifty to 

 one hundred feet thick, and plainly of igneous origin. It fractures 

 into slabs which have a clinking sound. The beds below are quite 

 variegated, of almost every color and texture, mostly fine sand, brick 

 red, deep yellow, rusty red, white-ash colored, dull black, &c. The rusty 

 iron layers sometimes form a sort of liaionite, but are composed largely 

 o± an aggregate of water-worn pebbles cemented with the silicate of 

 iron. There are also thick beds of quartzose sandstone, or an aggregate 

 of crystals of quartz and feldspar, so com^^act as to look like a coarse 

 granite. These large masses afibrcl good illustrations of the process of 

 weathering by exfoliation. 



The evidence is clear in a number of localities that at a late period in 

 geological history there were dikes or protrusions of igneous material 

 which flowed over these Monument Creek sandstones in broad sheets 

 or beds; and these broad, table-top buttes and mesas are the evidences 

 that are now left after erosion. 



This modern tertiary basin is very interesting as the introduction of 

 a new feature in the geology of this region. The appearance of the 

 country also undergoes a decided improvement. The great divide is 

 covered rather thickly with pine timber. It is full of excellent springs 

 and fertile valleys which give origin to numerous streams. The grass 

 is excellent and abundant, even upon the summits of the table lands. 

 For a distance of ten miles about the sources of Plum Creek the red 

 beds or triassic jut square against the sides of the metamori^hic foot- 

 hills of the mountains. The projecting summits of the upturned ridges 

 gradually fade out in importance. They have also lost their usual regu- 

 larity, and are siflit i\]) into an indefinite number of fragments of ridges, 

 varying in dip from ten to forty-five degrees. Near the water divide 

 these ridges gradually close up again toward the foot of the mountains 

 and are entirely concealed by the sands and arenaceous clays of the 

 Monument Creek group. 



In the valley of West Plum Creek and its branches, as they emerge 

 from the mountains, we have a fine exposure of the sedimentary beds. 

 The coarse, yellowish-gray sandstones and pudding-stones of the Monu- 

 ment Creek group incline slightly, perhaps three to five degrees. Then 

 come the sandstones of the lignite tertiary, inclining twenty-five degrees. 

 Then west of West Plum Creek are some ridges of cretaceous rocks. 

 The first ridge is made up of a rather impure limestone, filled with well- 

 defined species of Inoceramus and other shells, of No. 3 or middle creta- 

 ceous. The next ridge west is composed of No. 1, and the intermediate 

 valley is underlaid with the shales of No, 2. Among the brick-red 

 ridges is one low ridge composed almost entirely of gypsum — an unusual 

 development of this material — to the thickness of thirty or forty feet. 



There is an extensive series of low ridges of red and gray sandstones 

 extending up the base of the mountains. 



The high portion of country, which is plainly visible from Denver when 

 looking southward, and from the Arkansas River looking northward, 

 would seem to have been protected from erosion by causes which I can- 



