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GUIDEBOOK OF THE WESTERN UNITED STATES. 



sw. 



Near milepost 367 the valley opens and is irrigated, and the deep 

 red of the sandstone is relieved by the bright green of alfalfa, sugar 

 beets, and apple orchards, which are irrigated by water taken from 

 the creek that comes in from the right. Below this point the river 

 turns more toward the west, and it soon cuts through the red sand- 

 stone that has bordered the valley most of the way from Glenwood 

 Springs. 



As all the beds here dip toward the southwest the river cuts 

 through a formation from bottom to top and then passes into the 

 overlying formation. The top of the Triassic system is crossed at 

 milepost 369, or about three-quarters of a mile beyond the siding of 

 Chacra. The Grunnison formation, the next formation in the series 

 above the Triassic, is only about 300 feet thick, and as it dips at an 

 angle of about 45° it is soon crossed. It is characterized by a variety 

 of colors, but maroon, green, and white predominate. Across the 



4) river on the left there 



are some small conical 

 hills composed of this 

 formation, which are 

 capped on the far side 

 by massive beds of the 

 Dakota sandstone, 

 which marks the base 

 of the Upper Creta- 

 ceous series and is one 

 of the most persistent 

 and widespread formations in the Rocky Mountain region. It is 

 generally tliin, at few places exceeding 80 feet in thickness. It was 

 deposited on the surface of the Gunnison formation. During the 

 deposition of the Gunnison formation the region was land, though 

 probably of low relief, but the deposition of the Dakota marks the 

 end of land conditions and the beginning of the occupancy of the 

 region by the sea, which continued during the deposition of the suc- 

 ceeding thick shale. The Dakota sandstone is generally massive and 

 very resistant to erosion, so that where it is upturned at any con- 

 siderable angle it makes hogbacks, such as those seen back of Canon 

 City. Although the Dakota is not exposed near the railroad its beds, 

 concealed beneath the surface, are crossed by the track about halfway 

 between mileposts 369 and 370. The relation of the Dakota to the 

 rocks above is shown in figure 36. 



The rocks above the Dakota for a long distance are very soft shale 

 or shaly limestone, so they have been eroded into a wide valley that 

 lies between the little hogback formed by the Dakota sandstone and 

 the mountainous ridge on the left, which trends nearly parallel with 

 the line of the railroad and is composed of the Mesaverde forma- 



Horizontal scale 



Figure 36. — Section tJirough Grand Hogbac* at Newcastle. 



