DENVER & RIO GRAISTDE WESTERN ROUTE. 



85 



down from the high mountains at the back of the valley. The rail- 

 road follows the east side of the river, passing by 

 Pleasanton. the village of Pleasanton and hugging the granite 



Elevation G,48i feet, cliffs that border the valley on the northeast (right) . 

 Denver 196 miles. ^j^^ contact of the soft Tocks of the vallcy with the 

 granite or gneiss is not a normal contact but is due to a fault, the 

 granite having been elevated or the other rocks depressed an un- 

 known distance. 



In order to understand the meaning of the surface features along 

 the railroad from Pleasanton to Salida it is necessary to know the 

 geologic structure and the succession of hard and soft rocks. 



Mountains are usually formed either because they contain rocks 

 that are somewhat harder than the rocks in adjacent areas or be- 

 cause recent disturbances in the earth's crust have raised one part 

 of the crust with relation to another; or they may be formed by 



Figure 17. — Cross section of the Sangre de Cristo Range and the valley on its east side 

 at Pleasanton, showing the anticline of the mountain and the syncline on the oast. 



volcanic action. In the Rocky Mountains the principal ranges and 

 peaks have been formed by one or both of the two causes first stated. 

 The great Sangre de Cristo Range, which towers on the left a mile 

 above the railroad, is no exception, but this range, unlike many others 

 of this general region, is very narrow, being at no point more 

 than 12 or 15 miles wide. At many places its crest is composed 

 of granite and gneiss, which, being harder than the surrounding 

 rock, have remained at their present height, while the softer rocks on 

 either side have been washed away to lower levels. In general, the 

 structure of the mountain at the north end is that of a great anti- 

 clinal fold (arch), mainly in Carboniferous rocks, though it affects 

 the lower rocks down to and including the granite. At a point 

 farther south the fold crosses the range at a low angle, and from 

 that point southward the structure is entirely different. The section 

 shown in figure 17 represents in a general way the structure of the 

 rocks at the north end of the mountain — the anticline in the moun- 

 80097°— 22 7 



