DENVER & RIO GRANDE 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 6,481 feet. cliffs that border the valley on the northeast (right). 
Denver 196 miles. The contact of the soft rocks of the valley 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- 
canse recent disturbances in the earth’s crust have raised one part 
of the crust with relation to another; or they may be formed by 
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Ficurp 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 east. 
voleanic 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- 
80697°—22__7 
