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176 GUIDEBOOK OF THE WESTERN UNITED STATES. 
this climb the traveler wil] notice that the hard rocks through which 
the Gunnison has cut its canyon form a large, high mesa on the 
north (right), considerably higher than the summit over which the 
railroad passes. The shale was once probably at least as high as 
the granite, but it is so much softer that it has been worn away 
until it now lies distinctly below the hard rocks. It would thus seem 
that Gunnison River has gone out of its way to cut its canyon 
through the highest land and the hardest rocks in the region. This 
statement, however, represents merely the conditions as they appear 
to-day, but when Gunnison River first assumed this course it must 
have been flowing on the lowest land or it could not have remained 
there. At that time all this country probably stood at a much lower 
level and was nearly a plain, the hard rocks having been worn down 
as low as the soft rocks. Under such conditions the river found it 
as easy to flow over the granite as over the shale, and so its course 
was not in any sense abnormal. 
In making the climb to Cerro Summit the traveler will see on the 
south (left) the great mass of Tongue Mesa, which owes its preser- 
vation to a protecting cap of hard rock that was originally lava 
which came down from some of the numerous volcanoes in the San 
Juan Mountains, to the south, which are visible from the open 
valley near Montccae The traveler is now ap- 
proaching one of the most arid parts of Colorado, 
where water is the most valuable natural re- 
source. In order to irrigate a part of the great 
Uncompahgre Valley, which lies ahead, a long ditch has been dug 
to take water from far up on Cimarron Creek, carry it across Cerro 
Summit at a higher point than the railroad, and distribute it on 
the slopes to the west. Where this ditch crosses the summit it forks, 
and the right-hand branch, known as the Montrose and Cimarron 
ditch, passes under the railroad at the summit and is carried a long 
distance to the northwest to irrigate the broad terrace which the 
traveler will see later. 
From Cerro Summit and the slopes beyond an extended view to 
the west may be obtained. across the broad Uncompahgre Valley to 
the great Uncompahgre Plateau beyond. The ride down the slope 
is not particularly interesting, except as the traveler unfamiliar with 
the semiarid regions may see what it means to get water onto the 
land. The effect of irrigation is well illustrated by the verdant ter- 
race which the traveler may see on the right at an altitude of at least 
1,000 feet above the middle of the valley at Montrose. Where water 
is not available the surface is a desert, but where the land is supplied 
with all the water it needs, it will support a luxuriant vegetation. 
For a long time private enterprise was engaged in irrigating small 
parts of the Uncompahgre Valley from such streams as Cimarron 
Cerro Summit. 
Elevation 7,968 feet. 
Denver 335 miles. 
