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Our first stop on Saturday morning was Rock Park in Castle
Rock,CO. A hiking trail leads from the parking area up to the
top.
This feature is actually a small butte. The formation capping the butte, Castle Rock itself, is an eroded portion of sedimentary conglomerate layers. The Castle Rock area is along the eastern edge of the Rocky Mountains and sits atop a large alluvial basin filled with the eroded material from at least two mountain ranges which formed to the west. The light color haze in the atmosphere is smoke from the Hayman forest fire burning to the southwest. |
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Rock Park, looking south. Castle Rock, CO
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A large conglomerate boulder lies along the hiking path. Note the various gravel sizes; sand to cobble are easily visible. |
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Conglomerate boulder along Rock Park hiking trail. Castle Rock,
CO.
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A piece of Wall Mountain rhyolite embedded in the above boulder. The Wall Mountain rhyolite is a welded tuff that was created by the eruption of a stratovolcano named Mt. Aetna about 36.8 million years ago (MYA). Mt. Aetna was located far to the west near the present location of Mt. Robinson. The tuff was deposited here by waters running off of the Rocky Mountains just to the west. |
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Embedded tuff in Rock Park boulder.
Castle Rock, CO.
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The erosion resistant conglomerates above are being undercut as
the light colored, sandy deposits below erode away more rapidly.
There is a telling geologic story here, embedded in the layers of
Castle Rock.
The deposits forming Castle Rock are alluvial; placed here by streams or running water. Several factors affect the ability of streams to pick up and transport sediments. An important factor is the stream flow velocity and that is affected by gradient; the steepness of the land over which the stream flows. Up to a point, the steeper the gradient, the faster the stream flows and the larger the gravel it will transport. The light colored sand was transported a long distance over fairly flat terrain by relatively slow moving streams. The larger gravels in the conglomerates above required much faster moving water to erode them. The faster moving water is attributed to a steeper gradient in the drainage basin to the west caused by an uplift of the Rocky Mountains. The source of the deposits here can be pinpointed by their mineral content. Gold bearing rocks are found in these deposits, yet no gold has been discovered in the Castle Rock area. The gold identifies the source of these deposits as being many miles to the west, probably from the South Park area. |
| Castle Rock, from on top the butte, looking west. Castle Rock, CO. |
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Sabrina, an enthusiastic mountain climber, gives scale to the
layers shown here.
Clues, such as different gravel sizes and strange mineral content, contained in layers like these at Castle Rock help geologists to interpret the rock record and produce theories as to how the geology of an area evolved over long time periods. In this case, these deposits of tiny particles help tell the story of the formation of the massive Rocky Mountains themselves. |
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Castle Rock layering. Castle Rock, CO.
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