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Rattlesnake Dam

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Rattlesnake Dam lies in an erosional gap cut by Rattlesnake Creek.  This creek drains a synclinal depression cutting into the Precambrian core.  Compressional stresses and mountain uplift have developed folding and minor faulting at the site.  Schistose foliations within exposed rock strikes generally east-west across the regional mountain axis.  Exposed rock at the site consists of Precambrian schist, gneiss, and granite-gneiss.  The schist is hard, micaceous gray material  that is soft and decomposed at the surface.  The gneiss and granite-gneiss are hard, grey material, sometimes altered and broken.   The dam was built on mostly schistose-gneiss of the Precambrian Idaho Springs Formation.  This rock is generally grey to light gray, medium to fine grained, fractured, and thinly to very thinly foliated.  Near the ground surface the rock is weathered and decomposed.  Decomposed rock is soft, while harder sound rock occurs at increased depth.  Weathering and decomposition may be on the order of several tens of feet.   Overlying the bedrock are alluvium and colluvium associated with the present day Rattlesnake Creek erosion.  Average depth of these materials is about ten feet.  These materials transition in depth and gradation towards outcrops on the surrounding hills.  Alluvium generally consists of sands and gravels with a few cobbles and boulders residing in the stream bed area.  Colluvium consists of similar materials, but more towards the cobbles and boulders gradation, that are wasting downslope from the hill sides.

Last updated: Apr 02, 2013