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.TOURING

COLORADO GEOLOGY

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KENOSHA PASS TO FAIRPLAY

 Location Map           The Rocks              Geology Map

KENOSHA PASS   

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 TO FAIRPLAY

 

We resume our trip at Kenosha Pass.  Elevation is 10,000'; not terribly high by Colorado standards but high enough to give a spectacular view over South Park.
Just passed Kenosha Pass before you start down off to the left side of the road is an observation point and a plaque.  It affords a spectacular view of South Park. 

At first glance it looks very simple.  A flat valley surrounded by mountains.  In fact a very flat valley.  Geologically it is fairly complex.  It is a "complexly faulted, asymmetrical syncline whose axis trends slightly west of north." (Hutchinson, 1960). 

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Click on this picture to for a 360º view.
000723av Jefferson.jpg (83913 bytes)Jefferson In the above photo, the low hill on the left represents the Elkhorn Thrust (a very low angle fault block that has been shoved up over the rocks in the valley from the left towards the right).  Except for this and some intrusives, the rocks in the valley, below the loose till, are sedimentary rocks.  At this end of the valley, Jurassic through Tertiary in age.
As you drive down off the pass into the valley, you can see the first sedimentary rocks off to the right.   These are of the Morrison Formation (named for similarly aged rocks near the town of Morrison, Colorado).  Once down on the valley floor, you head south toward Jefferson.  Immediately to the right is a knobby hill.  This is an intrusive dyke (Baker Lake Porphyry) cutting through the sedimentary rocks.  You can see it in the picture to the right which was taken from Lost Creek Road which is off to the left of 285 about half way to Jefferson. 000723be.JPG (186497 bytes)
000723ae View east at Elkhorn Thrust about 20 miles back LCR.jpg (109311 bytes) If you turn on this road and drive back about 2.5 miles just where some hills start, you will find yourself on the edge of the Elkhorn Thrust which you saw from Kenosha Pass if you stopped at the Observation Point.  See photo to left.
At mile marker 199 is the town of Jefferson.  The road off to the left takes you on a nice drive back to Tarryall Reservoir.  Continue on through the town (watch your speed) and follow 285 south (actually southwest) down the middle of the valley.  At about mile marker 194 you cross Tarryall Creek where one of the first gold placer deposits was discovered in this area.  Just passed this, the road rises up onto a slightly higher plane.   This is the Como erosional surface.   

The mountains to the right (west side of South Park) are part of the Mosquito Range. 

On the right is the road and town of Como.  The road takes you across the Boreas Pass to Breckenridge.  Although the outcrops are rather poor here, they are Foxhills Sandstone.
   
Just passed mile marker 192 on the right are dark gray shales (very eroded) representing the Pierre Shale and 0.4 miles further along is a much more prominent outcrop seen in the photo to the right.  This is a quartz monzonite and represents one of those intrusives I mentioned above. 000723af Quartz Monzonite 13 miles south of Como on 285.jpg (165214 bytes)
000723aj General view of outcrop Dakota 56 miles s of Como.jpg (139431 bytes) Keep following 285 and at mile marker 188 you start to climb a little bit up to Red Hill Pass.  This is just a small ridge crossing the valley roughly north-south.  The rocks exposed here represent a nice section of sedimentary rocks.   The first prominent rocks are of the Dakota Sandstone (tan cross-bedded sandstones of Cretaceous age. 
In the first part of the outcrop on the right, some fine examples of burrows (ichnofossils) can be seen (photo to right).  This are most probably Corophium formed by burrowing crustaceans. 000723bh.JPG (292368 bytes)
000723ak More detailed view of outcrop Dakota 56 miles s of Como.jpg (71971 bytes) Better exposures of the sandstone, some showing crossbedding and ripplemarks are exposed just a few 100' up the side of the road.000723am Crossbedding in Dakota 56 miles s of Como.jpg (151405 bytes)
Between the Dakota and the next more resistant formation (Garo) is some multi-colored shales and sandstones, the Morrison Formation seen in the photo to the right. 000723an Morrison Fm 60 miles s of Como.jpg (132115 bytes)
000723ao Garo Fm 62 miles s of Como.jpg (113527 bytes) At the top of the pass (hogback) is a bright red-colored sandstone, the Garo Formation.  It weathers rather "knobby" in parts.

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In case you didn't realize it, the rocks have been getting older since Como (Foxhills - Pierre - Dakota - Morrison - Garo - Maroon), Upper Cretaceous to Permo-Pennsylvanian).  As we come down off the ridge, the soil is a deep maroon color.  This is due to the underlying formation from here to Fairplay - aptly called the Maroon Formation. 000723as Fairplay - till at river.jpg (84252 bytes)
000723at Fairplay sign with till behind.jpg (97601 bytes)

Fairplay lies ahead.  The most striking feature geologically about Fairplay is the gravel (sand, pebbles, cobbles, and boulders).   The whole town looks like it is sitting on top of a large gravel dump (no disrespect to Fairplay intended).  More about Fairplay on another trip.

Updated 04/24/03.

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