My current amateur geological exploit is to find the contact between the metamorphic proterozoic rock and overlying Fountain sandstone formation. This contact surface is where the rubber hit the road (or where the gneiss/schist hit the sandstone) in the latest episode of mountain building in Colorado. 

This contact has been found by geologists before and is in no way anything other than an excuse for Th’ Gaussling to get out and tramp around in the weeds. I do not hunt game or press flowers, so this will have to do.

The Rocky Mountains in Colorado are thought to be the result of crustal compression and upward thrusting caused by shallow subduction of oceanic plates under the western margins of the North American plate. This subduction causes the volcanism found along the west coast of North America. And it provides a motive force for the process of building mountain ranges.

Gneiss Unconformity within 10 Meters of Site 1

Gneiss Unconformity within 10 Meters of Site 1

After much hunting, I was able to locate a site visible from a road where the Fountain sandstone formation comes into contact with the up thrusted metamorphic rock. The clue was obvious- proterozoic rock, gneiss and schist in this case, in close proximity to a steeply dipping large scale sandstone formation.

On my initial survey yesterday, I was unable to see the exact contact point between the proterozoic rock with the sandstone just west of Loveland, Colorado. The actual contact was obscured by debris and soil- hence, no photo. After I get permission to cross a fence, a more detailed examination of the area will follow.

The photo above shows a mineral unconformity (or phase change) between gneiss (pronounced “nice”) and another common rock I have yet to identify. This is found a few meters away from the contact and is just an interesting feature not causally related to the contact.  To the right of this shot the darker rock is greatly distorted, apparently by shear, as it approaches the contact zone. I was able to locate the contact to within just a few meters.

In the photo below are samples taken across the diagonal interface seen in the photo above. The rocks were fractured to give unweathered surfaces. These are common mineral compositions found in the proterozoic zones. On the left is gneiss. The narrows area of the Big Thompson canyon is largely gneiss and schist according to the literature.

Site-1 Samples Across Unconformity

Site-1 Samples Across Unconformity

A few miles south at another contact zone I’ll call Site 2, a great example of sandstone cross-bedding can be seen in a road cut through the Fountain Formation. Examples of fluvial and dune cross-bedding can be found in other locations of this formation.

Site-2 Cross Bedding Fountain Formation

Site-2 Cross Bedding Fountain Formation

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