My search continued today for an exposed contact between the upthrusted proterozoic igneous rock of the Rock Mountains and the Fountain sandstone formation. I returned to an obscure roadcut site I had examined a few months ago. Three (male) cyclists outfitted in expensive cycling couture (Spandex) were standing there nibbling on dainty little energy snacks next to the spot I needed to be as I pulled up and exited my vehicle with a rock hammer in hand. One seemed taken aback momentarily as I walked towards them with the chisled masonry hammer. It didn’t dawn on me until after they left why they were acting strangely- I startled them. Sorry fellas!  \;-)

With rock hammer in hand I scrambled up a steep and unstable scree slope adjacent to what appeared to be disturbed layer next to a gneiss formation. Down below, along the roadcut, a contact was visible between the gneiss and what appeared to be schist.  This dark material has a preponderance of mica with little gross evidence of stratification. I wrongly concluded that I was not near the proterozoic contact.

But as I followed this discontinuity further up the mountain I found clear evidence of a stratified sedimentary formation adjacent to the igneous rock. On a ledge high above the road I found an actual contact between what appears to be modified sandstone and gneiss. I found a sample that has the gneiss fused onto the layered rock that fractures into thin sheets much like sandstone or shale. Regrettably, I left the camera in the Jeep.

What appears to have happened is that the sandstone layer has been thermally modified along the interface due to the intimate contact with the upthrusted igneous rock. I had half-expected to see a simple interface between sandstone and an igneous rock. Instead, what I seem to be seeing at this site is a modified sedimentary layer that shows evidence of some localized metamorphic modification.

A nearby thin layer of rock in the interface zone appears to be glassy or vitrified, as though it has been partially melted. I do not interpret this to be a result of weathering. A rapidly approaching lightning storm forced me to cut my exploration short and run for cover.

So, I have some hypotheses beginning to take shape. Now the question is, how do I falsify my interpretations? I certainly have much to learn about petrology.