The Front Range of Colorado is roughly comprised of those mountains that can be seen from the eastern plains. There is no precise definition that I am aware of, so this will have to do. 

Superficially, these mountains run north/south and appear to be organized into ranges, which are really just a series of roughly parallel ridge structures punctuated with the occasional high points that are refered to as peaks. The origin and orientation of these ranges is defined by the orientation of faults and with the effect of eons of erosion to form river channels. Erosion has the effect of removing the weakest materials and leaving behind the most resistant rock structures.

The present epoch of the Rocky Mountains are the result of the Laramide Orogeny, the most recent period of mountain building thought to have begun 70-80 million years ago in the late Cretaceous period. The cause of this mountain building episode is attributed to a shallow angle of subduction of the Kula and Farallon plates below the western margin of the North American plate.

Geologists propose that the shallow subducting slab of ocean bottom applied a drag on the root of overlying continental lithosphere. These forces lead to the broad belt of disturbance to the overlying rock leading to the formation of the Rocky Mountains.

As mountian building proceeded, overlying sedimentary formations were bent and fractured along the margins of the upward moving rock. Today these sedimentary formations are visible in the form of ridges of protruding lamanellar sandstone, mudstone, and shales whose surface planes sit at a high angle  relative to the horizon. The uppermost sedimentary formations are exposed further east in the plains, and as one moves a few miles closer to the mountains, the deeper and correspondingly older sedimentary formations are exposed. These parallel ridges of exposed, upthrusted sedimentary formations are collectively referred to as “foothills”.

Along much of the northern Colorado front range, the westernmost sedimentary formation that abutts the metamorphic rock is called the Fountain formation. Adjacent to this upthrust of metamorphic rock is a layer of disturbed Fountain formation that has been drug upwards to a near vertical orientation. If you have been to Boulder, Colorado, and have seen the Flatirons, you have seen the Fountain formation. Red Rocks Amphitheater and the Garden of the Gods are also part of the Fountain formation.

Here is my question- Somewhere, there should be an interface (I think geologists call it an unconformity) between the metamorphic and sedimentary formations. Where can it be inspected? A road cut or riverbed?

So, it turns out that Th’ Gaussling’s brother owns a spread that is comprised of Fountan formation sandstone. He has a mountain. And down from this mountain and into his yard come elk, deer, mountain lions, bear, and rattle snakes. One of his house cats, in fact, was last seen in the jaws of a cannibalistic mountain lion trotting off to a quiet spot to munch this fresh, tender kitty morsel.

To satisfy my curiosity about this interface, Th’ Gaussling was out in the brush scrambling over snow covered rocks, cactus, and yucca looking at examples of the Fountain formation and, nearby, a formation comprised of schist and gneiss. Not surprisingly, I did not find it in a single outing. But I was close- it’s buried in deep rubble, no doubt. The hunt continues.