When the Ancestral Rockies were uplifted, they immediately started to be eroded, just as happens to all mountains. Some of the Ancestral Rockies uplifts seem to have been pretty steep sided, high uplifts that dropped fairly precipitously down to the sea. A steep mountain front means that the sediments eroded will be dumped rapidly into basins along the mountains.
|The Flatirons west of Boulder, Colorado, made of Pennsylvanian Fountain Formation|
Part of the sedimentary pile that eroded off the Ancestral Rockies in central Colorado is called the Fountain Formation. It was named for a creek near Colorado Springs, Colorado, and we can infer that the origin of the sediment was a nearby high mountain range from the composition and texture of the rocks. The Fountain Formation is mostly a pinkish sandstone with coarse quartz and feldspar grains. The quartz is no surprise, since quartz is highly resistant and often enough, it’s the only significant mineral that survives the sedimentary process. Feldspar is common in igneous rocks like granite, and it’s relatively unstable in the conditions that prevail on the earth’s surface, so it tends to weather quite easily, into tiny silt fragments or even down to the basic chemicals that make it up, potassium, aluminum, and silicon, plus others for some varieties of feldspar. So the presence of big chunks of feldspar in the rocks tells us they didn’t have much time to weather, to alter the feldspar. The sediment was dumped quickly. Coarse-grained feldspar-rich sandstones like the Fountain Formation are called arkose. The definition of arkose is that it contains at least 25% feldspar. Because of the feldspar, we know that the rocks that were eroding to make this sediment were probably granite or similar rocks, and that’s exactly what we find in the cores of the Ancestral Rockies uplifts. In many places, the Fountain Formation lies directly on top of the Precambrian rocks that were uplifted, and from which the sediment was eroded to make the Fountain. That’s a pretty profound unconformity, representing the erosion or non-deposition of about a billion and a half years of rocks between the two.
The fact that the grains are coarse, some of them large pebbles and cobbles, making the rock a conglomerate, tells us that the sediment wasn’t transported very far. If it had been, the transportation would have broken the rock down in to finer particles.
Lastly, the fact that the Fountain Formation is pink, and even red in many places, says there was plenty of iron in the rock that was oxidized to rusty iron oxide and that there wasn’t time to winnow the iron out of the sediment. Some of the color does come from the pink feldspar in the rock, but a lot comes from iron.
The Fountain Formation was laid down in vast alluvial fans by a multitude of streams eroding the eastern flank of the Colorado Ancestral Rockies. Today, the sandstones, arkoses, and conglomerates that resulted lie along the Front Range in Colorado and Wyoming, where they form such striking features as the Red Rocks at Morrison, the Flatirons near Boulder, and the Garden of the Gods near Colorado Springs. They’ve been tilted by later deformation that made the present-day Rocky Mountains, deformation that came about 200 million years after the Ancestral Rockies were uplifted and eroded to make the Fountain Formation. The Ancestral Rockies were pretty much eroded away long before the modern Rocky Mountains formed, but the fact that there are some close correlations in geographic position between the two says that they were both probably exploiting the same or related weak zones. In all likelihood, the breaks in the crust that produced the Ancestral Rockies when Gondwana and North America collided provided the structures that would be rejuvenated much later to make the present-day Rockies.
—Richard I. Gibson
Photo by Richard Gibson
Guide Book to the Geology of Red Rocks Park and Vicinity, by Andrew M. Taylor
Mallory, W. W., 1958, Pennsylvanian coarse arkosic redbeds and associated mountains in Colorado, in Symposium on Pennsylvanian rocks of Colorado and adjacent areas: Rocky Mountain Assoc. Geol., p. 17-20.