Hiking the extensive trail system that connects Red Rocks Park with Section 16, west of Colorado Springs, one encounters an impressive array of ripple-marked sandstones, a beach back in the Mesozoic, now dipping at almost 90°. The so-called “White Acres Trail” should have been named “Ripple Mark Trail.” The steep east-facing slope of the hogback seems to be Niabrara sandstone, but is backed on the west by Dakota SS, both of which form hogbacks. But how about these classic ripple marks?
For those not tuned in to Southwest Geology, a sill is an igneous intrusive that has inserted itself parallel to the surrounding strata (a dike runs at angles to the strata). Palisade Sill is a huge intrusive in northern New Mexico between Raton and Taos on Highway 64. You won’t miss it when you head west into the canyon out of Cimarron. The sill is made of Monzonite, which is on the continuum between syenite (very light in color) and diorite (medium in color) and has plagioclase and orthoclase feldspars in approximately equal amounts. In short, it is a lighter than granite, but generally similar.
My favorite aspect of this imposing formation is that the joint systems are at nearly right angles, which leaves enormous faces and a ragged, blocky crest at the skyline. And, of course, lots of rubble along the stream below. Here are some images of the crestline.
Here are a few finds from yesterday’s pickings through the paper shales from Florissant. These are from 36 million years ago, caught in ash falls from nearby volcanoes and trapped in the mud on the bottom of the lake. A fly, a beetle, and a fly/ant/gastropod assemblage.
Precambrian granite (about 1.6 billion years old), distinct to the practiced eye from Pikes Peak Granite, sits directly under the Ordovician Manitou Limestone (~ 450 million years ago), which has fossilized scales from jawless fishes. This contact represents over a billion years that’s missing from the geologic record (at this site). That means that the old granite probably had a series of overburdens that may have represented many events, but the evidence has all eroded away, leaving the nearly flat erosional surface, upon which the seafloor sediments accumulated that became the limestone. On Shelf Road you can put your finger on over a billion years of missing time.
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