On Saturday, my family and I had the pleasure of taking a geological tour of the area at the base of the Front Range, just west of Denver. Our tourguide (it was just me and my family) was a USGS geologist who is a member of the Affiliation of Christian Geologists. We started with the unconformity between Precambrian metamorphic rocks (~1.75 billion years old) and the Pennsylvanian/Permian Fountain Formation (~300 million years) exposed behind Red Rocks Amphitheater. We ended up on Dinosaur Ridge, which has exposures containing dinosaur bones (in the Jurassic Morrison Formation) and dinosaur footprints (in the Morrison and in the Cretaceous Dakota Formation). I had been at most of the sites we examined, but of course there is always more to see.
I learned a lot, and my family had a great time too.
Grace and Peace
GeoChristian 
Ha! I’m looking at the possible related posts that wordpress has tagged to this post…hmmm Guitar Hero vs. Rock Band 2…I’m thinking, not really related…but what do I know?
Okay, you dont get off that easily Mr. Geologist. What are the current theories for the unconformity in the formation? Throw me a bone, I dont have all day to research and find the answer for myself…that and I’m lazy, I’ve been educated in American institutions after all!
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Matt:
Since you are a long-time resident of the area, I assume you’ve seen the unconformity, as it is marked by a prominent plaque behind the amphitheater. This marks an erosional surface, and a somewhat irregular surface at that.
The Rocky Mountains have been formed more than once. In the Pennsylvanian and Permian, the area was uplifted, and the Fountain Formation that forms the Red Rocks (and Flatirons and Garden of the Gods) was created from the erosional debris shed from this mountain range.
Here’s the evidence that this unconformity (erosional gap) represents over a billion years of geologic history:
1. The rocks that form the core of the Front Range are mostly metamorphic, with some igneous mixed in. These have been dated by various means (U-Pb, Rb-Sr) to be around 1.75 b.y. old. The dates are fairly consistent.
2. There are no rocks that have Cambrian through Mississippian fossils in the immediate area. The Fountain Formation has very few fossils (one wouldn’t expect a whole lot of preservation of fossils in a gravelly continental setting) so we broadly assign it to the Pennsylvanian and Permian. This isn’t just an arm wave, but based on the relation of this layer to other layers in other locations in Colorado.
3. The Fountain Formation appears to be deposited on top of a somewhat irregular surface. This represents an erosional episode that removed whatever Cambrian through Mississippian rocks that were probably once there.
4. At some locations along the Front Range (but not at Red Rocks) there is not only an erosional surface on the metamorphic rocks, but what appears to be a paleosol (ancient soil layer). The surface sat exposed to weathering for some time before the sediments that became the Fountain Formation were deposited. Soil features that are apparent include weathering of feldspars and micas to become clay, and migration of some elements out of the upper levels down to lower levels where they are concentrated, just like in a modern soil.
I hope this answers your question.
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Matt:
I thought the “possibly related post” called In Situ Carbonation of Peridotite for CO2 Storage looked interesting, but it may not be any more related to this post than the Guitar Hero ones.
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