South Canon Geology Hike
by Cindy Smith
In celebration of our new South Cañon Trails Geology Hike self-guided tour, 20 community members joined author Harold Taylor on a hike in the southern part of Ecology Park where Harold noted fossils, dinosaur tracks, and especially interesting geologic outcrops. ]
In this Cañon City area, a vivid succession of our geologic history is evident when looked at with an expert. The geology ranges from the relatively-recent Niobrara Formation along Temple Canyon Road at the beginning of the hike (which dates to about 70-80 million years old) to the Precambrian rocks that make up the Royal Gorge (dating to 1.7 billion years old). The hike takes you through several periods when Colorado was underwater and some when it was drier than it is now.
- The Great Unconformity. This outcrops several times and allows you to put your finger on a place where 1.3 billion years of history is missing. Above that fine line is the 450 million year old Ordovician; below that line is the 1.7 billion year old Precambrian. It can be an amazing feeling to ‘straddle’ that amount of missing time which either has eroded away or was not originally deposited here.
- Bentonite layers. Orange lines of bentonite (volcanic ash deposited as a sedimentary layer of clay) are important indicators of ancient volcanos, mostly erupting from the west. These are incredibly useful to geologists and paleontologists, as they are used for isotopic age-dating of rocks. The ash layers contain crystals of zircon (for uranium-lead ratios) and biotite (for potassium-argon ratios). Bentonite layers can also be marker beds allowing scientists to correlate rock layers over large distances. Bentonite has a fascinating characteristic which allows it to readily absorb water and expand up to 15-30% of its original volume, causing headaches when it swells under roadways, sidewalks, and house foundations. Hence its many applications in today’s world, including use in homemade toothpaste, skin products, kitty litter and excavation. Bentonite gets its name from Fort Benton, WY, where it occurs in large amounts.
- Morrison mudstone. Well into the hike and shortly after the intersection with the Redemption trail, you will encounter a very unusual Morrison outcrop. Normally the Morrison Formation is made up of very fine maroon or greenish-grey soils. However, this layer is light colored and peppered with pebble-to-cobble sized inclusions which indicate a high energy event such as a flood. These inclusions are still angular, indicating they were not transported very far, as transporting would have caused them to become rounded. We don’t know what caused this thick deposit in the Morrison. Likely possibilities are a large mudslide, faulting, or a major and localized flood.
- Morrison colors. Most of us think of dinosaur bones discovered in Garden Park when we think of the colorful Morrison Formation, which characteristically has large patches of neon green and maroon sediment. These can be easily seen from Highway 50 looking up at Skyline Drive
ridge. Typically, the maroon soils are higher up in the Morrison strata, and the green soils are lower down. Our Garden Park Morrison is the only place in which dinosaur bones can be found in both colored soils.
- The colors have to do with the iron and water content in the soil, and the resulting chemical changes that altered the color.
- Maroon soils. This color is due to oxidizing and a process we all know – rusting. This soil was originally deposited as a well-drained or drier soil. The soil has been oxidized, meaning the iron in the soil has rusted. Chemically, the ferrous iron (green) was exposed to oxygen and changed into ferric iron (red or brown).
- Greenish/grey soils. These soils have been reduced. They were deposited in a wetter environment or one where the water table was higher. This water-logged, saturated soil prohibited oxygen from entering. The ferric iron (red or brown) in the absence or removal of oxygen was reduced, and changed into ferrous iron (green). Organic material can also cause a reduction, and the fossil preservation is high.
The self-guided geology hike booklet can be purchased for a few dollars at several Cañon City locations, or it can be downloaded for free.