Local Geology & The Geology Time Trail

Dr. Harold Taylor with rock samples from Fremont County

It’s one thing to walk among rocks on the Geology Time Trail, and it’s another thing to walk among rocks on the Geology Time Trail with Harold Taylor.

Thirty students learned about rocks, Deep Time, and Cañon City geology at Harold’s “Local Geology” class at the PCC Mini-College. Harold explained the local geologic formations in and near Cañon City that we are so familiar with in our daily landscape, and which become much more meaningful with an understanding of how the Fountain Formation was deposited and why it is so red, where the Pierre Shale outcrops in Cañon City, and where to see the Great Unconformity and witness 450 million year old Ordovician rock perched directly on top of 1.7 billion year old Pre-Cambrian rock – showing a span of 1.2 billion years . . . . . missing.

Harold explained how to identify local rocks and passed around many samples, including Pre-Cambrian schist, gneiss (pronounced ‘nice’), and pegmatites, and then he moved on to more recent sandstone, limestone, conglomerate, stromatolites, and petrified wood.

Time in the classroom helped us understand the rocks along the trail

The class then walked the Geology Time Trail just outside the PCC classroom to view boulders from local geologic formations placed at their appropriate time period along the trail. At a scale of one foot equals one million years, it’s easier to understand the concept of Deep Time when distance is measured in feet instead of millions of years.

A huge block of travertine at the trailhead is less than 1,000,000 years old. Farther along the trail is a Dakota Sandstone bench with ripples indicating beachfront property 100 million years ago, Fountain Formation with obvious rounded rocks illustrating the river’s current at 300 million years ago, and the Harding Sandstone with its fish scales representing some of the earliest vertebrates ever found at 450 million years old.

Rocks along the trail mirror the backdrop to the East

In the mix are two stromatolites, sitting at the 265 million year mark showing the characteristic dome shape of blue-green algae mats. These fossils have been around since 3.7 billion years ago, are the earliest fossils to exist on Earth and the first example of photosynthesis, and as such are responsible for changing the atmosphere from toxic gases to an oxygen-rich environment that sustains us today.

At the 523 million year marker is a syenite boulder – not to be confused with cyanide. This igneous boulder intruded into much older 1.7 billion year rock about 10 miles southwest of Cañon City in the McClure Mountain Complex. This black and white speckled rock has a rare hornblende that is extremely consistent and can be used for uranium-lead age dating to determine an accurate age of a rock; pieces of this rock are found in laboratories around the world for just that.

The rocks along the trail add to our understanding of the formation of the Earth and the outcrops we see in Fremont County.

The syenite boulder helps tell the story of the uplift of the Wet Mountains