Why isn鈥檛 a cave a canyon? A brief history of Eagle County’s geology
Curious Nature
Adobe Stock Image/Licensed by Walking Mountains Science Center
Traveling around Eagle County, it is easy to wonder about the interesting geological features that dot the landscape. While we are blessed with abundant mountain peaks and canyons, there is one geologic feature whose elusive formations always fascinate me: caves.
While camping near the top of a canyon over the weekend, I stumbled upon some fissures in the ground that, upon further inspection, appeared to connect deeper into a much larger cave system. I found similar features in three different places. The caves were located near a large canyon, so I began to wonder why caves get carved out of the earth in the first place. What does it say about the geology of Eagle County? Why isn’t this cave a canyon?
To answer this question, we have to go back in geologic time. Colorado has been covered by shallow seas multiple times throughout its geological history, most notably during the Paleozoic Era (approximately 540 to 251 million years ago) and the Cretaceous Period (approximately 145 to 66 million years ago).
During these periods, warm, shallow seas advanced and retreated across the region, depositing thick layers of sediment that now form various rock formations like sandstone, limestone and gypsum. Between about 70 to 40 million years ago, an event called the Laramide orogeny took place, which is the formation of the modern Rocky Mountains. This caused the shallow sea-covered crust below present-day Colorado to uplift slowly. The uplift and subsequent erosion of this revealed geology explain the landscape we see today. But how does this relate to caves?
The key to understanding caves in Eagle County is understanding limestone. Many marine organisms, such as corals, shellfish or brachiopods, have shells or skeletons made of calcium carbonate (CaCO2). When these organisms die, their remains accumulate on the seafloor, forming layers of sediment.
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Over long periods of time, these layers of shell fragments are compacted and cemented together, often with the addition of calcium carbonate crystals from the surrounding water, creating limestone. Since it is made up of once-living organic material, limestone is classified as an “organic sedimentary rock.”
Limestone is very alkaline because of its calcium carbonate composition. However, rainwater is naturally slightly acidic due to dissolved carbon dioxide (CO2) from the atmosphere, which forms carbonic acid. When the acidic water interacts with the alkaline limestone, it causes a chemical reaction that dissolves the limestone into the water. In science class, you may have put acid onto limestone and watched it fizz, which is a great way to think about cave formation. The water dissolves the limestone, and over time, makes cavern features underground.
So why isn’t a cave a canyon? Caves and canyons are formed by similar but differing processes. Caves are formed by chemical weathering that dissolves rocks.
Canyons, on the other hand, are formed by mechanical weathering such as erosion, freeze-thaw cycles or abrasion from water. They both involve water moving through their geologic structures, but in different ways. Caves are more influenced by groundwater and canyons are more influenced by surface water. So, caves aren’t canyons because they are formed by different processes and involve groundwater rather than surface water.
Caves are sensitive environments that are easily susceptible to human disturbance. Since caves can take millions of years to form and are home to delicate structures and sensitive organisms, they are a non-renewable resource and particularly sensitive to damage from humans.
They can also be hazardous places to visit, and uninformed travelers should avoid entering cave systems. Caves provide fascinating geology and are exciting to find, but they should be enjoyed with an abundance of caution.
