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Local Knowledge: Ripples in time

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By Paul Swenson EBS CONTRIBUTOR

Starting the walk down Ousel Falls trail is quite exciting, you never know what you might see, especially if it’s your first time. Even for us old timers who have been there many, many times, its different every hike. There are birds, mammals, plants, fish, people, and dogs, along with the ever-changing level of the creek that make each visit unique. But one thing remains constant: The story contained in the rocks. It’s a tale that took hundreds of millions of years to make.

Like any good book, let’s start at the beginning. Leaving the parking lot, you’ll notice along the sides of the trail rounded cobbles, boulders, and gravel. These weren’t put here during the trail’s construction, but rather came from the outwash from melting glaciers at the headwaters of the South Fork. The end of the last ice age was about 10,000 years ago, so we know that this unconsolidated collection of stream-rounded rocks is that old. Pondering that fact for a minute—the creek was level with the parking lot only 10,000 years ago; since then it has carved the canyon we’re about to enter.

As we round the first major corner there’s a small outcropping of light tan, fine-grained sediment just below the gravels. This displays the properties of a paleo-soil (ancient soil) from just before the melting of the glaciers, perhaps from a meadow sitting at that spot.

Continuing down the trail, one starts to notice the absence of the cobbles seen along the top section of the canyon, and the start of a fine-grained, thinly bedded rock that’s called the Thermopolis Shale. This shale got its name from where it was first measured, described, and studied, outside Thermopolis, WY. The age of this shale is around 100 million years old. That means in a couple steps, we went back in time 100 million years. Cool.

 Some of the Thermopolis shale visible along the trail, with a lens cap for scale. PHOTO BY PAUL SWENSON

This shale represents muddy, fine-grained sediment that was deposited in a shallow sea that stretched from the Gulf of Mexico to the Artic Ocean. During this part of North America’s geologic history, major stress came from a newly formed subduction zone paralleling the continent’s West Coast, which caused the crust of the interior to warp downward, below sea level.

It’s this tectonic activity to the west that eventually leads to the formation of the Rocky Mountains. During this time the crust of the interior warps up and down, causing the surface to rise above sea level, then fall below several times over the span of tens of millions of years. Evidence of this now presents itself at the first bridge, a sandstone.

This sandstone is called the Muddy Sandstone and is an important reservoir rock for the oil industry in the west. Its extent is thousands of square miles and shows the telltale signs of being deposited in shallow water or tidally influenced bars, beaches, and dunes.

Ripple marks are visible in the rock under the first bridge. PHOTO BY PAUL SWENSON

As you cross the first bridge, look down next to the South Fork and you’ll see ripple marks. They are symmetric in shape which indicates the water currents that formed them moved back and forth due to the action of waves in shallow water. There are others in this formation that display asymmetry indicating water moving in one direction only, like in a stream flowing through a salt marsh, but not here.

As can be seen in the photos, there are lots of modern-day examples of ripple marks in shallow bays in oceans and lakes, or even a river’s backwaters. If you’re careful and walk down to these ripple marks (the rocks are slimy and slippery!) you’ll be standing on a 100-million-year-old beach.

As we continue our hike, we notice that this sandstone is more resistant to erosion than the shales above it, and it’s this difference in hardness that leads us to the falls. Now it’s time to return to the present and walk back up in time to our cars. Such a nice hike. Only took us 45 minutes, but hopefully you now appreciate it in a new light as it took nature 100 million years to create this natural spectacle.

Paul Swenson has been living in and around the Big Sky area since 1966. He is a retired science teacher, fishing guide, Yellowstone guide and naturalist. Also an artist and photographer, Swenson focuses on the intricacies found in nature. 

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