The 20-minute drive from the Boysen Dam to Thermopolis on U.S. Highway 20 goes through 2 billion years of natural history, making Wyoming’s Wind River Canyon the best time machine in the United States.
The walls of black and pink granite are from the Precambrian Period, an age of magma and volcanic intrusions more than 2.5 billion years ago. These aren’t only the oldest rocks in the canyon, but some of the oldest on the planet.
Meanwhile, the rock sitting on top of the granite is flathead sandstone from the Cambrian Period laid down around 500 million years ago.
Two billion years of history are missing in the Wind River Canyon, and to this day nobody knows exactly where it went.
It’s called the Great Unconformity by geologists. Evidence of the enormous gap in Earth’s geologic history can be seen in many other spots around the world, including the Grand Canyon. The Wind River Canyon might be the most accessible place where the stark contrast of the Great Unconformity can be seen and touched. There’s even a pull-off for photos.
Road signs along the highway highlight the most significant rock formations in the Wind River Canyon. It’s much easier to see the difference between the Gros Ventre Formation, the Gallatin Limestone and the Amsden Formation when a sign lets drivers know once they’ve reached it. The signs include the name and age of each formation. There are more layers than there are signs, but the signs that are there are very good at pointing out the major formations of time.
At least 12 rock layers are exposed in the canyon spanning the Palaeozoic, Precambrian, Cambrian, Ordovician, Devonian, Mississippian, Pennsylvanian and Permian epochs. The Wind River Canyon is an excellent teacher of that history, as at least one rock layer from each major period is exposed along its walls.
The most remarkable thing about the Wind River Canyon’s journey through time is that everything is laid out chronologically. The trip from Shoshoni to Thermopolis takes drivers forward in time, with each successive layer getting closer to today.
Conversely, a driver from Thermopolis to Shoshoni takes drivers back in time. No matter which way the drive is taken, the canyon is a geologically perfect succession through Earth’s history.
There’s also plenty of life in the Wind River Canyon, both on the road and in the rocks. Several formations exposed in the canyon have produced fossils from the prehistoric oceans that covered Wyoming at the time.
On the canyon’s southern end, the Phosphoria Formation contains fossils of tiny marine creatures that lived in the Early Permian Period around 280 million years ago. The rocks preserve fossils of the youngest trilobites, the famous pill bug-like invertebrates that were completely extinct by the end of that period.
Trilobites first appeared 520 million years ago and went extinct around 250 million years ago, a 270-million-year existence that is preserved in the rocks inside the Wind River Canyon. It makes that 20-minute drive a journey through the rise and fall of one of the most iconic and important species in Earth’s history.
Other formations in the canyon preserve traces of soft-bodied marine creatures. While their bodies disappeared hundreds of millions of years ago, their burrows and trackways from the prehistoric ocean floor are still visible in the rock.
Drivers are greeted by the blood-red rock of the Chugwater Formation at the northern end of the Wind River Canyon, a familiar sight for Wyomingites. But even as the canyon ends and opens up into the Bighorn Basin, the journey through time continues.
Visitors emerging from the northern end of the canyon arrive at the most famous geologic age in Earth’s history. The rocks on that horizon come from the Age of the Dinosaurs and beyond, and anyone can keep traveling forward or even go back in time without leaving their car or even turning around.
As you keep going, you see the bright-red Triassic-age rock, into Jurassic and Cretaceous formations. Then, you can see a mirror of the Wind River Canyon on the other side (of the Bighorn Basin) near Cody or the Ten Sleep Canyon on the eastern edge of the Bighorn Basin.
The rock layers in the Ten Sleep Canyon are harder to see, which in some people’s opinion makes the Ten Sleep Canyon prettier, and in others makes the Wind River Canyon prettier.
While the Grand Canyon and others might be more picturesque at a distance, the Wind River Canyon is the West’s best time machine. There is nowhere else in Wyoming, or possibly the United States, where such a complete history of the planet is visible and easy to find, right outside the window during a 20-minute drive on U.S. Highway 20.