Not a crater . . .
A few years ago, while looking around on Google Earth, I noticed a strange circular feature on BLM land 10 miles south of Hays. The satellite view showed a distinct series of concentric circles, resembling a big target a few miles across (labeled "Dome" on th image above). Interestingly, the target shape of the landform is not noticeable from the ground – only from above. A little research helped me understand how it was formed. At the time I was teaching at Hays-Lodgepole High School, so I decided to go explore it and take some photos, including several with my drone.
Below: Drone photo of the same dome that is labeled on the Google Earth image above. The label "sandstone cliffs" marks the area I spent quite a bit of time exploring. Click on image to enlarge.
The strange feature is an “igneous dome”, caused by magma forcing layers of sedimentary rock upward tens of millions of years ago. The magma never made it to the surface, instead hardening below the layers. In the millions of years that followed the dome has eroded, leaving the concentric circular ridges of rock. As it turns out, there are several domes in the area, all created by plumes of ancient magma that pushed layers of rock upward – each one is unique. In fact the Little Rocky Mountains north of the isolated domes were formed by magma that intruded there 50-55 million years ago.
Right: A dome features concentric rings of strata. As the top of a dome is eroded, the formation resembles a bullseye when viewed from above with the youngest rock layers at the outside and each ring progressively older moving inwards.
Sandstone cliffs . . .
One of the most scenic aspects of the dome are the beautiful sandstone cliffs found on the perimeter of the dome. The cliffs are composed of nearly horizontal layers of late Cretaceous age Eagle Sandstone - the same formation that makes up the Rimrocks in Billings. The sandstone that covered the dome has been eroded away, leaving only the upturned portion on the perimeter of the dome. I spent quite a bit of time exploring the cliffs on the southwest edge of the dome, where there is an abundance of incredible concretions. Go to Bigskywalker.com to access my blog and photo tour, which includes photos of the concretions and many more photos of the dome.
Below: Here are the scenic cliffs on the southwest edge of the dome (drone view, looking northeast). A portion of the Little Rockies can be seen in the backgroiund.
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