The Geology P.A.G.E.: Geological Destination: Doughnut Falls, Salt Lake City, UT

Friday, October 09, 2020

Geological Destination: Doughnut Falls, Salt Lake City, UT

In a continued effort to catch up on my things that I wanted to post on, I am now currently up to September of 2016, when the family and I hiked up to Doughnut Falls (I've also seen it spelled as "Donut Falls"). The hike is part of the Mill D South Fork within the Wasatch Mountains. The trail itself is 3.5 miles out and back, with a lovely view of the falls once you get there. It really is a fantastic hike. You can find information on it at Utah.com. There is tons of geology going on within the area including lots of glacial geology evidence. However, I only took pictures of the falls themselves so we will focus on the rock unit of the falls.

Inside of the carved out cave, looking up at the inside of the "doughnut".

The rock unit that makes up the falls is called the Doughnut Formation, named after the falls. It is a Late Mississippian in age (~330 million years old) thinly bedded limestone, interbedded with shales. During the Late Mississippian, Utah was under water, allowing for deeper water deposits to accumulate, specifically limestones, which are the accumulated remains of marine organisms. Because of this, there are abundant brachiopods and bryozoan fossils within the formation.

Another view inside the cave with the exit cave also in view with the "doughnut".

As you can see in the first two pictures, the beds of the limestone are a few feet thick at the top of the falls, creating an erosion resistance surface. Eventually the stream broke through the thicker limestone layers, likely along a crack that got widened over time through erosion, and started eroding into the underlying shale layers, creating this cave. The layers are also severely tilted, which happened during the build up of the Wasatch Mountains. This period of time is known as the Sevier-Laramide Orogenic events, that took place from ~160 to ~35 million years ago as the Farallon Plate off the western coast of the US was pushing up against North America and was being subducted underneath the North American plate.

The "doughnut" part of Doughnut Falls from the top.

From the top you can see that the stream is running down the surface of the limestone, which often happens with waterfalls, until it gets to the hole in the rock. Waterfalls are created by erosion resistant rock layers, usually limestone or sandstone, that are underlain by more erodible layers, like shale. Once the water can break through the erosion resistant layer, it quickly erodes away the lower layers, leaving a cliff edge where the break happened that slowly erodes upstream over time.