Exploring Paleozoic Sediments in Southern Utah

Just before the pandemic locked everything down, I went on a geology hike near Comb Ridge in southern Utah with Professor Gary Gianniny, Chairman of the Department of Geosciences at Fort Lewis College in Durango, Colorado.  I’m working on a book about the canyon country of the Southwest and this was a great opportunity to learn from an expert in its geology.  While the hike primarily focused on geology, you can’t tour this region without seeing fossils.  All our fossil collecting was catch-and-release.  I took photos but did not bring back any physical specimens.  Here’s a quick overview of our hike.

We started from a point on the southeast side of a feature called the Monument Upwarp, about 16 miles west of the town of Bluff, Utah.  Here is an annotated Google Maps satellite view of the area:

Our starting point overlooked an abandoned meander of the San Juan River.  The river long ago took a shortcut by punching through a narrow wall of the meander (out of site behind the near slope), leaving it now abandoned.

From there, we turned east and went down the side of cliffs bordering Comb Wash.  During the first part of the hike on the top of the Monument Upwarp we were in the Honaker Trail Formation, a marine limestone of the Pennsylvanian Period.  Gary has done quite a bit of research on these limestones. He pointed out numerous examples of fossils in this formation.  The first was a brachiopod (I didn’t get the species).  Notice the spring-like squiggly lines along the hinge line at the top.  This is called a spiralia. Gary said it is indicative of it being a brachiopod:

Here is an example of crinoid pieces.  Gary said this limestone is made up of about 90% crinoid fragments:

As we descended, the layers got younger because of the way they were tilted.  When we got to the red layers, Gary explained we were now crossing into the Halgaito Formation, terrestrial sandstones of the Permian Period.  He pointed out fossils of plant roots encrusted in calcium carbonate (the light gray areas).  These are called rhizoliths.  He said you can still see rhizoliths forming today on the roots of modern plants.

Like most geologists who frequently get out onto the landscape, Gary was in good physical condition.  I however, was another story.  The 600-foot return climb across gullies, boulder fields, and loose scree was about the limit of my sorry physical condition.  But it was a great experience and one I wouldn’t hesitate to do again.