Stop 9: Jurassic Rocks of the San Rafael Swell
The hill nearby shows several layers. On top is the Buckhorn Fm, like where we camped. Below are the three members of the Morrison Fm. From the top, the Brushy Basin member showing banding, the Salt Wash, then the Tidwell member. This is on top of the Summerville Fm, distinguished by ripple marks indicating the back and forth movement of a tidal flat on the edge of a sea. Below is the Curtis Fm, which we can't see from here, but is marine and marginal marine sediments. We are standing on the eroded Carmel Fm. The Navajo dune deposit Fm is covered by these formations. In the mid Jurassic this area was submerged so few dinosaurs have been found, but marine fossils have been. The whole top and center of Utah at this time was shallow sea.
We drive further along the canyon to where a bare exposure of the walls are shown.
Stop 10: Navajo Slickrock
This is early Jurassic crossbedded sandstone. We can see large sets of sandstone with smaller crossbeds. This is known as "large scale tangential" crossbedding. Tangential means there are sweeping internal sets. This is an erg deposit that is about 800 feet thick here. It can be deduced that these are wind borne sand deposits by looking at the way sand dunes are formed today. As the wind blows sand grains along they bounce until a small obstruction stops them, such as a clump of grass. This then stops more grains and forms a slip face on the leeward side. As this continues a gentle slope forms on the windward side and a steep slope on the leeward side due to gravity. In cross section the layers are tangential to the surface which proves windborne and not water borne sand. The dune moves and when the wind stops or shifts, the sand particles become cohesive again and a new sand dune forms.
We proceed farther down the canyon and stop near a small level ledge on the left side of the road. This is the lower level of the Navajo sandstone. On the ledge is a single three-toed dinosaur footprint. The age is slightly later than the tracks we saw back at the Warner tracksite. This is a very deep track with a slight ridge at the back indicating the pushoff of the heel. We need another track to determine the stride and hip width of the animal. This is said to be probably a prosauropod, but the fourth toe is not seen. Sierra College has acquired the rights from the BLM to make casts of this track.
We continue farther down the canyon with a side stop to look at Native American petroglyphs. Farther down the canyon we stop at a BLM pictograph site. These were painted about 2000 years ago. This is a shear cliff with a lot of desert varnish on some of the walls. This is a mix of manganese, iron oxide and clay that interact with moisture to form a hard black finish on the sandstone.