This is the upstream end of Roark Bluff at the beautiful deep swimming hole across from the campground. The dashed line indicating the boundary between the Newton Member of the Everton Formation and the lower part of the Everton Formation is approximate. Mark Hudson, one of the geologists on our team, has sampled tufa at the base of this cliff, and has verified that it is the lower Everton Formation (Oel on the geology maps). At the base of the Newton Sandstone Member (Oen), there is a change to more massive, rounded bedding higher up on the cliff. The exact contact is an estimate in the graphic. The differences visible in the photo are more massive beds and rounded texture in the Newton. In the lower Everton the beds are less massive, thinner, and are more angular.
For most, the overhanging ledge that marks the lower quarter of the bluff stands out. That is an erosional feature and its location is more likely related to hydrology instead of stratigraphy. During floods, the rocks below are eroded, undercutting the overhanging rocks which then become unstable and under the forces of gravity detach via rockfall.
Check out our Geosites Map for the location of this site.