Pierre Shale, Upper Shale Member
Late Cretaceous, about 84 to 75 million years old. 1 crag on this site has its map point on it.
What the map says
Gray concretionary silty shale. Thickness about 865 m
1:100,000 sheet: Colton, Roger B., 1976, U.S. Geological Survey Geologic Investigations Series I-855-G (1 crag). A contact line at that scale is about 100 m wide on the ground.
Served by Macrostrat (CC BY 4.0). The wording is the map's own, quoted as a fact about the mapped unit, not about any one wall.