In this geological map of Nebraska, the blue and gray areas are the oldest.
a circa 1932 article in nebraska history magazine
1919 comment about a local find
a fossil at the peabody museum of natural history,
Fossil at the Peabody Museum of Natural History at Yale University, collected from "brick yards [clay pit] at Table Rock, United States." It is from the Late Pennsylvanian period. It was collected in 1932 and is the species "Lissochonetes geinitzianus." The taxonomic classification is Animalia -- Brachiopoda -- Rugoschonetidae -- Lissochonetes.
The reference for this fossil is a 1932 publication by "Carl O. & Condra, George E. 1932. Brachiopoda of the Pennsylvanian System in Nebraska. Bull. Neb. Geol. Surv. Bulletin of the Nebraska Geological Survey. ser.2 (n.5): 377pp.,25tf.,4tbl.,44 pls."