Geologic units in Holt county, Nebraska

Pierre Shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 15 % of this area: Mostly medium to dark-gray, brownish-gray, and black, fissle clay shale. Locally grades to thin beds of calcareous, silty shale or claystone, marl, shaly sandstone, and sandy shale. Locally contains thin seams of gypsum and sparse selenite crystals. Approx. max thickness 1970 ft.
Lithology: shale; evaporite
Ogallala Group or Formation (Phanerozoic | Cenozoic | Tertiary | Miocene) at surface, covers 85 % of this area: Silt, sand, sandstone, gravel and conglomerate. Predominantly interfingered fine- to coarse grained, poorly sorted, arkosic, fluvial deposits of light-gray, light-olive-gray, and grayish-green calcareous silt and sand, and locally poorly consolidated conglomerate, sandstone, and siltstone.
Lithology: mudstone; sandstone; volcanic ash