Geologic units in Trego county, Kansas

Loess (Holocene to Pleistocene) at surface, covers 31 % of this area

eolian silts

Niobrara Chalk (Late Cretaceous) at surface, covers 30 % of this area

Interbedded, soft, light-gray calcareous shale and chalk

Ogallala Formation (Miocene) at surface, covers 26 % of this area

massive to cross-bedded, generally arkosic sand, silt and gravel, locally cemented with calcium carbonate; also contains limestone, volcanic ash, diatomaceous marl, opaline sandstone and bentonitic clay

Alluvium (Holocene to Pleistocene) at surface, covers 10 % of this area

unconsolidated sand, silt, clay, and gravel

Carlile Shale (Late Cretaceous) at surface, covers 3 % of this area

Chalky shale with bentonite and thick chalk beds, dark gray fissile shale containing septarian concretions, and fine-grained sandstone.