Geologic units in Logan county, Kansas

Loess (Holocene to Pleistocene) at surface, covers 81 % of this area

eolian silts

Alluvium (Holocene to Pleistocene) at surface, covers 7 % of this area

unconsolidated sand, silt, clay, and gravel

Pierre Shale (Late Cretaceous) at surface, covers 4 % of this area

Thin bedded gray to brownish gray shale; contains concretions, selenite crystals, thin beds of bentonite and locally chalky beds.

Niobrara Chalk (Late Cretaceous) at surface, covers 4 % of this area

Interbedded, soft, light-gray calcareous shale and chalk

Ogallala Formation (Miocene) at surface, covers 3 % 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