Geologic units in Thomas county, Kansas

Loess (Holocene to Pleistocene) at surface, covers 92 % of this area

eolian silts

Ogallala Formation (Miocene) at surface, covers 7 % 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 1 % of this area

unconsolidated sand, silt, clay, and gravel