Geologic units in Washington county, Colorado

Eolian deposits (Quaternary) at surface, covers 50 % of this area

Includes dune sand and silt and Peoria Loess

Ogallala Fm (Tertiary) at surface, covers 32 % of this area

Loose to well-cemented sand and gravel

Pierre Shale--Upper unit (Cretaceous) at surface, covers 10 % of this area

Pierre Shale--Upper unit

White River Fm or Group (Tertiary) at surface, covers 4 % of this area

East: Ashy claystone and sandstone. Includes Castle Rock Conglomerate in region southeast of Denver. Northwest: Ashy claystone in North Park

Gravels and alluviums (Quaternary) at surface, covers 2 % of this area

Includes Broadway and Louviers Alluviums

Modern alluvium (Quaternary) at surface, covers 0.7 % of this area

Includes Piney Creek Alluvium and younger deposits