Geologic units in Logan county, Colorado

Ogallala Fm (Tertiary) at surface, covers 29 % of this area

Loose to well-cemented sand and gravel

Eolian deposits (Quaternary) at surface, covers 26 % of this area

Includes dune sand and silt and Peoria Loess

White River Fm or Group (Tertiary) at surface, covers 24 % of this area

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

Modern alluvium (Quaternary) at surface, covers 7 % of this area

Includes Piney Creek Alluvium and younger deposits

Pierre Shale--Upper unit (Cretaceous) at surface, covers 7 % of this area

Pierre Shale--Upper unit

Older gravels and alluviums (Quaternary) at surface, covers 5 % of this area

Includes Slocum, Verdos, Rocky Flats, and Nussbaum Alluviums in east, and Florida, Bridgetimber, and Bayfield Gravels in southwest

Arikaree Fm (Tertiary) at surface, covers 2 % of this area

Sandstone; contains abundant volcanically derived material

Gravels and alluviums (Quaternary) at surface, covers 0.6 % of this area

Includes Broadway and Louviers Alluviums

Ogallala Group or Formation (Miocene) at surface, covers 0.2 % 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.