Geologic units in Weld county, Colorado

Laramie Fm (Cretaceous) at surface, covers 25 % of this area

Shale, claystone, sandstone, and major coal beds

White River Fm or Group (Tertiary) at surface, covers 20 % of this area

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

Eolian deposits (Quaternary) at surface, covers 18 % of this area

Includes dune sand and silt and Peoria Loess

Ogallala Fm (Tertiary) at surface, covers 7 % of this area

Loose to well-cemented sand and gravel

Older gravels and alluviums (Quaternary) at surface, covers 7 % of this area

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

Gravels and alluviums (Quaternary) at surface, covers 6 % of this area

Includes Broadway and Louviers Alluviums

Modern alluvium (Quaternary) at surface, covers 6 % of this area

Includes Piney Creek Alluvium and younger deposits

Pierre Shale--Upper unit (Cretaceous) at surface, covers 4 % of this area

Pierre Shale--Upper unit

Fox Hills Sandstone (Cretaceous) at surface, covers 3 % of this area

Fox Hills Sandstone

Denver Fm or lower part of Dawson Arkose (Tertiary-Cretaceous) at surface, covers 2 % of this area

Arkosic sandstone, shale, mudstone, conglomerate, and local coal beds

Laramie Fm and Fox Hills Sandstone (Cretaceous) at surface, covers 0.9 % of this area

Laramie Fm and Fox Hills Sandstone

Denver and Arapahoe Fms (Tertiary-Cretaceous) at surface, covers 0.5 % of this area

Sandstone, mudstone, claystone, and conglomerate; Denver is characterized by andesitic materials

Arikaree Fm (Tertiary) at surface, covers < 0.1 % of this area

Sandstone; contains abundant volcanically derived material

Upper Miocene Rocks (Upper Miocene) at surface, covers < 0.1 % of this area

Southwest Wyoming: South end of Wind River Range--Siliceous, arkosic, and locally radioactive sandstone, claystone, and conglomerate. Fission-track age about 27 Ma. Recent work suggests that part of these deposits may be of Eocene age. Pliocene and Miocene (as originally defined 2) South Pass Formation. Saratoga Valley--White to greenish-gray tuffaceous sandstone, siltstone, and claystone; locally conglomeratic. North Park Formation; Central Wyoming: Arkosic sandstone, conglomerate, and siltstone; some light-colored tuffaceous radioactive claystone and white cherty limestone. North of Sweetwater River in Granite Mountains--Light-colored tuffaceous radioactive claystone, siltstone, sandstone, and arkose. Moonstone Formation; East Wyoming: Light-colored tuffaceous claystone, sandstone, and conglomerate. Ogallala Formation in Denver Basin.

Ogallala Group or Formation (Miocene) at surface, covers < 0.1 % 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.

White River Formation (Oligocene) at surface, covers < 0.1 % of this area

White to pale-pink blocky tuffaceous claystone and lenticular arkosic conglomerate.

Gravel, pediment, and fan deposits (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Mostly locally derived clasts. Includes some glacial deposits along east flank of Wind River Range. Locally includes some Tertiary gravels.

Alluvium and Colluvium (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Clay, silt, sand, and gravel in flood plains, fans, terraces, and slopes.