Geologic units in Custer county, Colorado

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 34 % of this area

Includes metabasalt, metatuff, and interbedded metagraywacke; locally contains interlayered biotite gneiss. Derived principally from volcanic rocks

Santa Fe Fm (Tertiary) at surface, covers 15 % of this area

Siltstone, sandstone, and conglomerate

Gravels and alluviums (Quaternary) at surface, covers 13 % of this area

Includes Broadway and Louviers Alluviums

Sangre de Cristo Fm (Permian-Pennsylvanian) at surface, covers 10 % of this area

Arkosic conglomerate, sandstone, and siltstone

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 8 % of this area

Includes Silver Plume, Sherman, Cripple Creek, St. Kevin, Vernal Mesa, Curecanti, Eolus, and Trimble Granites or Quartz Monzonites; also, San Isabel Granite of Boyer (1962) and unnamed granitic rocks

Pre-ash-flow andesitic lavas, breccias, tuffs, and conglomerates (Tertiary) at surface, covers 4 % of this area

Includes several named units

Glacial drift (Quaternary) at surface, covers 3 % of this area

Includes some unclassified glacial deposits

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

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

Minturn and Belden Fms (Pennsylvanian) at surface, covers 2 % of this area

Minturn and Belden Fms

Intra-ash flow quartz latitic lavas (Tertiary) at surface, covers 2 % of this area

Intra-ash flow quartz latitic lavas

Dakota, Purgatoire, Morrison, Ralston Creek, and Entrada Fms in southeast. Dakota, Morrison, and Entrada Fms in central mountains. Dakota, Burro Canyon, Morrison, Wanakah, and Entrada Fms in Gunnison River area. Dakota, Morrison, Curtis, And Entrada Fm (Cretaceous-Jurassic) at surface, covers 1 % of this area

Dakota, Purgatoire, Morrison, Ralston Creek, and Entrada Fms in southeast. Dakota, Morrison, and Entrada Fms in central mountains. Dakota, Burro Canyon, Morrison, Wanakah, and Entrada Fms in Gunnison River area. Dakota, Morrison, Curtis, And Entrada Fm

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 0.8 % of this area

Intermediate to felsic compositions

Fountain Fm (Permian-Pennsylvanian) at surface, covers 0.7 % of this area

Arkosic sandstone and conglomerate

Ash-flow tuff of main volcanic sequence (Tertiary) at surface, covers 0.6 % of this area

Includes many named units

Carlile Shale, Greenhorn Limestone, and Graneros Shale (Cretaceous) at surface, covers 0.6 % of this area

Carlile Shale, Greenhorn Limestone, and Graneros Shale

Mafic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 0.6 % of this area

Gabbro and mafic diorite and monzonite

Minturn Fm in west-central and south-central and other units (Pennsylvanian) at surface, covers 0.5 % of this area

Arkosic sandstone, conglomerate, shale, and limestone. Includes Madera Fm and Sharpsdale Fm of Chronic (1958) in Sangre de Cristo Range and Gothic Fm of Langenheim (1952) in Elk Mountains. Other units of Middle Pennsylvanian age.

Pierre Shale--Lower unit (Cretaceous) at surface, covers 0.4 % of this area

Sharon Springs Member (organic-rich shale and numerous bentonite beds) in lower part

Intra-ash flow andesitic lavas (Tertiary) at surface, covers 0.4 % of this area

Intra-ash flow andesitic lavas

Mesozoic and Paleozoic rocks (Mesozoic-Pennsylvanian) at surface, covers 0.2 % of this area

Mainly as in Mesozoic unit (MZ) plus Permian and Pennsylvanian Fms

Niobrara Fm (Cretaceous) at surface, covers 0.2 % of this area

Calcareous shale and limestone

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 0.2 % of this area

Includes Boulder Creek -M.Y. AGE GROUP (AGE 1650-1730 M.Y.)--Includes Boulder Creek, Cross Creek, Denny Creek, Kroenke, Browns Pass, Powderhorn, Pitts Meadow, Bakers Bridge, and Tenmile Granites, Quartz Monzonites, or Granodiorites; also, unnamed granitic rocks

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite (Tertiary) at surface, covers 0.2 % of this area

Includes basalts of Hinsdale Fm in San Juan Mountains - Servilleta Fm in San Luis Valley and many other occurrences

Intrusive rocks (Cambrian) at surface, covers 0.2 % of this area

Alkalic and mafic intrusive rocks in small plutons, and diabase dikes

Eocene prevolcanic sedimentary rocks (Tertiary) at surface, covers 0.1 % of this area

South-central: Arkosic sand and bouldery gravel of Echo Park Alluvium. Southwest: includes Telluride Conglomerate and Blanco Basin Fm (arkosic mudstone, sandstone, and conglomerate)

Mesozoic rocks (Lower Cretaceous-Triassic) at surface, covers < 0.1 % of this area

Mainly Lower Cretaceous, Jurassic, and Triassic Fms

Pierre Shale, undivided (Cretaceous) at surface, covers < 0.1 % of this area

Pierre Shale, undivided

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

Shale and sandstone. Includes Farisita Conglomerate in northwestern Huerfano County