Geologic units in Fremont county, Colorado

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 19 % of this area

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

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 17 % 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

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 6 % 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 5 % of this area

Includes several named units

Niobrara Fm (Cretaceous) at surface, covers 4 % of this area

Calcareous shale and limestone

Minturn and Belden Fms (Pennsylvanian) at surface, covers 4 % of this area

Minturn and Belden Fms

Older gravels and alluviums (Quaternary) at surface, covers 3 % of this area

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

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 3 % of this area

Locally contains minor hornblende gneiss, calc-silicate rock, quartzite, and marble. Derived principally from sedimentary rocks

Ash-flow tuff of main volcanic sequence (Tertiary) at surface, covers 3 % of this area

Includes many named units

Sangre de Cristo Fm (Permian-Pennsylvanian) at surface, covers 2 % of this area

Arkosic conglomerate, sandstone, and siltstone

Dakota Sandstone and Purgatoire Fm (Cretaceous) at surface, covers 2 % of this area

Sandstone and shale

Dakota Group and Morrison And Ralston Creek Fms at mountain front between Boulder and Colorado Springs. Dakota, Purgatoire, Morrison, and Ralston Creek Fms in Canon City area (Cretaceous-Jurassic) at surface, covers 2 % of this area

Dakota Group and Morrison And Ralston Creek Fms at mountain front between Boulder and Colorado Springs. Dakota, Purgatoire, Morrison, and Ralston Creek Fms in Canon City area

Fountain Fm (Permian-Pennsylvanian) at surface, covers 2 % of this area

Arkosic sandstone and conglomerate

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite (Tertiary) at surface, covers 2 % of this area

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

One or more Ordovician fms (Ordovician) at surface, covers 2 % of this area

Fremont Limestone, Harding Sandstone, and Manitou Limestone

Pierre Shale--Upper unit (Cretaceous) at surface, covers 2 % of this area

Pierre Shale--Upper unit

Leadville Limestone, Williams Canyon Limestone, and one or more of: Fremont Limestone, Harding Sandstone, and Manitou Limestone (Mississippian-Ordovician) at surface, covers 2 % of this area

Leadville Limestone, Williams Canyon Limestone, and one or more of: Fremont Limestone, Harding Sandstone, and Manitou Limestone

Vermejo Fm and Trinidad Sandstone (Cretaceous) at surface, covers 2 % of this area

Shale, sandstone, and major coal beds; sandstone

Pierre Shale--Lower unit (Cretaceous) at surface, covers 2 % of this area

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

Gravels and alluviums (Quaternary) at surface, covers 1 % of this area

Includes Broadway and Louviers Alluviums

Modern alluvium (Quaternary) at surface, covers 1 % of this area

Includes Piney Creek Alluvium and younger deposits

Intrusive rocks (Cambrian) at surface, covers 1 % of this area

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

Laramide intrusive rocks (Tertiary-Cretaceous) at surface, covers 1 % of this area

Mainly intermediate to felsic compositions; some mafic

Carlile Shale, Greenhorn Limestone, and Graneros Shale (Cretaceous) at surface, covers 1 % of this area

Carlile Shale, Greenhorn Limestone, and Graneros Shale

Santa Fe Fm (Tertiary) at surface, covers 1 % of this area

Siltstone, sandstone, and conglomerate

Eocene prevolcanic sedimentary rocks (Tertiary) at surface, covers 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)

Colorado Group (Cretaceous) at surface, covers 1.0 % of this area

Consists of Niobrara Formation (Kn) and either Benton Shale or Carlile, Greenhorn, and Graneros Fms (Kcg)

Mafic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 0.9 % of this area

Gabbro and mafic diorite and monzonite

Raton Fm (Tertiary-Cretaceous) at surface, covers 0.9 % of this area

Arkosic sandstone, siltstone, and shale; contains major coal deposits in Raton Basin

Landslide deposits (Quaternary) at surface, covers 0.9 % of this area

Locally includes talus, rock-glacier, and thick colluvial deposits

Dry Union Fm (Tertiary) at surface, covers 0.7 % of this area

Siltstone, sandstone, and conglomerate. Includes Wagontongue Fm (Miocene) in South Park

Oligocene sedimentary rocks (Tertiary) at surface, covers 0.6 % of this area

Northwest: includes Duchesne River Fm (sandstone and shale; includes some rocks of Eocene age) and Bishop Conglomerate near Utah border. South-central: includes Florissant Lake Beds (tuffaceous shale and tuff) and Antero Fm (limestone tuff, tuffaceous sandstone, and conglomerate). Southwest: includes Creede Fm (tuffaceous siltstone, sandstone, conglomerate) and gravels interbedded with volcanic rocks northeast and southeast of Gunnison

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 0.6 % of this area

Intermediate to felsic compositions

Intra-ash flow andesitic lavas (Tertiary) at surface, covers 0.5 % of this area

Intra-ash flow andesitic lavas

Glacial drift (Quaternary) at surface, covers 0.5 % of this area

Includes some unclassified glacial deposits

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

In Boulder-Fort Collins area, contains Richard, Larimer, Rocky Ridge, Terry, and Hygiene Sandstone Members; elsewhere shale between zones of Baculites reesidei and B. scotti

Wall Mountain Tuff (Tertiary) at surface, covers 0.4 % of this area

Early ash-flow tuff of Sawatch Range provenance

Evaporitic facies of Minturn And Belden Fms in South Park and southward (Pennsylvanian) at surface, covers 0.4 % of this area

Gypsum, siltstone, and shale

Pierre Shale, undivided (Cretaceous) at surface, covers 0.4 % of this area

Pierre Shale, undivided

Morrison and Ralston Creek Fms (Jurassic) at surface, covers 0.3 % of this area

Ralston Creek Fm: Claystone, sandstone, limestone, and gypsum

Poison Canyon Fm (Tertiary) at surface, covers 0.2 % of this area

Arkosic conglomerate, sandstone, and shale

Quartzite, conglomerate, and interlayered mica schist (Early Proterozoic) at surface, covers 0.2 % of this area

Quartzite, conglomerate, and interlayered mica schist

Dakota and Morrison Fms (Cretaceous-Jurassic) at surface, covers 0.1 % of this area

Dakota and Morrison Fms

Lykins Fm (Triassic-Permian) at surface, covers 0.1 % of this area

Red siltstone, shale, and limestone

Eolian deposits (Quaternary) at surface, covers < 0.1 % of this area

Includes dune sand and silt and Peoria Loess

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

Mainly Lower Cretaceous, Jurassic, and Triassic Fms

Lykins Fm and Lyons Sandstone (Triassic-Permian) at surface, covers < 0.1 % of this area

Lykins Fm and Lyons Sandstone

Dakota Sandstone or Group (Cretaceous) at surface, covers < 0.1 % of this area

Dakota Sandstone or Group