Geologic units in Grand county, Colorado

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 17 % of this area

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

Coalmont Fm (Tertiary) at surface, covers 16 % of this area

Arkosic sandstone, conglomerate, and shale; coal in lower part; in North Park

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 12 % 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

Troublesome Fm (Tertiary) at surface, covers 9 % of this area

Sandstone and siltstone; in Middle Park

Pierre Shale, undivided (Cretaceous) at surface, covers 8 % of this area

Pierre Shale, undivided

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

Glacial drift (Quaternary) at surface, covers 5 % of this area

Includes some unclassified glacial deposits

Colorado Group (Cretaceous) at surface, covers 4 % of this area

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

Volcanic rocks in northwestern Colorado (Tertiary) at surface, covers 4 % of this area

Mainly of intermediate compositions

Landslide deposits (Quaternary) at surface, covers 3 % of this area

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

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 2 % of this area

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

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

Includes Broadway and Louviers Alluviums

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 1 % of this area

Intermediate to felsic compositions

Windy Gap Member of Middle Park Fm (Cretaceous) at surface, covers 1 % of this area

Andesitic breccia and conglomerate

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

Includes Piney Creek Alluvium and younger deposits

Dakota Sandstone or Group (Cretaceous) at surface, covers 1.0 % of this area

Dakota Sandstone or Group

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite (Tertiary) at surface, covers 0.8 % of this area

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

Mesozoic rocks (Lower Cretaceous-Triassic) at surface, covers 0.8 % of this area

Mainly Lower Cretaceous, Jurassic, and Triassic Fms

Bouldery gravel on old erosion surfaces in Front Range and Never Summer Mountains (Tertiary) at surface, covers 0.7 % of this area

Bouldery gravel on old erosion surfaces in Front Range and Never Summer Mountains

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

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

Morrison Fm and Sundance Fm (Jurassic) at surface, covers 0.6 % of this area

Sundance Fm: Sandstone, shale, claystone, and limestone

Dakota, Morrison, and Sundance Fms (Cretaceous-Jurassic) at surface, covers 0.5 % of this area

Dakota, Morrison, and Sundance Fms

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

Older glacial drift

Morrison Fm (Jurassic) at surface, covers 0.4 % of this area

Variegated claystone, mudstone, sandstone, and local beds of limestone

Dakota and Morrison Fms (Cretaceous-Jurassic) at surface, covers 0.4 % of this area

Dakota and Morrison Fms

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 0.3 % 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

Minturn Fm in west-central and south-central and other units (Pennsylvanian) at surface, covers 0.3 % 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.

Chinle and Chugwater Fms (Triassic) at surface, covers 0.2 % of this area

Chinle and Chugwater Fms

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

Gabbro and mafic diorite and monzonite

Chugwater Fm (Triassic) at surface, covers 0.2 % of this area

Red sandstone, siltstone, shale, and local limestone and gypsum

Ash-flow tuff of main volcanic sequence (Tertiary) at surface, covers 0.2 % of this area

Includes many named units

Jelm, Lykins, Lyons, and Satanka Fms (Triassic-Permian) at surface, covers 0.1 % of this area

Red siltstone, shale, and sandstone

Browns Park Fm (Tertiary) at surface, covers 0.1 % of this area

Sandstone and siltstone; west of Park Range

Leadville, Gilman, Dyer, Parting, and Sawatch Fms in west-central and south-central. Leadville Limestone, Ouray Limestone, Elbert Fm, and Ignacio Quartzite in far southwest (Mississipian-Cambrian) at surface, covers < 0.1 % of this area

Elbert Fm: shale and sandstone.

Chinle Fm (Triassic) at surface, covers < 0.1 % of this area

Red siltstone, sandstone, and limestone-pellet conglomerate

Chinle And State Bridge Fms (Triassic-Permian) at surface, covers < 0.1 % of this area

Red siltstone and sandstone

Rhyolitic intrusive rocks and flows of late-volcanic bimodal suite (Tertiary) at surface, covers < 0.1 % of this area

Rhyolitic intrusive rocks and flows of late-volcanic bimodal suite

Morrison Fm and Entrada Sandstone (Jurassic) at surface, covers < 0.1 % of this area

Morrison Fm and Entrada Sandstone

Laramide intrusive rocks (Tertiary-Cretaceous) at surface, covers < 0.1 % of this area

Mainly intermediate to felsic compositions; some mafic

Maroon Fm (Permian-Pennsylvanian) at surface, covers < 0.1 % of this area

Arkosic sandstone, siltstone, conglomerate, and local limestone

Upper Tertiary intrusive rocks (Tertiary) at surface, covers < 0.1 % of this area

Intermediate to felsic compositions