Geologic units in Summit county, Colorado

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 24 % of this area

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

Glacial drift (Quaternary) at surface, covers 14 % of this area

Includes some unclassified glacial deposits

Pierre Shale, undivided (Cretaceous) at surface, covers 12 % of this area

Pierre Shale, undivided

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 8 % 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 7 % 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

Laramide intrusive rocks (Tertiary-Cretaceous) at surface, covers 6 % of this area

Mainly intermediate to felsic compositions; some mafic

Minturn Fm in west-central and south-central and other units (Pennsylvanian) at surface, covers 4 % 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.

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)

Maroon Fm (Permian-Pennsylvanian) at surface, covers 4 % of this area

Arkosic sandstone, siltstone, conglomerate, and local limestone

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 3 % of this area

Intermediate to felsic compositions

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 3 % 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

Older glacial drift (Quaternary) at surface, covers 2 % of this area

Older glacial drift

Morrison Fm (Jurassic) at surface, covers 2 % of this area

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

Dakota and Morrison Fms (Cretaceous-Jurassic) at surface, covers 2 % of this area

Dakota and Morrison Fms

Landslide deposits (Quaternary) at surface, covers 1 % of this area

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

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

Includes Broadway and Louviers Alluviums

Dakota Sandstone or Group (Cretaceous) at surface, covers 0.6 % of this area

Dakota Sandstone or Group

Modern alluvium (Quaternary) at surface, covers 0.6 % of this area

Includes Piney Creek Alluvium and younger deposits

Dry Union Fm (Tertiary) at surface, covers 0.5 % of this area

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

Leadville Limestone, Williams Canyon Limestone , Manitou Limestone, and Sawatch Quartzite (Mississippian-Cambrian) at surface, covers 0.4 % of this area

Leadville Limestone, Williams Canyon Limestone , Manitou Limestone, and Sawatch Quartzite

Volcanic rocks in northwestern Colorado (Tertiary) at surface, covers 0.3 % of this area

Mainly of intermediate compositions

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

Sandstone and siltstone; in Middle Park

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 0.2 % 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

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

Red siltstone, sandstone, and limestone-pellet conglomerate