Geologic units in Boulder county, Colorado

Additional scientific data in this geographic area

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 20 % 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

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 16 % 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

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 13 % of this area

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

Laramie Fm and Fox Hills Sandstone (Cretaceous) at surface, covers 8 % of this area

Laramie Fm and Fox Hills Sandstone

Pierre Shale--Upper unit (Cretaceous) at surface, covers 7 % of this area

Pierre Shale--Upper unit

Glacial drift (Quaternary) at surface, covers 6 % of this area

Includes some unclassified glacial deposits

Modern alluvium (Quaternary) at surface, covers 3 % of this area

Includes Piney Creek Alluvium and younger deposits

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

Fox Hills Sandstone

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

Includes dune sand and silt and Peoria Loess

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

Laramide intrusive rocks (Tertiary-Cretaceous) at surface, covers 3 % of this area

Mainly intermediate to felsic compositions; some mafic

Pierre Shale--Middle unit (Cretaceous) at surface, covers 2 % 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

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

Includes Broadway and Louviers Alluviums

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

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

Dakota, Morrison, and Sundance Fms

Colorado Group (Cretaceous) at surface, covers 2 % of this area

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

Lykins Fm and Lyons Sandstone (Triassic-Permian) at surface, covers 2 % of this area

Lykins Fm and Lyons Sandstone

Fountain Fm (Permian-Pennsylvanian) at surface, covers 1 % of this area

Arkosic sandstone and conglomerate

Lykins Lyons and Fountain Fms (Triassic-Pennsylvanian) at surface, covers 0.8 % of this area

Red siltstone, sandstone, and conglomerate

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 0.8 % of this area

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

Ingleside Fm and Fountain Fm (Permian-Pennsylvanian) at surface, covers 0.4 % of this area

Ingleside Fm: Limestone and calcareous sandstone. Fountain Fm: arkosic sandstone and conglomerate

Laramie Fm (Cretaceous) at surface, covers 0.3 % of this area

Shale, claystone, sandstone, and major coal beds

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 0.2 % 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

Bouldery gravel on old erosion surfaces in Front Range and Never Summer Mountains (Tertiary) at surface, covers 0.2 % of this area

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

Denver and Arapahoe Fms (Tertiary-Cretaceous) at surface, covers 0.1 % of this area

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

Quartzite, conglomerate, and interlayered mica schist (Early Proterozoic) at surface, covers < 0.1 % of this area

Quartzite, conglomerate, and interlayered mica schist