Geologic units in Gilpin county, Colorado

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 44 % of this area

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

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 23 % 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 22 % 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

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

Includes some unclassified glacial deposits

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

Mainly intermediate to felsic compositions; some mafic

Bouldery gravel on old erosion surfaces in Front Range and Never Summer Mountains (Tertiary) at surface, covers 0.9 % of this area

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

Mafic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 0.8 % of this area

Gabbro and mafic diorite and monzonite