Geologic units in Alamosa county, Colorado

Unclassified surficial deposits and underlying Alamosa Fm in San Luis Valley (Quaternary-Tertiary) at surface, covers 81 % of this area

Alamosa Fm: gravel, sand, silt

Eolian deposits (Quaternary) at surface, covers 13 % of this area

Includes dune sand and silt and Peoria Loess

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 5 % of this area

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

Minturn Fm in west-central and south-central and other units (Pennsylvanian) at surface, covers 0.4 % 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.

Mafic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 0.3 % of this area

Gabbro and mafic diorite and monzonite

Pre-ash-flow andesitic lavas, breccias, tuffs, and conglomerates (Tertiary) at surface, covers < 0.1 % of this area

Includes several named units

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

Intermediate to felsic compositions

Glacial drift (Quaternary) at surface, covers < 0.1 % of this area

Includes some unclassified glacial deposits