Geologic units in Conejos county, Colorado

Unclassified surficial deposits and underlying Alamosa Fm in San Luis Valley (Quaternary-Tertiary) at surface, covers 28 % of this area

Alamosa Fm: gravel, sand, silt

Pre-ash-flow andesitic lavas, breccias, tuffs, and conglomerates (Tertiary) at surface, covers 19 % of this area

Includes several named units

Ash-flow tuff of main volcanic sequence (Tertiary) at surface, covers 19 % of this area

Includes many named units

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite (Tertiary) at surface, covers 15 % of this area

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

Los Pinos Fm (Tertiary) at surface, covers 7 % of this area

Volcaniclastic conglomerate interbedded with basalt flows of Hinsdale Fm (Tbb) on east flank of San Juan Mountains. Grades laterally into Santa Fe Fm of San Luis Valley

Intra-ash flow andesitic lavas (Tertiary) at surface, covers 3 % of this area

Intra-ash flow andesitic lavas

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

Includes Broadway and Louviers Alluviums

Landslide deposits (Quaternary) at surface, covers 2 % of this area

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

Modern alluvium (Quaternary) at surface, covers 2 % of this area

Includes Piney Creek Alluvium and younger deposits

Older glacial drift (Quaternary) at surface, covers 1 % of this area

Older glacial drift

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

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

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 0.9 % of this area

Intermediate to felsic compositions

Intra-ash flow quartz latitic lavas (Tertiary) at surface, covers 0.3 % of this area

Intra-ash flow quartz latitic lavas

Glacial drift (Quaternary) at surface, covers 0.2 % of this area

Includes some unclassified glacial deposits

Eocene prevolcanic sedimentary rocks (Tertiary) at surface, covers < 0.1 % of this area

South-central: Arkosic sand and bouldery gravel of Echo Park Alluvium. Southwest: includes Telluride Conglomerate and Blanco Basin Fm (arkosic mudstone, sandstone, and conglomerate)

Dakota, Burro Canyon, Morrison, and Wanakah Fms (Cretaceous-Jurassic) at surface, covers < 0.1 % of this area

Dakota, Burro Canyon, Morrison, and Wanakah Fms

Mancos Shale (Cretaceous) at surface, covers < 0.1 % of this area

In northwest and west-central: Intertongues complexly with units of overlying Mesaverde Group or Fm; lower part consists of a calcareous Niobrara equivalent and Frontier Sandstone and Mowry Shale Members; in areas where the Frontier and Mowry Members (Kmfm), or these and the Dakota Sandstone (Kfd) are distinguished, map unit (Km) consists of shale above Frontier Member. In Southwest: Lower part contains Juana Lopez Member (Kmj)

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers < 0.1 % 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

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

Basalt flows (Quaternary) at surface, covers < 0.1 % of this area

Basalt flows

Lower middle Tertiary volcanic rocks (Lower Oligocene to upper Eocene (older than 31 Ma)) at surface, covers < 0.1 % of this area

Mostly intermediate lavas of the lower Datil Group and intermediate volcaniclastic sediments of the lower Spears Group (Tla+Tvs). Locally includes ash-flow tuffs of the upper Datil Group (Tlrp). Includes intermediate volcaniclastic sedimentary rocks of the Conejos Formation in northern New Mexico.

Landslide deposits and colluvium (Holocene to Pleistocene) at surface, covers < 0.1 % of this area

Landslide deposits on western flanks of Socorro Mountains not shown for clarity.

Paleogene sedimentary units (Paleogene) at surface, covers < 0.1 % of this area

Includes Baca, Galisteo, El Rito, Blanco Basin, Hart Mine, Love Ranch, Lobo, Sanders Canyon, Skunk Ranch, Timberlake, and Cub Mountain Formations.

Lower middle Tertiary rhyolitic to dacitic pyroclastic rocks of the Datil Group, ash-flow tuffs (Lower Oligocene to upper Eocene (31-36 Ma)) at surface, covers < 0.1 % of this area

Regional ash-flow tuffs include Hell's Mesa, Kneeling Nun, Caballo Blanco, Datil Well, Leyba Well, Rock House Canyon, Blue Canyon, Sugarlump, Oak Creek, Bluff Creek, Gillespie, Box Canyon, Cooney and Chiquito Peak Tuffs; the tuffs of Steins Mountain, Black Bill Canyon, Woodhaul Canyon, and Farr Ranch; tuffs of the Organ cauldron; and lower tuffs in the Bell Top Formation. Includes some locally erupted lavas and tuffs within thick intra-caldera units; includes minor volcaniclastic sedimentary units and lavas between thin outflow sheets.

Piedmont alluvial deposits (Holocene to lower Pleistocene) at surface, covers < 0.1 % of this area

Includes deposits of higher gradient tributaries bordering major stream valleys, alluvial veneers of the piedmont slope, and alluvial fans. May locally include uppermost Pliocene deposits.