Flows and cinder cones of olivine tholeiite basalt in and near Snake River Plain. Largely Pleistocene (<2.6 Ma) but includes flows as old as 3 Ma. Covered with 1-3 m (3-10 ft) of loess. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).
Rhyolite tuffs and flows of Heise volcanic field (6.6-4.5 Ma; includes Blacktail Creek, Walcott, Conant Creek, and Kilgore tuffs), and rhyolite domes and flows of Magic Reservoir area (6.6-3 Ma). (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).
Deposits in valleys consisting of gravel, sand, and silt. Includes younger terrace deposits. May contain some glacial deposits and colluvium in uplands. (Quaternary Sediments).
Gravel and subordinate sand and silt deposited at mouths of canyons; largest fans are in Basin and Range Province in east-central and southeastern Idaho. (Quaternary Sediments).
Marine limestone of Mississippian carbonate banks and turbiditic sandstone, mudstone, and conglomerate of Antler flysch trough; includes Madison and Lodgepole formations and Chesterfield Range Group of southeastern Idaho; McGowan Creek Formation, White Knob Limestone, and overlying carbonate bank of Lost River Range; and Copper Basin Group of Pioneer Mountains. Includes poorly dated Salmon River assemblage east of Stanley, consisting of argillite, siltstone, calcareous sandstone, and limestone. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Fluvial conglomerate and sandstone of the Beaverhead Formation northwest of Dubois. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Marine phosphorite, shale, and chert of Phosphoria Formation, fine-grained sandstone and mudrock of Wells, Quadrant, Amsden, and Shedhorn formations, and fine-grained sandstone, carbonaceous mudstone, and limestone of the Snaky Canyon Formation and Sun Valley and Oquirrh groups. Located in south-central and eastern Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Older gravel, sand, and silt deposited in fans, streams, and lakes. Includes older terrace gravels and Tuana Gravel northwest of Twin Falls. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).
Marine and deltaic sandstone and shale of Cordilleran foreland basin (includes Gannett Group, Frontier, Mowry, Kootenay, and Thermopolis formations) in eastern Idaho thrust belt. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Fluvial and lacustrine deposits; includes conglomerate, sandstone, and shale near Salmon; Pass Creek Gravel, Wet Creek Gravel, and Donkey Fanglomerate north of Mackay; Medicine Lodge beds of southern Beaverhead Mountains; and isolated deposits in northern Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).
Fluvial, fan, and lacustrine deposits and intercalated volcanic rocks of the Basin and Range Province (~16-2 Ma); consolidated to weakly consolidated sandstone, siltstone, arkose, conglomerate, mudstone, tuffaceous sediment, basalt, basaltic tephra, and rhyolite tuff. Includes deposits of Lake Idaho (Idaho Group) in western Snake River Plain and Salt Lake Formation deposited in Basin and Range Province of east-central Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).
Rhyolite tuffs, flows, and domes; includes Yellowstone Group (2.0-0.6 Ma) and isolated domes on Snake River Plain and north of Soda Springs (less than 2.0 Ma). (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).
Feldspathic fine-grained quartzite, siltite, and subordinate argillite and carbonate-bearing rocks in east-central Idaho; includes Apple Creek, West Fork, Inyo Creek, Big Creek, and Gunsight formations. (Mesoproterozoic Metasedimentary Rocks of Lemhi Subbasin of Belt Basin).
Marine dolostone and limestone and sandstone in east-central and southeastern Idaho (Summerhouse, Kinnikinic, and Swan Peak sandstones, Fish Haven, Laketown, Jefferson, Three Forks, and Darby formations) and deep-water carbonaceous mudrocks of Phi Kappa, Trail Creek, and Milligen formations east of Ketchum. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Fine- and medium-grained sand dunes in southern Idaho; includes Bruneau and St. Anthony dune fields. (Quaternary Sediments).
Flows and cinder cones of olivine tholeiite basalt, and shallow basalt intrusives (~15-3 Ma); includes basalt in Owyhee County and southwest of Twin Falls, basalt of Weiser (basalt to andesite), basalt of Cuddy Mountain (alkali basalt and picro-basalt) north of Cambridge, basalt in Mount Bennett Hills north of Gooding, andesite at Square Mountain near Magic Reservoir, and Cub River diabase sill east of Preston. Includes gabbro at depth in cross section D-D’-D”. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).
Red mudrock and fine-grained sandstone and marine limestone; includes Dinwoody, Woodside, Thaynes, and Ankareh formations in eastern Idaho thrust belt. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Only shown in Henrys Lake area and in cross section. Consists of Ms and PlPs (PPAs). (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Marginal marine and marine sandstone and limestone; includes Nugget, Twin Creek, Preuss, Morrison, and Stump formations in eastern Idaho thrust belt. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Upper part consists of shallow marine and fluvial quartzose sandstone, and minor siltite, shale, and limestone of the Brigham Group of southeast Idaho, Wilbert Formation of east-central Idaho, and quartzites of unknown affinity west of Borah Peak. More highly metamorphosed equivalents are quartzite, metaconglomerate, metasiltite, calc-silicate marble, and schist of Gospel Peaks successions C and D in central Idaho; mature quartzite, biotite schist, and minor calc-silicate rocks of Syringa metamorphic sequence east of Moscow; and schist and quartzite in the Albion Range (Elba Quartzite, schist of Upper Narrows, quartzite of Yost, schist of Stevens Spring, quartzite of Clarks Basin, schist of Mahogany Peaks, and Harrison Summit Quartzite). Lower part consists of diamictite, immature sandstone, and bimodal volcanic rocks related to continental rifting. Includes Pocatello Formation (700-665 Ma), formation of Leaton Gulch near Challis, Shedroof Conglomerate in extreme northwest Idaho, and metamorphic equivalents (schist, marble, calc-silicate rocks, metaconglomerate, and ~686 Ma metavolcanic rocks) of Gospel Peaks successions A and B in central Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).
Dacite, andesite, and rhyolite tuffs and flows and subordinate basalt and latite flows; covers large area in south-central Idaho. Includes Absaroka Volcanic Group near Henrys Lake and scattered volcanic rocks in eastern and northern Idaho. (Eocene Challis Magmatic Complex and Related Sedimentary Rocks).
Largely fine-grained sediment, in part playa deposits of evaporative lakes; includes Snake River Group and Bruneau Formation in central Snake River Plain and glacial lake deposits in Bonners Ferry area of northern Idaho. Also includes travertine and tufa northeast of Bancroft and Lake Thatcher sediments in the Gem Valley south of Grace. (Quaternary Sediments).
Unsorted gravel, sand, and clay of landslide origin; includes rotational and translational blocks and earth flows. (Quaternary Sediments).
Reddish gray conglomerate with limestone and quartzite clasts, gray limestone, and grayish brown sandstone. Syntectonic alluvial fan and braided stream. Thickness as much as 3,250 m (10,663 ft).
Western Montana: gray, fine- to medium-grained, locally conglomeratic sandstone with interbeds of dark gray carbonaceous shale. Local conglomerate, porcellanite, and coal. Central Montana: dark gray, carbonaceous shale interbedded with siltstone and sandstone, chert-pebble conglomerate, and coal. Big Elk Sandstone Member: light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black, clayey shale. Boulder River Sandstone Member: gray to greenish gray, very fine-grained to conglomeratic, glauconitic sandstone. Brackish to nonmarine. Thickness exceeds 2,135 m (7,005 ft) in Lima Peaks region, and exceeds 915m in the Greenhorn, Snowcrest, Gravelly, and Pioneer Mountains of southwestern Montana. Elsewhere thickness as much as 215 m (705 ft).
Mafic volcanic rock
Felsic volcanic rock
Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft).
Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.
Dark gray, chert-bearing, fine grained limestone interbedded with subordinate light brown, calcareous, quartzose siltstone to fine sandstone. Marine. Thickness at type section 685 m (2,247 ft).
Woodside Formation: Maroon and red siltstone, dolomite, and shale. Tidal flat. Thickness as much as 250 m (820 ft). Dinwoody Formation (TRd): Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).
Bluebird Mountain Formation: Medium gray to brownish gray quartzite interbedded with yellowish brown, very fine-grained sandstone, gray calcareous siltstone, and medium-gray sandy limestone. Marine. Thickness 275 m (902 ft). Railroad Canyon Formation: Medium gray and brownish black, phosphatic mudstone, shale, limestone, limestone conglomerate, and medium gray sandstone. Marine. Thickness 260 m (853 ft). Surrett Canyon Formation: Dark gray, massive limestone with much scattered white bioclastic debris. Marine. Thickness at type section 67 m (220 ft). South Creek Formation: Dark gray, thin-bedded, chert bearing limestone that alternates with dark gray, argillaceous limestone. Marine. Thickness at type section 91 m (300 ft).
Sediment or sedimentary rock (no unique unit description on map).
Beaverhead Mountains, southwestern Montana - Light gray quartzite with subordinate argillite, siltite, and calc-silicate rocks; unresolved unit that has been variously assigned to Yellowjacket Formation, Lemhi Group, Swauger Formation, or Belt Supergroup. Thickness as much as 2,700 m (8,858 ft).
Shedhorn Formation (Psh): Orangish brown, very fine- to medium-grained, well-sorted, quartzose sandstone. Nearshore marine shelf. Thickness as much as 70 m (230 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft).
Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft).
Upper part: pinkish gray, gray, or brown welded or unwelded tuff that contains abundant phenocrysts of sanidine and quartz, and uncompacted pumice fragments at the top.
Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).
Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft).