Cassia County, Qs Idaho 113o00’10” o Qw 42 35’05” 86 Qw ke iver PPsP Sna R o PPs 114 03’50” Qa Burley o Qb Sublett Rge. 42 30’55” Declo Qs 77 PPsP Qm PPsP Tov Zs Wm QTb Qa PPs QTb Tpf Idahome PPsP Qs Qs PPsP 27 Albion Tps Qs QTb Ps Qs 84 k Zs e Qw Qs

e

r Tpf 81 TsR Qw C Zs Sublett P Tpf Wm PPs e Malta Qs

s Tov Cassia o Wm

o Zs Wm Pzu G Tpf Mtns. or Mt. Harrison South Hills PPsP PPsP PPsP Qa Oakley PzZm Elba Ps Ps

Qs Qs r e Pzu Tpf Zs Raft River Valley Ps

PPsP Ps Blackpine Rge. v

Zs i

R Tps Tpf Qs Tpf Wm t PzZm f PPs Tpf Zs Tpf a Trapper Pk. Cache Pk. Tms R Qa Tpf Toi Toi PPsP Tpf Zs Tpf Toi Qs Qs Almo Tpf Ms PPsP Tpf Albion Mtns. Wm PPs Zs Zs Toi PPs PPs Almo Qs Qa PzZm Pluton Raft RiverTpf Narrows. Qa Tpf Zs City of Tms Qs Tps Tpf PPsP Qbo Qs Rocks Tpf Qs o o 42 00’04” 41 59’32” o 114o16’55” 113 00’05”

Digital Atlas of Idaho, Nov. 2002 0 5 10 miles http://imnh.isu.edu/digitalatlas Compiled by Paul K. Link, 0 8 16 kilometers Idaho State University, Geosciences Dept. http://www.isu.edu/departments/geology/ Cassia County Cassia County, on the south side of the Snake River forms much of Idaho’s southern boundary with Utah and Nevada on the west. It contains a diverse assemblage of rocks, including the oldest rocks in Idaho, the metamorphic Green Creek gneiss in the Albion Mountains core complex. Most of the people in Cassia County live in irrigated agricultural areas near Burley, Declo and Oakley. This area, on the southern edge of the Snake River Plain, is underlain by Quaternary basalt, including shield volcanoes visible today, overlain by windblown loess and alluvium. Thick cobble gravel derived from the Albion Range in Pleistocene time underlies the Oakley valley. The Albion Mountains, south of Albion, and west of Elba and Almo, are one of a series of turtleback-shaped uplifts of metamorphosed middle crust that are found along the axis of the Mesozoic Cordilleran orogenic belt, generally at the location of maximum crustal thickening during compressional tectonics. They contain Archean gneiss overlain by Proterozoic Elba quartzite and Paleozoic Clarks Basin and Harrison Summit schist and quartzite, below a low-angle detachment fault. Above the fault are less metamorphosed, folded Paleozoic rocks. The Raft River Mountains, just to the south across the Utah border are a similar feature. The Oligocene Almo Pluton, which underlies the City of Rocks, was intruded 25 million years ago into this domal uplift. The City of Rocks contains distinctive granite monoliths that were a landmark to the California Trail and are now a rock climber’s playground. The Raft River detachment fault, an east dipping structure, exhumed the Albion core complex and downdropped the Raft River Valley and the volcanic rocks of the Jim Sage and Cottrell Mountains between Elba and Malta. The U.S. Department of Energy drilled a several-thousand-foot geothermal well south of Malta in the 1980s, but the project did not demonstrate economic potential for electrical generation. Southeast of the Raft River Valley are the Black Pine Mountains, underlain by folded and thrust-faulted Paleozoic rocks in the hanging wall of the Raft River detachment. These rocks formerly were on top of the City of Rocks area, and have been downfaulted to the east to their present location. They contain gold mineralization that was mined in the 1990s and at several previous times in the 20th century. The Sublett Range, east of the Raft River Valley, contains thick upper Paleozoic limestones. To the west of the Albion Range is Middle Mountain, which contains schistose quartzite that is mined for “Oakley Stone” a decorative flagstone shipped across the world. West of there, across Goose Creek, is the extensive Miocene Trapper Creek basin filled with tuffs and rhyolite ignimbrites from the Twin Falls volcanic center. The Cassia Mountains or South Hills, on the western edge of the county, contain spectacular cliffs in deep canyons, all cut from these rhyolite ignimbrites and fallout tuffs. See several sections of text in Rocks Rails and Trails. P.K. Link, 10/02

Descripton of Geologic Units for Cassia County, Idaho Qa Quaternary alluvial deposits

Qm Quaternary moraine (unsorted boulders, cobbles and sand) and glaciofluvial outwash (bedded stream deposits formed from streams draining active glacial ice). Qs Quaternary surficial cover, including colluvium, fluvial, alluvial fan, lake, and windblown deposits. Included fluveolian cover on Snake River Plain, (Snake River Group). Qw Quaternary windblown deposits; sand dunes and loess.

Qbo Lake Bonneville deposits, fine-grained lake beds.

Qb Pleistocene basalt lava, 2 million to 12,000 years old, flows have some vegetation and surface weathering. QTb Pleistocene and Pliocene basalt lava and associated basaltic tuff (deposited close to basaltic vent). Tps Pliocene and Upper Miocene stream and lake deposits (Salt Lake Formation, Starlight Formation, Idaho Group). Tpf Pliocene and Upper Miocene felsic volcanic rocks, rhyolite flows, tuffs, ignimbrites. (in Owyhee County and Mt. Bennett Hills, this should be Tmf). Tms Pliocene and Upper Miocene sedimentary rocks and interlayered basalts (parts of Starlight Formation, Salt Lake Formation). Tov Oligocene volcanics; Potlatch volcanics, basalt and trachytic pyroclastic rocks [alkali-rich basalts]) and Salmon Falls Creek volcanics [andesites]. Toi Oligocene City of Rocks granite (About 25 Ma).

TR s Triassic sedimentary rocks.

Pzu Upper Paleozoic sedimentary rocks.

Ps Permian sedimentary rocks.

PPPs Permian and Pennsylvanian sedimentary rocks.

PPs Pennsylvanian sedimentary rocks.

Ms Mississippian sedimentary rocks.

PzZm Paleozoic/Neoproterozoic metasedimentary rocks, mainly quartzose sandstone (includes formation of Leaton Gulch). Zs Neoproterozoic sedimentary rocks undivided.

Wm Archean gneiss and schist, Albion Range, and Priest River complex west of Sandpoint.

Symbols

Geologic unit contacts with unit Overturned anticline: trace of axial Yp designation. plane.

Normal fault: certain; dashed where Overturnedsyncline: trace of axial approximately located; dotted where plane. concealed. Location of ISU Rockwalk rock Thrust fault: certain; dashed where from each county. approximately located; dotted where concealed. Cities Detachment fault: certain; dashed Feature location where approximately located; dotted where concealed. Roads Anticline: trace of axial plane: large Interstate Route arrow indicates direction of plunge. 15 U.S. Route Syncline: trace of axial plane: large 95 arrow indicates direction of plunge. 1 State route