Fremont County, Idaho Xm Mds
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Fremont County, Idaho Xm MDs Xm Qs PPP Ms Qf Qf C Henrys o TR s n Lake t Xm PPP Ms i n 20 en t a Xm Sawtell l D Centennial Mtns Tcv iv Ms Peak id e Tcv Xm Pzl Tcv Qf PPP Ms Ms Qs 44o 30 TR s Qf Qf Qb Island Park 111o 40 Qa IslandRes. Park Qs Qf 44o 20 Island Park Caldera Qf Big Bend Ridge Qb Qa Mesa Falls Qf 20 Qf Yellowsone National Park Yellowsone Qb Qs Qf Qf Warm Qb River Qw Qw Qw Ashton St. Anthony Qb k Sand Dunes r o Squirrel F Qf Qb Qb Big Grassy s Qs y Butte r Qa o en 32 44 00 H Qb Qs Qw St. Anthony Qf 112o 00 Teton Dam Qs 20 Site 111o 10 Teton 111o 30 ver Qb Teton Ri 0 5 10 miles 0 8 16 kilometers 1:500,000 Digital Atlas of Idaho, Nov. 2002 http://imnh.isu.edu/digitalatlas Compiled by Paul K. Link, Idaho State University, Geosciences Dept. http://www.isu.edu/departments/geology/ Fremont County Fremont County occupies the northeast corner of the Snake River Plain and includes the western fringe of Yellowstone National Park. It is mainly underlain by volcanic rocks associated with the Snake River Plain and Yellowstone hot spot system. Quaternary sediments overlies these volcanic rocks and allow irrigated farming. The windblown St. Anthony sand dunes contain sand blown northeastward from now-dry lakes to the southwest on the Snake River Plain. The Island Park area forms the central part of Fremont County, and consists of the subsided Island Park Caldera, that erupted about 1.2 million years ago to form the Mesa Falls tuff, and then subsided since its underpinnings were withdrawn. As one drives north on the Ashton grade, one ascends the wall of the caldera. The Centennial Mountains, on the north edge of Fremont County, on the border with Montana, form the Continental divide. These mountains contain Paleoproterozoic gneiss basement, overlain by Cambrian through Triassic sedimentary rocks. These Paleozoic rocks are similar to those of the Teton Mountains, and much thinner than those of the Beaverhead Range to the west. On the southeast edge of Fremont County, south of Ashton, flows the Teton River, cut in a steep canyon through 2 million year old Huckleberry Ridge rhyolite welded tuff. It was here, in 1975, that the newly build Teton Dam broke, causing death and destruction downstream toward Newdale, Teton and Rexburg. P.K. Link, Descripton of Units for Idaho County Geologic Maps 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. Qf Pleistocene silicic volcanic rocks; rhyolite lava and ash-flow tuff (includes Yellowstone Group). Qb Pleistocene basalt lava, 2 million to 12,000 years old, flows have some vegetation and surface weathering. Tcv Eocene Challis Volcanic Group, volcanics and volcaniclastics; Older andesitic lavas, intermediate age dacite lava and tuff and younger rhyolite flows and tuffs; 51 to 44 Ma. (Includes Potato Hill and Kamiah volcanics of northern Idaho). TR s Triassic sedimentary rocks. PPPMs Permian and Carboniferous sedimentary rocks (Snaky Canyon Formation). Ms Mississippian sedimentary rocks. MDs Mississippian and Devonian sedimentary rocks. Xm Paleoproterozoic gneiss in the Centennial Mountains. 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.