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Fremont CountyFremont 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that erupted about 1.2 million years ago to form the Mesa Falls tuff, and then subsided since its underpinnings were withdrawn.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, 9/02 Additional ReadingRocks Rails and Trails: pages The Snake River Plain-Yellowstone Hot Spot and its Effect on Drainage Patterns Challis Volcanic Group & Intrusive Rocks References on Idaho Geology |
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Click here to see a correlation of geologic units, and the associated time scale. Click here for a printable version of this map.
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