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Island Park, situated between the northeastern end of the Snake River Plain and the western margin of the Yellowstone Plateau, is traversed by U.S. 20-191 between Ashton, Idaho, and West Yellowstone, Montana (Fig. 1) and, in part, by the Mesa Falls Road (Idaho 47). All of the physiographic and geologic features described in this guide can be reached during the summer months by passenger car on those two paved highways or on short gravel roads that extend from them. During the winter, U.S. 20-191 remains open, but other winter travel is possible only by skis or skimobile. One quarry locality described later is privately owned but has generally been freely accessible.

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