 The geology of Gallatin National Forest (GNF) was mapped as part of an evaluation of its mineral resource potential by the U.S. Geological Survey (USGS). Map A, the western part of GNF, south of Interstate Highway 90 (I-90) and west of the Yellowstone River, includes the Gallatin and Madison Ranges and the West Yellowstone area. Map B, the northern part of GNF, north of I-90, includes the Bridger Range and Crazy Mountains. The eastern part of GNF, south of I-90 and east of the Yellowstone River, was examined previously by the USGS as part of the mineral resource potential study of the Absaroka-Beartooth study area and is published separately. The colored maps of this report supersede a black-and-white version. Both maps were digitized, compiled, and produced using the USGS software program GSMAP and unreleased proprietary updates to it. All original source maps were digitized at their original scales and map projections, then merged into the final digital map formats. After editing, the digital maps were filtered to remove all points not necessary for improving resolution at a scale of 1:126,720 (2 mi per in., the scale used in this report and by the U.S. Forest Service). Rocks range in age from Archean to Holocene. For purposes of resource assessment, many rock units have been combined on these maps. Only brief descriptions of rock units are presented here; the reader is referred to original sources for detailed rock unit descriptions.
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