The geology of western and northern Gallatin National Forest was mapped in two parts as part of an evaluation of its mineral resource potential by the USGS. Previously, the western part was released as Map A and the northern part as Map B of USGS Geologic Investigations Series Map I-2584. These maps were compiled, digitized, and produced using the USGS software program GSMAP. All the original source maps were simplified on mylar overlays, digitized at their original map scales and projections, and then merged into the final maps. 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 by the USDA Forest Service). For purposes of resource assessment, many rock units have been combined on these maps. For details, the user should consult the original source maps.