Aeromagnetic and gravity maps of part of eastern Montana indicate the existence of several distinct crustal units whose influence must be considered in the interpretation of seismic data recorded at the Large Aperture Seismic Array. The magnetic map suggests the area contains 5 major crustal units separated by fault-like boundaries. The principal features of the gravity map are a marked high over the Porcupine dome, and a 40- by 70-mile gravity low to the northeast of Porcupine dome.