A regional gravity survey of western and central Montana comprising data from more than 2,700 gravity stations was interpreted by automated computer techniques, including use of least-square polynomial surfaces as approximations to the regional gravity field.

The Bouguer gravity anomaly map of western and central Montana shows a large northwest-trending gravity low of -175 mgal across the fold-and-thrust belt of the Montana Rockies and two northwest-trending zones of gravity contours that increase sharply eastward. One of the zones is in the Lewis thrust area and the other is east of the Disturbed Belt. An increase in the Bouguer gravity anomalies continues eastward from the Montana Rockies to the Great Plains of eastern Montana, where gravity values of more than -55 mgal are found.

Two prominent northeast-trending residual gravity anomalies of +20 mgal in the western Montana plains probably reflect igneous ridges at the top of the basement. Northeast-trending residual anomalies of +20 mgal are associated with the Bearpaw arch; they are believed to be reflections of large near-surface intrusive bodies, perhaps cupola features of a deep-seated parent body. Distinctive positive residual anomalies over the Big Snowy and Women's Pocket uplifts and over the Porcupine and Bowdoin domes are attributed to uplifted crustal blocks that produced arching in the sedimentary sequence. Several significant negative residual anomalies are believed to be expressions of structural basins between positive features. The general northeast trend of the regional gravity anomalies of wes ern and central Montana suggests a genetic relation with the pre-Paleozoic structural grain of southern Canada and the north-central United States.