During the period 1960–1964 a gravity survey covering 25,000 km2 in southwestern Montana was made to determine (1) the source, subsurface shapes and extents of the Boulder and Tobacco Root batholiths, and (2) the pre-fill configurations and possible origin of several Cenozoic basins.
Approximately 6250 gravity stations were occupied using two gravity meters: a C. H. Frost with a scale constant of 0.754 mgal per scale division and a Pioneer model Worden meter with scale constant of 0.0934 mgal per scale division. Most elevations were determined using Paulin altimeters controlled by bench mark and map elevations; some vertical control was established with a Dumpy level. The final Bouguer gravity values are referenced to Pendulum Station 110 [Duerksen, 1949] in Boulder, Montana, and the resultant contour map (Figure 1) has been tied to maps published by Davis et al. [1963; 1965a, b] and Kinoshita et al. [1964 a, b].