A regional gravity survey in the eastern Snake River Plain was conducted in the early summer of 1961. Seven hundred and seven gravity stations were established between latitudes 42°15'N and 44°30'N between longitudes 111°30'W and 114°30'W. Three hundred and twenty-five of these stations were located in 2,700 square miles of the eastern part with an average density of one station per 8.3 square miles. The remaining 9,300 square miles were covered by several lines, with an average lineal density of one station per 2.0 miles. A simple-Bouguer gravity contour map has been made of the area by standard methods. The low gravity relief and broad high of the eastern Snake River Plain strongly contrasts with the high amplitude anomalies of the western plain. The major anomalies of the eastern plain consist of 1) a broad high, which is an extension of the large gravity highs of the western plain, 2) a set of elongated alternating lows and highs that trend normal to the axis of the eastern plain, 3) a series of small, local highs on the boundary of the plain, and 4) a prominent low centered over Mud Lake in the northern part of the surveyed area. The basalts of the eastern plain have probably filled troughs or valleys in an undulating subsurface floor rather than a large regional graben.
