A regional gravity survey of the Cuyuna Iron Range, Minnesota, was conducted during the summer of 1955 by the U. S. Geological Survey. It was believed that gravity data
would aid in the understanding of the major structures of the range.
It was found that synclinal and steeply dipping structures produced positive gravity anomalies while anilclinal structures produced negative anomalies. This principle was noted in areas of well known geology and then applied to outlying areas of the district. The outstanding gravity feature is a narrow positive anomaly extending from south of Brainerd eastward through Aitkin, the axis of the anomaly being somewhat south of the South Range. This gives support to the theory that the Biwabik formation passes under the stratigraphically higher South Range member as a synclinal structure. However, this anomaly is also explained using one main iron formation and assuming an anticlinal structure between the North and South Ranges and a syncline between the South Range and Bay Lake. A large magnitude negative gravity anomaly north of Mille Lacs is postulated to result from an intrusive mass extending to the erosion surface. Aeromagnetics are used to strengthen the gravity interpretation.
The need is realized for additional regional gravity coverage to the east of the area Included in this survey and a detailed gravity study of the area between the North and South Ranges.