A set of aeromagnetic profiles for a strip 100 mi. wide extending from the Rocky Mountains to the edge of the continental shelf off the eastern coast of the United States reveals large anomalies of major crustal significance. The data consist of 20 profiles flown at a barometric altitude of approximately 5,000 feet, centered along a line from Denver, Colorado, to Washington, D.C., and spaced approximately 5 mi. apart.

Several distinct anomalous patterns reflecting basement lithology can be clearly recognized. These include the Piedmont province in the eastern United States and the basalt flows in Iowa and Nebraska (mid-continent gravity "high"). The most intense anomalies occur in central and western Nebraska. Two anomalous areas in eastern Iowa and western Ohio probably are related to a single tectonic province and are correlatable with a pronounced horseshoe-shape gravity feature extending from eastern Iowa through eastern Minnesota across northern Lake Michigan and through the central part of Michigan and western Ohio. A linear magnetic anomaly at least 100 mi. long and 35 mi. wide is present over the Appalachian plateau and parallels the Appalachian structural trends.

The large amplitude and areal extent of the magnetic anomalies and their obvious correlation with known gravity anomalies suggest intrabasement units of correspondingly high acoustic velocities. It is clear that these vertical lithologic discontinuities must be taken into account in deep refraction studies of the crust and upper mantle; otherwise, seismic interpretations based on horizontal discontinuities alone yield incorrect crustal thicknesses.

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