These maps show variations in the Earth's magnetic field caused primarily by the uneven distribution of the mineral magnetite in the rocks that make up the upper part of the Earth's crust. The features and patterns of the maps reveal details of subsurface geology including the locations of buried faults, the location of magnetite-bearing rocks, which include many kinds of rocks of interest to mineral exploration and environmental studies, and the thickness of surficial sedimentary rocks.

The map is constructed from grids that combine information collected in 65 separate aeromagnetic surveys conducted over the past 40 years. Data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. An index plot gives an overview of the flightline spacing of the original surveys. A data table summarizes the detailed specifications of the surveys.

This project was supported by the Mineral Resource Program of the USGS.