Two aeromagnetic anomalies of regional extent outline two previously unknown buried masses of highly magnetic, probably Precambrian, rocks in southeastern New York and adjacent Vermont, Massachusetts, and Connecticut. The northern mass extends northeastward from Albany, New York, to Bennington, Vermont, where it appears to be buried beneath weakly magnetic Precambrian rocks of the Green Mountains. The southern mass extends north-northeastward from Beacon, New York, through Stissing Mountain, to Copake, New York, and appears to be the buried northeast extension of the Reading Prong. The shape of the Beacon-Copake magnetic anomaly indicates that the source is near the surface and has a sharp boundary, probably a fault, on its northwestern side; the shape also indicates that the source becomes deeply buried to the southeast and thus supports a parautochthonous interpretation for the northern part of the Reading Prong. In southwestern Massachusetts, the highly magnetic Beacon-Copake mass appears to be overstepped on the east by a buried slice of weakly magnetic Precambrian rocks which, in turn, is overstepped on the east by imbricate slices of weakly magnetic Precambrian rocks exposed along the western front of the Berkshire Highlands.

Precambrian rocks exposed in the Green Mountains, the Berkshire and Housatonic Highlands, the eastern part of the Hudson Highlands, and the Manhattan Prong have a remarkably lower amplitude magnetic pattern than those in the Adirondack Mountains and the Reading Prong. This difference in magnetic character appears to represent more than different thicknesses of Precambrian rocks and may reflect a different sequence of Precambrian rocks to the east, a lower grade of Precambrian metamorphism to the east, or possibly a reduction in magnetite content in the eastern Precambrian rocks because of Paleozoic metamorphism.