Pendulum stations occupied by the U. S. Coast and Geodetic Survey at locations chosen for their geologic significance provide a net in southern New England, eastern New York, and northern New Jersey comprising about 120 stations. Numerous supplemental stations have been added with a modem gravimeter. Specific gravities of representative rocks near the pendulum stations have been determined.
Isanomaly maps based on isostatic, Bouguer, and free-air anomalies are remarkably alike in their major features. Some groups of anomalies reflect bedrock units that have exceptional densities. When corrections are applied for these local masses, regional tendencies are clarified. Two belts of negative anomalies are separated by a belt of positive anomalies. The negative belts correspond in a general way to axes of subsidence during Paleozoic periods. The entire region experienced orogenic deformation, followed by peneplanation.
Since the groups of Bouguer anomalies are little affected by the ordinary isostatic corrections, and are related only in minor degree to the visible bedrock, it appears that they either indicate considerable departures from isostatic balance or reflect differences in density that are concealed. Glennie's hypothesis of crustal warping, in a modified form, offers an attractive explanation of the outstanding anomalies.