Anomaly designates lithosphere transition
The structure of the lithosphere along the east coast involves
a transition from continental lithosphere to oceanic lithosphere.
Ancient subduction and collision thickened the continental lithosphere creating the Appalachian Mountains. The continental crust is less dense: it rises high and creates a positive anomaly.
Atlantic Ocean spreading formed the oceanic lithosphere. Sediments shed from North America have since buried part of the younger oceanic crust and loaded it down. The oceanic crust is denser: it sinks low and creates a negative anomaly.
Because these different parts of the lithosphere have differing densities, the margin between them can be seen on a gravity anomaly map.