Teleseismic P wave arrival times recorded by the Northeastern Seismic Network are used ot invert for lateral crust and upper mantle structure to depths of 500 km. Three-dimensional inversion of the travel time data between two ancient orogenic provinces suggests that structures down to possibly 200 km and greater can be correlated with surficial geologic and tectonic features. This has the important implication that major orogenic belts have effects that reach well into the lithosphere which are stable for extended periods of time, perhaps as long as 1 b.y. The crust beneath the Paleozoic Appalachian Province is characterized by slightly greater thicknesses and lower average velocities than that of the Precambrian Grenville Province. The higher average velocities associated with the Grenville Province extend to depths of 200 km and appear to be maximum beneath the Adirondack dome. A relatively low velocity anomaly extending to depths in excess of 200 km and dipping to the northwest shows a spatial correlation with the Bronson Hill--Boundary Mountains Anticlinorium in central New Hampshire and Maine. These structures occupy the sites of a complex series of island arc sequences last active in Early Devonian time prior to the Acadian orogeny. This low-velocity region may represent subducted oceanic lithosphere which has undergone post-orogenic radioactive heating.