Crust and mantle discontinuities across the eastern margin of the North American craton were imaged using P to S converted phase receiver functions recorded by the Missouri to Massachusetts Broadband Seismometer Experiment. Crustal structure constrained by modeling Moho conversions and reverberations shows a variation of Moho depth from a minimum of 30 km near the Atlantic coast to depths of 44--49 km beneath the western Appalachian Province and 38--45 km beneath the Proterozoic terranes in the west. The variation in crustal thickness is substantially greater than that required for local isostasy, unless lower crustal densities are >3110 kg/m3. In the upper mantle, Ps phases corresponding to a discontinuity at depths of 270--280 km were clearly observed beneath the eastern half of the array. Beneath the western third of the array, the receiver function stacks indicate more complex scattering, but weak Ps phases may be generated at depths of roughly 320 km. The transition between these two regions occurs across the eastern edge of the North American lithospheric keel imaged by tomography. The observed phases may be interpreted as conversions from the base of a low-velocity asthenosphere.