Using teleseismic P wave tomography we detect very large (~5%) variation in the compressional velocity of the upper mantle beneath the Proterozoic--Paleozoic boundary in the Northeastern US. In this region, the upper mantle is heterogeneous on the 100 km scale to a depth of 300 km. Below this depth, the mantle appears to be laterally homogeneous, at least on the scale of 200--250 km. Given the tectonic history of the region, the large amplitude, short wavelength heterogeneities can not be caused by temperature variations, but may be due to chemical heterogeneity or variations in the amount and orientation of flow-induced seismic anisotropy. ¿ American Geophysical Union 1995