We show that the two-dimensional coherence between topography and Bouguer gravity is anisotropic throughout most of the Canadian Shield, excepted Hudson Bay Basin. We interpret the axis of maximum coherence as the direction where the lithosphere is weakest. In the southeastern Shield and the Appalachians, the azimuthally averaged coherence cannot be fitted by the thin elastic plate model. In the other regions, it yields the effective elastic thickness. The average coherence increases in the direction perpendicular to the main tectonic boundaries, the axis of high electrical conductivity, and the seismic fast axis. This consistency suggests that the same stress field affected the entire lithosphere during the last tectonic event.