On the basis of two three-dimensional (3-D) models of S-wave velocity, PUMO [Hara et al., 1993> and SH425.2 [Su and Dziewonski, 1991>, we show that the deep roots of cratons and mid-ocean ridges provide a satisfactory explanation of the pattern and amplitude of the Earth's long wavelength lateral heterogeneity in the depth range from 300 to 400 km. ¿ American Geophysical Union 1994