We have investigated the potentially important effects arising from lateral heterogeneities on postglacial uplifts. Time‐dependent solutions for Maxwell rheology with lateral variations in the upper mantle are obtained by finite‐element techniques in cylindrical geometry. The displacements in the central portion of the ice sheet are not at all influenced by these lateral variations, whereas the uplifts near the edge of the ice load are extremely sensitive to lateral heterogeneities in the upper mantle. For ice sheets characteristic of the Laurentide ice sheet, deformations around the peripheral bulge can still be affected by an asthenosphere as far as twenty‐five degrees away. The continental lithosphere under the former ice load would appear to be thicker with the presence of a neighboring asthenosphere. Recent estimates of the North American plate thickness would exceed 250 km under these circumstances.