Thirty-nine 50 MHz radar polarization experiments were performedin 1991–92 near Upstream B Camp, Antarctica, along two lines perpendicular to flow,1.4 and 2 km long and 900 m apart. For each of the experiments, which were at 100m intervals, the receiving antenna was held fixed, alternately parallel and perpendicular to flow, while the transmitting antenna was rotated in 15° increments through a full circle twice, once for each orientation of the receiving antenna. The data consist ofecho-amplitude measurements from the bottom of the ice. Assuming a model of the ice sheet as a crystalline medium with axial symmetry, the azimuths of the symmetry axis and the cosines of the phase shifts between extraordinary and ordinary waves can beestimated from the variations in amplitude with orientation of the transmitting antenna. The results from bottom echoes show an abrupt change in the axis of symmetry in a distance of only 100m. This suggests that the experimental lines crossthe boundary between two blocks of ice with different stress histories.