The depth of any know time horizon in an ice sheet is a valuable datum against which to check models of surface mass balance and ice dynamics. Where deep cores have been collected, the Holocene/Wisconsin climatic boundary (about 12 000 B.P.) can be identified by changes in a large number of physical and chemical parameters. Unfortunately, there are only a few deep bore holes in the polar regions so other methods, such as geophysical techniques, are needed. One potentially powerful technique is electrical resistivity sounding. The resistivity of polar ice is strongly dependent on temperature, and several physical and chemical properties in the ice. By measuring the apparent resistivity of the ice sheet along a surface profile, a model of resistivity versus depth can be obtained. The measurements and model studies at several locations in Antarctica (Ice Stream B, Ross Ice Shelf, and Dome C) have resulted in the discovery of a rather sharp change in resistivity at a depth identified with the Holocene/Wisconsin transition at two locations (J9 on the Ross Ice Shelf and Dome C in East Antarctica), where core studies have been made.

The change in resistivity in polar ice sheets can be attributed primarily to changes in crystal size. In general, crystal size increases with depth down to the transition zone, decreases rather sharply over a short depth, and then gradually increases again. Resistivities show a similar behavior, and indeed there is a good correlation between the two at Dome C and J9. At a location on Ice Stream Β where the ice is about 1060 m thick, our best model, using that correlation, shows the Holocene–Wisconsin boundary at 750 m depth. Furthermore, the shape of the resistivity change with depth (after correction for temperature) shows a good correlation with the crystal-size profile across that boundary at Byrd Station.

Another pronounced increase in resistivity occurs deeper in the ice, especially at locations on the Ross Ice Shelf where the ice has come from West Antarctic ice streams or the major outlet glaciers of East Antarctica. We attribute this increase also to a large increase in crystal size, such as occurs in the basal ice at Byrd Station, up-stream from Ice Stream D, and at Little America Station V on the Ross Ice Shelf.