Bottom crevasses are fractures that extend upward into floating ice shelves. They form when seawater penetrates the base of the ice shelf and ruptures the ice up to the level at which englacial stresses equal the stress of the seawater. For a freely floating ice shelf, the penetrating level of closely spaced crevasses is estimated at about half the ice thickness h; for an isolated crevasse the level is about &pgr;h/4. However, an analysis of the heights and locations of bottom crevasses in the Ross Ice Shelf shows that none of the crevasses approach the predicted limits, perhaps because the existing theory does not include the back stress &sgr;b, which is present in bounded ice shelves. By reformulating the theory to include a back stress term, &sgr;b can be evaluated experimentally from radar measurements of crevasse height and ice thickness. The magnitude of &sgr;p (2 bars in the grid northwest corner of the ice shelf) suggests the ice shelf is playing an important role in buttressing the inland ice sheet.