The first deep-tow boomer survey on the Antarctic continental margin has revealed relict subglacial deltas on the outer continental shelf off the Antarctic Peninsula. Progradation of subglacial deltas is thought to take place at the grounding lines of ice streams which flow on deforming subglacial till. Acoustic characteristics and estimation of likely sediment transport rates suggest that these features were produced by late-stage readvance of grounding lines during the waning of the last ice sheet that covered the shelf. This readvance could have taken place during the Younger Dryas (12.9–11.6 ka), but changes in sea level and climate may not be the only important controls on subglacial delta formation. The discovery of relict subglacial deltas on the outer shelf is consistent with the hypothesis that the grounded ice sheet in these areas was low profile and fast flowing. If low-profile grounded ice extended to the shelf edge in many places around Antarctica at times of glacial maximum, this could explain the greater glacial maximum ice-sheet extent in interpretations based on offshore data, compared with reconstructions based on onshore data and numerical glaciological models.