Small-scale morainal ridges on the middle continental shelf of the Whales Deep Basin partly record the rapid opening of a 200 Km grounding line embayment slightly after ~11.5 cal Kyr BP. The overlapping pattern of backstepped moraines indicates that the grounding line oscillated back and forth during the retreat. Published sediment fluxes were combined with new sediment volume mapping of the moraines to infer the rates at which the grounding line retreated. The analyses indicate that the rates of grounding line retreat on the eastern flank of the embayment ranged from ~490 m a^-1 to 1300 m a^-1. At the maximum rate of retreat, the embayment would have taken less than two centuries to open. The quantitative analysis provides a paleo-perspective on modern retreat. A paleo-modern comparison validates concern that glaciers in the Amundsen Sea sector are already experiencing an unstable retreat.