However, erratics at all levels are traceable to sources in the Ford Ranges, indicating that ice flow never crossed the divide that currently separates Marie Byrd Land from interior West Antarctica. In all cases, the exposure ages decrease with elevation, tracking the retreat of ice from the peaks. An erratic from the summit of The Billboard gives an exposure age of 105 kyr BP, suggesting that this peak stood above the ice sheet throughout the glacial maximum. We did not locate a well defined trimline or weathering limit marking the uppermost level reached by glacial maximum ice, but an exposure age of 3.2 kyr BP for a fresh erratic on ice-scoured bedrock at 415m sets a firm minimum limit. All of the other sites we examined were overrun by the ice sheet.
Further analyses are in progress to constrain the lowering history of the ice surface at each site.
Evidently, major recession of the ice sheet occurred well after the climatic warming phase recorded in Antarctic ice cores, and post-dated all but the final stages of global eustatic sea level rise.