We attempt to explain why the seafloor profile that presumably existed before the advance of grounded ice on the Antarctic Peninsula continental shelf changed from seaward-dipping to the present-day landward-dipping and overdeepened profile. Paleobathymetric profiles and past internal configuration of the margin are calculated by reverse modelling starting from a cross-section derived from a depth-converted multichannel seismic profile, in which the boundaries between units of marked geometric difference have been identified and digitised. The reverse modelling includes sediment decompaction and isostatic compensation after progressive removal of each sedimentary unit, and recovery of the post-rift thermal subsidence. We present the initial results of these these models and sensitivity tests obtained with the age constraints available from the initial results of ODP Leg 178. We conclude that the outer shelf became overdeepened with a landward-dipping profile at c. 4.5 Ma (early Pliocene), coinciding with the onset of progradation and aggradation, a major break in the margin's architecture.