During recent years interest in the West Antarctic Ice Sheet (WAIS) increased among the geoscientific community because: (1) its bed is located mainly below sea level and drops towards the interior of the Antarctic continent, making the WAIS inherently unstable in a future warming world, (2) extensive thinning and associated grounding-line retreat of the Pine Island and Thwaites glaciers draining into the Amundsen Sea Embayment (ASE) have suggested major changes are underway in the sector, increasing contributions to current sea level rise, (3) a complete collapse of the WAIS in the future would raise global eustatic sea level by 3.4-5 m, while melting of the ASE drainiage basin alone would raise sea level by 1.2-1.5 m. A sea level rise of that magnitude would cause major global socio-economical and ecological problems. Detailed knowledge of the long-term behaviour of the WAIS in the ASE during the recent geological past (Last Glacial Maximum [LGM] to present) will contribute to a better understanding of current glacier dynamics and help to improve numerical ice-sheet models, which aim to predict future sea-level rise. Previous marine geoscientific studies in the ASE focused on the main palaeo-ice stream troughs to reconstruct the LGM extent of the WAIS on the continental shelf and its subsequent retreat history. However, little is known about elevated marinal areas of the palaeo-ice streams in the ASE, where ice retreat most likely lagged behind that in the troughs. Here we present results from multibeam swath bathymetry surveys, high-resolution seismic and sedimentological investigations from the former bed in an inter-ice stream area between the Pine Island-Thwaites palaeo-ice stream and a fast-flow tributary emanating from the area now occupied by the Cosgrove Ice Shelf. The data show an unusual assemblage of glacial morphological features including crevasse-squeeze ridges, large-scale hummocks perpendicular to the palaeo-ice flow, associated recessional moraines, and hill-hole pairs. This combination of bedforms has not been described before from the Antarctic shelf and indicates a more complex ice flow behaviour for the eastern ASE than suggested by the pattern of bedforms in the palaeo-ice stream troughs alone. Our data indicate that slow flowing ice masses covered the topographical highs adjacent to the Pine Island-Thwaites palaeo-ice stream (PITPIS) during the LGM. These ice masses most likely stagnated during a phase of general stillstand of the PITPIS. Here we introduce a six-phase formation model based on these interpretations. New radiocarbon ages indicate a pre-Holocene deglaciation of the inter-ice stream ridge between ~11.6 and 16 ka BP. This new information can be used as a reference dataset for interpreting more inter-ice stream areas in future studies, since they are key areas for stabilising ice streams, and form a large part of the ice sheet in general. New insights into ice dynamics here may help improve ice flow models.