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The reconstruction of former ice sheets is important for testing Earth system models that can assess interactions between polar ice sheets and global climate, but information retrieved from contemporary glaciated margins is sparse. In particular, we need to know when ice sheets began to form marine outlets and the mechanisms by which they advance and retreat over timescales from decades to millions of years. Here, we use a dense grid of high-quality two-dimensional seismic reflection data to examine the stratigraphy and evolution of glacial outlets, or palaeo-ice streams, that drained the northwest Greenland Ice Sheet into Baffin Bay. Seismic horizons are partly age constrained by correlation with cores from drill sites. Progradational units separated by onlap surfaces record 11 major phases of shelf-edge ice advance and subsequent transgression since the first ice-sheet expansion 3.3-2.6 million years ago. The glacial outlet system appears to have developed in four stages, each potentially caused by tectonic and climatic changes. We infer that an abrupt change in ice-flow conditions occurred during the mid-Pleistocene transition, about 1 million years ago, when ice movement across the shelf margin changed from widespread to a more focused flow (ice streams), forming the present-day glacial troughs.