What happens beneath a glacier affects the way it flows and the landforms left behind when it retreats. Direct observations from beneath glaciers are, however, rare and the subglacial environment remains poorly understood. We present new, repeat observations from West Antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record. We interpret erosion at a rate of 1 m a−1 beneath a fast-flowing ice stream, followed by cessation of erosion and the formation of a drumlin from mobilized sediment. We also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed. All these changes occurred on time scales of a few years or less. This variability suggests that an ice stream can reorganize its bed rapidly, and that present models of ice dynamics may not simulate all the relevant subglacial processes.