Abstract. Jakobshavn Isbrae is Greenland's fastest moving outlet glacier and a major contributor to sea level rise. We analyze new gravity and magnetic profiles across the glacier, extending from the mouth of the outlet fjord to 64 km inland of the 2008 grounding line. Our results provide new insights into Jakobshavn Isbrae's geologic underpinnings and controls on the basal velocities. Earlier studies of basal processes minimized basal slip as a fast flow mechanism. Currently, velocities are up to double those considered in these studies, necessitating a reanalysis of the basal conditions. The gravity field along the glacier's main trunk cannot be attributed to the gravitational effect of bed topography and the overlying ice sheet. We interpret the remaining gravity signal as evidence of up to 2400 m of low density sediment beneath the main trunk. Examining recent velocities, we find basal slip is a major contributor to ice flow along most of the sediment filled trough. Within 54 km of the grounding line, only isolated 1–3 km wide regions have velocities that possibly result solely from internal deformation of the ice. We conclude soft bed sliding over the thick sediment wedge beneath Jakobshavn Isbrae is the dominant mechanism of fast flow.