Shallow discontinuous valleys crease the upper portion of the continental slopes in the region where the Mississippi Delta has built across the shelf. These valleys attest to the instability of the forward-growing slope despite its average inclination of only 1 per cent. Mass movements probably of the earthflow type appear to be the cause of the valleys rather than turbidity currents because deposits from valley floors show no concentration of sand by currents, and many of the valleys are not located at points where large discharge of sediment-laden water could produce turbidity currents. Furthermore, depressions and hills along the courses of the valleys and their short length differentiate them from the long continuous slightly incised valleys which are outer continuations of submarine canyons and which are generally ascribed to turbidity currents. The delta-front valleys shift position as the slopes are built forward.