Ice-rafted debris in sediment cores from the North Atlantic suggests that the Laurentide ice sheet (LIS) periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7,000 years. Here, I propose that Heinrich events (i.e., what these episodes are called) were caused by free oscillations in the flow of the Laurentide ice sheet which arose because the floor of Hudson Bay and Hudson Strait is covered with soft, unconsolidated sediment that forms a slippery lubricant when thawed. The proposed Heinrich event cycle has two phases. The growth phase occurs when the sediment is frozen and the LIS is stranded (immobile) on a rigid bed. The volume of the LIS slowly grows during this phase at a rate dictated by snow accumulation. The purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway (i.e., an ice stream such as those which occur in West Antarctica today) develops through Hudson Strait. The volume of the LIS rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the Labrador Sea. The periodicity T=&pgr;/&kgr;(-k&thgr;sl/2G˜)2≈7000 years of the proposed Heinrich event cycle is a function of the thermal conductivity and diffusivity of ice, k and &kgr;, respectively, the atmospheric sea level temperature &thgr;sl (in degrees Celsius), and the excess geothermal heat flux defined by G˜=G-k&Ggr;, where &Ggr; is the atmospheric lapse rate, and G is the geothermal heat flux. Agreement between the predicted T and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism I propose. An alternative mechanism in which Heinrich events are forced by periodic variations in external climate is implausible, because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet. ¿ American Geophysical Union 1993