The transition in the middle Pleistocene (~0.9 Ma) seen in Δ18O deep-sea-core records from relatively low-amplitude, high-frequency (41 kyr) to high-amplitude, low-frequency (100 kyr) ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft-bedded to a mixed hard-soft bedded Laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock. A one-dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant (23 and 41 kyr) orbital forcing. Progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change (100 kyr) reflects nonlinear deglaciation processes. In model simulations over the last 3 Ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records. ¿ 1998 American Geophysical Union