Near Toronto, in Southern Ontario, Canada, glaciolacustrine muds and shoreface sands of Late Pleistocene age (Early Wisconsin glaciation; ≈45 ka) were deformed by icebergs ploughing the floor of an ancestral ice-dammed Lake Ontario. Outcrop mapping and a ground-penetrating radar survey identify ice scour structures up to 4 m deep, up to 10 m wide and at least several hundred metres long with high-standing lateral margins (berms). Ploughing of lake-floor sediment is also recorded by ice-keel turbated facies in which storm-deposited shoreface sands have been mechanically mixed with underlying muds. Palaeoenvironmental information for one prominent scour, such as water depths, estimates of the mass and draft of the ice mass and scouring forces, can be determined using a simple finite element numerical model. A mass of 0·04 million tonnes, a keel draft of between 10 and 30 m and a scouring force of 5 × 103 kN is consistent with scouring by a small iceberg (‘bergy bit’) or pressure-ridged lake ice; the former is preferred because of the considerable depth of the scour and the overall glacial depositional setting. Estimates of ice-mass dimensions agree with independent reconstructions of palaeobathymetry (no greater than 20 m) derived from storm-deposited hummocky and swaley cross-stratified facies that fill the scour. Data presented here aid the identification of ice-scour structures and turbated sediment in the ancient record; despite being a common process on modern high-latitude shelves, descriptions of ancient structures and facies are few.