In northern Manitoba, intersecting grooves 300–1800 m long are ice-scour marks created by the dragging of iceberg keels along rises in the bed of a glacial lake whose water plane was at about 305 m asl. The lake was bounded by glacial ice on its northern and eastern margins. The occurrence of scours on topographic divides indicates that a single extensive lake, thought to be a northern extremity of Lake Agassiz, occupied the area as far north as Seal River at the time the ice scours were formed. The lake extended as far west as Sprott Lake and eastwards into the Hudson Bay Lowlands into an area later occupied by Tyrrell Sea. The preservation of the scour marks suggests that the lake drained suddenly.Ice-scour marks are easily recognized on air photographs and provide a means of identifying areas that have been inundated by glacial lakes. Scours in emerged marine sediment are generally obliterated by littoral processes.