Remarkable insoluble rock forms resulting from differential hydraulic erosion of Grenville marble have been discovered at the bottom of lakes in the Laurentian region (Québec, Canada). Some of these forms are several decimetres tall. Given the results of studies on the chemical dissolution rate of Canadian Shield marble, it is unlikely that these insoluble units were only released from the encasing marble during the Holocene. Instead, it is proposed that they likely result largely from differential hydraulic erosion caused by pockets of meltwater fed by the retreating Laurentide ice sheet. The authors also consider the possibility that the insoluble rocks were released from their marble matrix by running water well before the last deglaciation, and that this retreat of the ice would have been preceded by one or more subglacial lakes.