The fossil ice wedges which have been found during the last eight years in the St. Lawrence Lowlands provide evidence of permafrost development in the emerged lands of Southern Québec following deglaciation. Palynology yields evidence that a boreal forest was developing during that period. Such conditions account for the essential features of late glacial geomorphogeny as characterized by the preponderant importance of underground ice. A major role was performed by underground ice in the shaping of two areas which have been studied in detail by the author, viz. Arundel and Piedmont. In the former area, the underground ice has governed the formation of thermokarstic depressions associated with a polygonal network of fossil ice wedges, with loess deposits, and with periglacial vales. In the latter, the underground ice has determined spectacular collapse features in a marine fill bottom, inducing the Champlain Sea clay to become vertically arranged. In both areas, the involved permafrost dynamics have interfered with the entrenching of the drainage network.