By far the greater part of Quebec, more than 500,000 square miles, is underlain by the hard old Precambrian rocks of the Canadian Shield. Because its nature was first known and studied in the rugged plateau-like highlands north of the St. Lawrence River, the name Laurentian is often used for the whole region. The rocks are, largely, granites, diorites, quartzites, gneisses, schists, and slates. Except on rounded hill tops and in stream gorges, however, the surface material is mostly of glacial origin, or the sand, gravel and clay deposits of the post-glacial period. The skyline as seen from the air is "monotonously even". The main trend of relief is a plateau-like surface arising from the sea-level, on the shore of James and Hudson Bays, to 1,000 feet in Abitibi, 1,500 feet in the Laurentian mountains, and above 2,000 feet along the Labrador boundary. Scattered monadnocks rise a few hundred feet above the upland in the interior, but the most conspicuous summits are found toward the eastern edge of the Shield in the Laurentide National Park north of Quebec City (3,900 feet), and Mont-Tremblant Park west of Montreal (3,150 feet).
Numerous cycles of erosion have reduced the Laurentian Upland to its present level. The history of peneplanation dates back to Precambrian Times. On the western margin of the Shield, fragments of old peneplains are buried beneath the Paleozoic sediments. Then a marine invasion occurred over most of the Laurentian Upland, and several hundred feet of Paleozoic sediments were accumulated. As the land was uplifted at the close of the Paleozoic period, a new series of erosion cycles, lasting for some 500 million years, removed the overburden of sedimentary rocks. But before the last glaciation, the region was uplifted again, and the rivers were rejuvenated. The active streams deepened their valleys, and with their tributaries have excavated large basinlike areas. Those cycles of erosion that took place before Quaternary glaciation shaped the basic features of the present physiography of the land. The three main peneplains of Quebec, referred to later, were formed before the occurrence of ice sheets.