Abstract

The following observations suggest that erosion by continental ice sheets removed stratigraphic cover to exhume some of the world's largest areas of exposed Precambrian crystalline rock.

1. Of the world's large areas of exposed Precambrian rock, two were centrally located beneath great Pleistocene continental ice sheets that no longer exist, are proportionate to those ice sheets in size and similar in shape, and are great lowlands holding central seas. Most of the others are in areas that were covered by Paleozoic ice sheets and are presently overlain only by stratigraphic cover that is younger than those ice sheets.

2. Around Laurentian and Fennoscandian shields, arcs of exhumation control major topography and drainage. These are arcuate lowlands of continental scope encircling the shields at the Precambrian-Paleozoic contact where glaciation skinned Paleozoic rocks off crystalline basement. They are geometrically similar and are proportional in size to the ice sheets that excavated them. They form the St. Lawrence and upper MacKenzie valleys, including America's greatest lake basins—Great Bear, Great Slave, Athabaska, Reindeer, Winnipeg, Winnipegosis, Lake of the Woods, Rainy Lakes, Great Lakes, and St. Lawrence Trough. European counterparts are Skagerrak, Baltic, Gulf of Finland, Lakes Ladoga and Onega, Onega Bay, Gulf of Dvinsk, and White Sea.

3. Peripheral parts of shields that were inundated by Pleistocene ice sheets show exhumed pre-Paleozoic terrain with a regional slope that is radially outward, down-glacier. Central parts are flat-floored basins that slope radially inward up-glacier and have no monad-nocks because glacial erosion cut deeply into pre-Paleozoic basement.

4. Erosion by the Laurentide and Fenno-scandian ice sheets has carved great hierarchies of ellipsoidal basins. In North America, the master basin holds Hudson Bay; the lesser radiating basins, farther down-glacier, hold the Great Lakes. A similar but lesser hierarchy has eastern Lake Ontario as its master basin and the Finger Lakes of New York as its lesser radiating basins.

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