submarine glacial trough in the eastern continental shelf of North America, the most impressive such feature on Earth. It extends from the mouth of the St. Lawrence River eastward through the Gulf of St. Lawrence to the edge of the continental shelf, about 190 miles (306 km) south of Newfoundland. It has a mean width of 50 miles (80 km) and a depth as great as 1,700 feet (518 m) below sea level.

The topography of the Laurentian Trough is believed to have been greatly modified by glacial activity during the Pleistocene Epoch (1,600,000 to 10,000 years ago). The depression of the Earth's crust owing to the forward movement of glaciers caused silt blankets and submerged shorelines along the trough. The floor of the trough forms a basin, and the trough has both tributaries and distributaries.