These hills were formed by glaciation. 这些山丘是冰川作用形成的。
The Great Lakes basins were excavated by the ice, and many present landforms are the result of this continental glaciation. 五大湖盆地都是冰挖掘出来的，如今许多现有地形都是这种大陆冰川作用的结果。
Pleistocene glaciation caused the extinction of many species. 第四纪冰期，使许多物种绝灭。
