Glaciers were formed which move down valleys with great erosive power. These glaciers carved new s cenery. There are three main types of glacial erosion - plucking, abrasion and freeze thaw. Plucking is when meltwater from a glacier freezes around lumps of cracked and broken rock. When the ice moves downhill, rock is plucked from the back wall.
Abrasion is when rock frozen to the base and the back of the glacier scrapes the bed rock.
Freeze-thaw is when meltwater or rain gets into cracks in the bed rock, usually the backwall. At night the water freezes, expands and causes the crack to get larger. Eventually the rock will break away. This a valley which was V-shaped but has been eroded by ice. The velley sides are steeper and the valley floor flatter after the ice melts.