A loch is the name for a valley that has been enlarged by glaciers and then flooded. If the flooding is due to the sea, then it is called a sea loch.

All lochs began as river valleys. During the Ice Age, tongues of ice (glaciers) filled the valleys where the rivers had once flowed. Ice then scraped away at the bottom of these valleys, turning them from a V shape to a U shape.

Glaciers also scraped away softer rocks faster, and so deeper, than harder rocks. This created long, narrow basins. It is these basins that filled up with river water or sea water when the Ice Age ended.

Like glaciated valleys elsewhere, lochs give spectacular scenery, especially because they have steep sides. Most lochs are also very deep.