During an Ice Age, water from the sea is locked up as ice on the land. As a result, the sea level falls.
Glaciers (and beyond the ice, rivers) flow down valleys, scooping them deeper as they make their way to the lower sea.
When the Ice Age ends, the ice melts and sea levels rise again, flooding the deep glacier-cut valleys, and making fjords (called lochs and forths in Scotland). In lowlands flooded valleys are called estuaries.
