These old fold mountains in the north and north-west extend from Norway and Sweden to Scotland. They are made up of very hard crystalline rocks and were once very lofty and rugged. However, after facing the erosive action of glaciers for millions of years, they have been reduced to low hills with gentle slopes. In the coastal regions, fiords have been formed because of the drowning of glacier valleys. Fiords are narrow but deep inlets of water with steep sides that break and indent the coastline, making it irregular.