Corries are amphitheatre-shaped mountain-side hollows found where the build-up of snow was greatest (in the northern hemisphere this is typically on north or east facing mountain slopes). They are formed when lying snow accumulates over a number of years and compresses lower layers into ice (firn) in a hollow. Once it has developed a sufficient volume, gravity causes it to move in a rotational manner out of the hollow and down the mountain side, eroding the floor by plucking and abrasion. Corries are the sources of Alpine glaciers.