They may occur through a build-up of sedimentation as shoals, and become true islands through changes in the level of the lake. They may have been originally part of the lake's shore, and been separated from it by erosion, or they may have been left as pinnacles when the lake formed through a raising in the level of a river or other waterway (either naturally, or artificially through the damming of a river or lake). On creation of a glacial lake a moraine can form an island.