The term crannóg comes from the Irish word for tree, crann. Crannógs were man-made islands mostly made in shallow lakes and bogs on which people built houses and kept animals in ancient times. They were built by placing layers of stone, brushwood or tree trunks on the lakebed with wooden pilings on top and this platform was covered with a layer of earth or sand. On this, the dwellers built their thatched houses of wattle and mud.  Crannógs are found in shallow lakes and bogs and occasionally in rivers.