Are sometimes formed of masses of driftwood on which débris, vegetation, etc., gradually form a soil, but are more commonly portions of river banks detached by the force of the current when swollen and drifted put, sometimes as much as 100 m., to sea, carrying with them plants, reptiles, and larger animals, and thus contributing to the distribution to distant shores of animal and vegetable life; they are to be met with off the mouths of the larger American, Asian, and African rivers, and sometimes in inland seas and lakes; Derwent Lake, in England, has a notable one, which sinks, and rises periodically; they are also made artificially in districts subject to floods as asylums of refuge.