The source of the River Waveney is a ditch on the east side of the B1113 road between the villages of Redgrave, Suffolk and South Lopham, Norfolk. The ditch on the other side of the road is the source of the River Little Ouse which continues the county boundary and, via the Great Ouse, reaches the sea at King's Lynn. It is thus claimed that during periods of heavy rainfall Norfolk can be considered to be an island. The explanation of this oddity is that the valley in which the rivers rise was formed not by these rivers, but by water spilling from the periglacial lake known as Lake Fenland. This was a periglacial lake of the Devensian glacial period, fifteen or twenty thousand years ago. The ice sheet closed the natural drainage from the Vale of Pickering, the Humber and The Wash so that a lake of a complex shape formed in the Vale of Pickering, the Yorkshire Ouse valley, the lower Trent valley and the Fenland basin. This valley was its spillway into the southern North Sea basin, thence to the English Channel basin.