During the end of the last Devensian glacial period the formation of Lake Humber in the lowest reaches of the river, meant that substantial lake bed clays and silts were laid down to create the flat landscape of the Humberhead Levels. These levels extend across the Trent valley, and include the lower reaches of the Eau, Torne and Idle. In some areas, successive layers of peat were built up above the lacustrine deposits during the Holocene period, which created lowland mires such as the Thorne and Hatfield Moors.