The county has a varied geography. The south-west contains part of the Fens, a naturally marshy region which has been drained for agriculture, and the south-east is an upland region. A wide vale runs north-south from the centre to the north of the county. To its east, the chalk hills of the Lincolnshire Wolds, which have been designated a national landscape, occupy the north-east, with a coastal plain and the Lincolnshire Marsh beyond. The west of the vale is demarcated by the Lincolnshire Edge, a long escarpment; at its northern end are the Coversands, an area of heath. Beyond the edge, the western border of the county contains the eastern part of the Trent Valley and, in the north, part of the Humberhead Levels, with the River Trent itself forming part of the border.