The Shropshire Hills Area of Outstanding Natural Beauty covers about a quarter of the county, mainly in the south. Shropshire is one of England's most rural and sparsely populated counties, with a population density of 136/km2 (350/sq mi). The Wrekin is one of the most famous natural landmarks in the county, though the highest hills are the Clee Hills, Stiperstones and the Long Mynd. Wenlock Edge is another significant geographical and geological landmark. In the low-lying northwest of the county overlapping the border with Wales is the Fenn's, Whixall and Bettisfield Mosses National Nature Reserve, one of the most important and best preserved bogs in Britain. The River Severn, Great Britain's longest river, runs through the county, exiting into Worcestershire by the Severn Valley.