the Pennines, an upland chain sometimes referred to as "the backbone of England", which stretches from the Tyne Gap to the Peak District. Other uplands in the North include the Lake District with England's highest mountains, the Cheviot Hills adjoining the border with Scotland, and the North York Moors near the North Sea coastline. The geography of the North has been heavily shaped by the ice sheets of the Pleistocene era, which often reached as far south as the Midlands. Glaciers carved deep, craggy valleys in the central uplands, and, when they melted, deposited large quantities of fluvio-glacial material in lowland areas like the Cheshire and Solway Plains. On the eastern side of the Pennines, a former glacial lake forms the Humberhead Levels: a large area of fenland which drains into the Humber and which is very fertile and productive farmland. Much of the mountainous upland remains undeveloped, and of the ten national parks in England, five – the Peak District, the Lake District, the North York Moors, the Yorkshire Dales, and Northumberland National Park – are located partly or entirely in the North. The Lake District includes England's highest peak, Scafell Pike, which rises to , its largest lake, Windermere, and its deepest lake, Wastwater. Northern England is one of the most treeless areas in Europe, and to combat this the government plans to plant over 50 million trees in a new Northern Forest across the region. Urban Uniquely for such a large urban belt in Europe, these cities are all as recent as the Industrial Revolution; most of them previously scattered villages. Vast urban areas have emerged along the coasts and rivers, and they run almost contiguously into each other in places. Near the east coast, trade fuelled the growth of major ports and settlements (Kingston upon Hull, Newcastle upon Tyne,, Middlesbrough, Newcastle and Sunderland) to create multiple urban areas. Inland needs of trade and industry produced an almost continuous urbanisation from the Wirral Peninsula to Doncaster, taking in the cities of Liverpool, Manchester, Leeds, and Sheffield, with a population of at least 7.6 million. Analysis by The Northern Way in 2006 found that 90% of the population of the North lived in and around: Liverpool, Central Lancashire, Manchester, Sheffield, Leeds, Hull and Humber Ports, Tees Valley and Tyne and Wear. At the 2011 census, 86% of the Northern population lived in urban areas as defined by the