England until the Industrial Revolution caused a manufacturing and development shift to the Midlands and the North. During the Second World War, the Royal Air Force and the United States Army Air Force constructed many airbases in East Anglia for the heavy bomber fleets of the Combined Bomber Offensive against German-occupied Europe. East Anglia was ideally suited to airfield construction, as it includes large areas of open, level terrain and is close to mainland Europe. Many of the airfields can still be seen today, particularly from aerial photographs, and a few remain in use, the most prominent being Norwich International Airport. Pillboxes were erected in 1940 to help defend the nation against invasion, and they can also be found throughout the area at strategic points. Similarly, but from the Napoleonic Wars, a number of Martello towers can be found along the coast. Geography East Anglia is bordered to the north and east by the North Sea, to the south by the estuaries of the rivers Orwell and Stour, and shares an undefined land border to the west with the rest of England. Much of northern East Anglia is flat, low-lying and marshy (such as the Fens of Cambridgeshire and Norfolk), although the extensive drainage projects of the past centuries actually make this one of the driest areas in the UK. Inland, much of the rest of Suffolk and Norfolk is gently undulating, with glacial moraine ridges providing some areas of steeper relief. The highest point in Norfolk is the Beacon Hill; the supposed flatness of Norfolk is noted in literature, including Noël Coward's Private Lives – "Very flat, Norfolk". On the north-west corner East Anglia is bordered by a bay known as The Wash, where owing to deposits of sediment and land reclamation, the coastline has altered markedly within historical times; several towns once on the coast of the Wash (notably King's Lynn) are now some distance inland. Conversely, over to the east on the coast exposed to the North Sea the coastline is subject to rapid erosion and has shifted inland significantly since historic times. Major rivers include the River Nene and Suffolk's Stour, running through country beloved of the painter John Constable. The River Cam is a tributary of the Great Ouse and gives its name to Cambridge, whilst Norwich sits on the River Yare and River Wensum. The River Orwell flows through Ipswich and has its mouth,