 German invasion plans envisaged that the main thrust of their advance inland would cross the North Downs at the gap in the ridge formed by the Wey valley, thus colliding with the defence line around Guildford.Between the wars Croydon Airport, opened in 1920, served as the main airport for London, but it was superseded after the Second World War by Heathrow, and closed in 1959.Gatwick Airport, where commercial flights began in 1933, expanded greatly in the 1950s and 1960s, but the area occupied by the airport was transferred from Surrey to West Sussex in 1974.