The remainder of its area was traversed by three significant rows of hills: the North Downs, the Wealden Ridge and the South Downs. Thus Rastrick's Brighton Main Line of 1841, included one of the largest cuttings in the country at Merstham, significant tunnels at Merstham, Balcombe, Clayton and Patcham as well as the famous Ouse Valley Viaduct. The major tunnels on the SECR network were at Merstham, Sevenoaks and Shakespeare Cliff.