Portsdown Hill is a long chalk ridge in Hampshire, England. The highest point of the hill lies within Fort Southwick at 131m above sea level. The ridge offers good views to the south over Portsmouth, the Solent, Hayling Island and Gosport, with the Isle of Wight beyond. The hill is on the mainland, just to the north of Ports Creek, which separates the mainland from Portsea Island, on which lies the main part of the city of Portsmouth, one of the United Kingdom's main naval bases. To the north lies the Forest of Bere, with the South Downs visible in the distance. Butser Hill can be seen on a clear day. The hill is formed from an inlier of chalk which has been brought to the surface by an east-west upfold of the local strata known as the Portsdown Anticline.