Pewit Island is a small island located in the north western section of Portsmouth Harbour

Portsmouth Harbour

Portsmouth Harbour is a large natural harbour in Hampshire, England. Geographically it is a ria: formerly it was the valley of a stream flowing from Portsdown into the Solent River. The city of Portsmouth lies to the east on Portsea Island, and Gosport to the west on the mainland...

. Historically it appears to have been connected to the mainland via a manmade shingle causeway, parts of which still exist. The island has been colonised by oak

Oak

An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...

 and blackthorn

Blackthorn

Prunus spinosa is a species of Prunus native to Europe, western Asia, and locally in northwest Africa. It is also locally naturalised in New Zealand and eastern North America....

 scrub.The island is a Hampshire and Isle of Wight Naturalists’ Trust nature reserve.

In 1857 plans drawn up to improve the defences of Portsmouth

Portsmouth

Portsmouth is the second largest city in the ceremonial county of Hampshire on the south coast of England. Portsmouth is notable for being the United Kingdom's only island city; it is located mainly on Portsea Island...

included building a fortification on the island. The plans were latter modified in the face of improved artillery technology and the fortification was never built.