The green and fertile island of Shapinsay lies at the centre of the Orkney Island group to the north of the eastern section of Mainland Orkney. 6 miles (9.5 km) in length and generally flat, it rises to a height of 64m (210 feet) at Ward Hill in the centre. Largely made up of Middle Old Red Sandstone, the island's most interesting features include its 'ayres' or storm beaches which are strips of sea water shut off from the ocean by narrow necks of land. Intensively cultivated, the island's anchorage at Elwick was used by King Haakon's fleet in 1263 before sailing south to Largs. Mill Dam in the east is an RSPB nature reserve which is home to many breeding birds including the water rail, pintails, waders and black headed gulls. In 1805 the minister of Shapinsay first noted here the Orkney vole, Microtus arvalis orcadensis, as a sub-species of the European vole. Sites of archaeological interest include Odin's Stone, Mor Stein standing stone, Castle Bloody chambered cairn and the Broch of Burroughston. Buildings of note include Balfour Castle, Elwick Mill, 12th-century Linton Chapel and Shapinsay Heritage Centre which is located in an old smithy. Quholm in the northeast was the birthplace of the father of American writer Washington Irving. The population of the island fell from just over 900 in 1891 to 416 in 1961. A slow decline has continued; 346 (1971), 329 (1981), 322 (1991), 300 (2001) and 307 (2011), with most of the islanders now engaged in the farming of sheep and cattle, boat building, knitwear production and craft industries such as wood carving. Shapinsay is linked to Kirkwall by car ferry service.