rolling mountain chain that is less rugged and more forested than the Highlands. Shorter than the Highlands, the tallest mountain in the range is Merrick, at . There are ranges of gentler, rolling hills even in the central lowlands of Scotland. Such ranges include the Ochil Hills, near Stirling, the Campsie Fells outside Glasgow, and the Lomond Hills in Fife. Mountains in Scotland are categorised by their height. Peaks over are known as Munros. There are 282 Munros in Scotland, all within the Highlands. Corbetts are peaks with an altitude of between , with a relative height of at least . The classification of peaks in Scotland is kept under periodic review by the Scottish Mountaineering Club. Coastline Mainland Scotland has of coastline. Including the numerous islands, this increases to some . The west coast in particular is heavily indented, with long promontories separated by fjordlike sea lochs. The east coast is more regular, with a series of large estuarine inlets, or firths, and long sandy beaches, for example at Aberdeen. Parts of the Scottish coastline consist of a machair formation, a dune pasture land formed as sea levels subsided. The east coast has several significant estuaries and is home to nature reserves including the Ythan Estuary and Fowlsheugh, both of which have been designated as Important Bird Areas. Firths around the Scottish coast include the Solway Firth, Firth of Clyde, and Firth of Lorne on the west coast, and the Cromarty Firth, Moray Firth, Firth of Tay, and Firth of Forth on the east coast. The Pentland Firth is not an inlet, but the strait that separates the Orkney Isles from the mainland. Major sea lochs include Loch Fyne, Loch Long, Loch Ryan, Loch Linnhe, Loch Torridon, Loch Ewe and, on the Isle of Lewis, Loch Seaforth. Islands Scotland has over 900 islands, over 100 of which are inhabited. Most of the islands are located off the northern and western coast of the country. The northern and western islands of Scotland can be found in three main groups: Shetland, Orkney and the Hebrides which can be divided into the Inner Hebrides and the Outer Hebrides. Shetland and Orkney, together with Fair Isle and Stroma are referred to as the Northern Isles. With a total land area of Lewis and Harris (referred to as two islands but actually just one) is the largest Scottish island. Many of these offshore islands