Firth is the Lowland Scots word used to denote various coastal waters in Scotland. It is usually a large sea bay, which may be part of an estuary, or just an inlet, or even a strait. It is cognate to fjord (both from Proto-Germanic *ferþuz) which has a more narrow sense in English, whereas a firth would most likely be called a fjord if it were situated in Scandinavia. Bodies of water named "firths" tend to be more common on the east coast, or in the southwest of the country, although the Firth of Lorn is an exception to this. The Highland coast contains numerous estuaries, straits and inlets of a similar kind, not called "firth", e.g. the Minch, and Loch Torridon; these are often called sea lochs.
A firth is generally the result of ice age glaciation and is very often associated with a large river, where erosion caused by the tidal effects of incoming sea water passing upriver has widened the riverbed to an estuary. Demarcation can be rather vague. The Firth of Clyde is sometimes thought to include the estuary as far upriver as Dumbarton, but the Ordnance Survey map shows the change from river to firth occurring off Port Glasgow, while locally the change is held to be at the Tail of the Bank where the river crosses a sandbar off Greenock at the junction to the Gare Loch, or even further west at Gourock point.
However, some firths are exceptions. The Cromarty Firth on the east coast of Scotland, for example, resembles a large loch with only a relatively small outlet to the sea and the Solway Firth and the Moray Firth are more like extremely large bays. The Pentland Firth is a strait rather than a bay or an inlet.
These are connected to, or form part of, the North Sea.
The Northern Isles were part of Norway until the 15th century, and retain many Norse names. In Shetland in particular, "firth" can refer to smaller inlets, although geo, voe and wick are as common. In Orkney, "wick" is common.
In the Scottish Gaelic language, linne is used to refer to most of the firths above; it is also applied to the Sound of Sleat, Crowlin Sound, Cuillin Sound, Sound of Jura, Sound of Raasay, and part of Loch Linnhe.
The following is a selection of other bodies of water in Scotland which are similar to various firths, but which are not termed such -
Likewise, in the Northern Isles, the words "firth" and "sound" are often used arbitrarily or interchangeably. Bluemull Sound for example, is very similar to some of the firths in the Shetland Islands.
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