Firth is a word in the English and Scots languages used to denote various coastal waters in the United Kingdom, predominantly within Scotland. In the Northern Isles, it more often refers to a smaller inlet. It is linguistically cognatetofjord (both from Proto-Germanic *ferþuz), which has a more constrained sense in English. Bodies of water named "firths" tend to be more common on the Scottish east coast, or in the southwest of the country, although the Firth of Clyde is an exception to this. The Highland coast contains numerous estuaries, straits, and inlets of a similar kind, but not called "firth" (e.g. the Minch and Loch Torridon); instead, these are often called sea lochs. Before about 1850, the spelling "Frith" was more common.
A firth is generally the result of ice ageglaciation 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 into 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. In navigation terms, the dredged River Clyde Channel for shipping meets the Firth of Clyde Channel at the Tail of the Bank, where the river crosses a sandbar off Greenock as the estuary widens at the junction to the Gare Loch. Locally, the river can be described as extending even further west to 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.
InScottish Gaelic, the Firth of Clyde is treated as two bodies, with the landward end being called Linne Chluaidh (IPA:[ˈʎiɲəˈxl̪ˠuəj]; meaning the same as the English), while the area around the south of Arran, Kintyre and Ayrshire/GallowayisAn Linne GhlasScottish Gaelic pronunciation:[əˈʎiɲəˈɣl̪ˠas̪].
Cliffs in Saviskaill Bay on Rousay, looking northward to Westray across Westray Firth
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.
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.
^Anderson, Joseph (Ed.) (1893) Orkneyinga Saga. Translated by Jón A. Hjaltalin & Gilbert Goudie. Edinburgh. James Thin and Mercat Press (1990 reprint). ISBN0-901824-25-9
