View from the north of Edinburgh looking over the Forth

The Firth of Forth is an inlet of the North Sea that separates Fife in the north and Edinburgh and the Lothians to the south. 'Firth' originates from the Old Norse Word "fjoeror", which also became the modern Norwegian word of fjord. A firth (or fjord) is made up of undiluted sea water whereas an estuary has a gradation of salt water dilution as the river water mixes with the sea. Westwards from Queensferry, the Firth of Forth becomes the Forth Estuary and then the River Forth upstream of Stirling.