The Firth of Forth, the estuary of the Forth, extends 48 miles from Alloa to the North Sea. Flowing between Clackmannanshire and Fifeshire on the north and Stirlingshire, East Lothian and West Lothian on the south, it expands to a width of 2.5 miles at Borrowstounness but contracts again to a width of one mile at Queensferry. Here the river is spanned by the famous cantilever bridge, the Forth Railway Bridge, completed in 1887. From here, the estuary gradually widens to 18 miles at its mouth.
The tide of the North Sea is perceptible in the Firth as far as Stirling. Scenery along the Forth is picturesque and rich in historical interest. Above Queensferry on the Fifeshire coast is the sheltered bay of St. Margaret's Hope.