Foundations are being laid in the Forth for the three giant towers which will carry the cable-stayed structure across the river.
The central tower will be located on Beamer Rock, with the other two in water depths of up to 67ft (20m).
The towers will soar nearly 700ft (207m) above water level and 166ft (50m) above those of the adjacent Forth Road Bridge.
The foundations are based on huge caissons being sited below the river bed, which is understood to have significantly reduced the cost compared to the traditional piling method of digging foundations, which was favoured by the unsuccessful rival bidder.
One engineering expert said: “The critical path is getting this bit of work done. If it works, it will be a major feat of engineering.”
The bridge is expected to start taking visible shape by the end of the year, but tree clearance has started on either side of the Forth to make way for new approach roads from the M9 and M90.
Forth Crossing Bridge Constructors – the consortium which won the £790 million main contract – comprises Spanish firm Dragados, Hochtief of Germany, American Bridge International, along with Scottish company Morrison Construction, the junior partner with a 16 per cent stake.
The bridge is due to be opened by the end of 2016, although the American Bridge website yesterday referred to the “estimated completion date” as 14 June, 2017.
The road links and an electronic traffic control system are being built as separate contracts.
The project has become embroiled in a political row over major sub-contracts, such as the supply of steelwork, which have gone to foreign firms.
The Scottish Government’s Transport Scotland agency has said no Scottish firms bid for the order, but they have won numerous smaller sub-contracts and many further orders have still to be awarded.