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The Traeth Mawr (Welsh for "big sands") is a polder near Porthmadog in Gwynedd in Wales. It was formerly the tidal estuary of the Afon Glaslyn, and many travellers sank in its quicksands trying to cross it. Much of it is between high mountains. Pont Aberglaslyn is at its upper end.
Around 1798, William Madocks bought the Tan-yr-Allt estate near Penmorfa Marsh. He reclaimed an area of sand and built Tremadog on it, protecting it from the sea and the river by building a bank from Clog-y-Berth to Portreuddyn. He used wooden railways to assist in its construction, and it was later used by the Croesor Tramway. Madocks obtained an Act of Parliament (1 August 1807 - 47 George III Cap. 71) permitting him to complete the reclamation. Between 1808 and 1811 he constructed an embankment called "the Cob" from the island of Ynys Towyn (now part of Porthmadog) near the Caernarfonshire shore to Boston Lodge on the Meirionnydd shore in order to cut off the estuary from the sea; this gained 1,500 acres (607 ha). It was 1,600 yards (1,500 m) long, 90 feet (27 m) wide at the bottom, tapering to 18 feet (5.5 m) at the top, which was 21 feet (6.4 m) above the level of the river. The cost of construction was £60,000, compared to his original estimate of £23,500. Soon after, in 1812 the embankment broke in a violent storm, but the breach was mended by the end of September 1814.
The Ffestiniog Railway has crossed the embankment since 1836 when a carriageway was constructed at a lower level on the inland side to take the public road, which is now the A487. The embankment is officially regarded as a bridge and a toll was payable by road vehicles until September 2003, when the Cob was bought by the Welsh Assembly Government. That tollgate was notorious for causing traffic jams at peak holiday travel times: it was not exceptional for the queue to back up all the way to Minffordd, 1 mile (1.6 km) away. The road on the Cob was widened in 2002, and a separate path added for walkers and cyclists. This path now forms part of Lôn Las Cymru, the national cycle route from Holyhead to Cardiff.
At its seaward end, Traeth Mawr joins "Traeth Bach" ("little sands"), the estuary of the Afon Dwyryd.
- Boyd, James I.C. (1975) . The Festiniog Railway 1800 - 1974; Vol. 1 - History and Route. Blandford: The Oakwood Press. ISBN 0-8536-1167-X. OCLC 2074549.
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