is a [[polder]] near [[Porthmadog]] in [[Gwynedd]] in [[Wales]]. The area was formerly the large tidal [[estuary]] of the ''[[Afon Glaslyn]]''. It was created after large-scale land reclamation occurred in the late 18th and early 19th centuries. A large embankment, called the Cob, separates the area from the sea and carries a road and railway line.
Around 1798, William Madocks bought the Tan-yr-Allt estate near Penmorfa Marsh. Soon afterwards he reclaimed an area of sand from the sea and the river by building a earthen bank from Prenteg to Clog-y-Berth (now Porthmadog). The township of Tremadog was founded within the new area. A wooden-tracked railway was used in the dyke's construction. The rails were later used by the Croesor Tramway.
Soon after completion in 1812, the embankment was breached in a violent storm. Repairs were completed by the end of September 1814. Although the original estimate to complete the Cob was £23,500 (£16.9million in 2014), it eventually cost Madocks £60,000 (£43.3million in 2014) to finish. At its seaward end, Traeth Mawr joins "Traeth Bach" ("little sands"), the estuary of the River Dwyryd.
A carriageway was also constructed at a lower level on the inland side to take a public highway. Until September 2003, when the Cob was bought by the Welsh Assembly Government, all vehicles crossing the embankment were required to pay a Road pricing. The collection of fees often caused at peak holiday travel times: it was not exceptional for queues to back up to in each direction. In 2002, the highway across the Cob was widened while a separate path was added for walkers and cyclists. The pedestrian route now forms part of Lôn Las Cymru, the national cycle route from Holyhead to Cardiff.
In 2010 work started on the Porthmadog, Minffordd and Tremadog bypass to reduce the amount of through traffic in the town. Welsh Assembly Government : A487 Porthmadog, Minffordd and Tremadog Bypass On completion the original course of the A487 across the Cob was renumbered as the A4971. [[File:Twilight. Snowdon seen across Traeth Mawr, n. Wales.jpeg|thumb| Twilight. Snowdon seen across Traeth Mawr ]] In 2012, of the embankment were widened on the seaward side of the Porthmadog end to allow a second platform to be constructed at the Ffestiniog & Welsh Highland Railway's Harbour Station.
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