The Cob

Traeth Mawr (Welsh for "big sands") 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 century and the early 19th century. A large embankment runs across the edge of the area separating it from the sea; it is named the Cob and carries a road and railway line.

The original estuary of the Afon Glaslyn was a dangerous place; many people were said to have died in the quicksands trying to cross the Traeth Mawr. In the late 18th century, various landowners around the edge of the estuary began to systematically reclaim land of between 50 acres (20 ha) and 100 acres (40 ha). Between 1770 and 1800 this resulted in the creation of about 1,500 acres (607 ha) of new land.

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 2 mi (3.2 km) 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 dykes construction. The rails were later used by the Croesor Tramway.

In 1807 Madocks obtained a Private Act of Parliament permitting him to complete the reclamation of Traeth Mawr. Between 1808 and 1811 an embankment called "the Cob" was constructed from the island of Ynys Towyn (now part of Porthmadog) to Boston Lodge in the Meirionnydd. The massive stone-lined earthwork 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 work completed the reclamation of 1,500 acres (607 ha) of Traeth Mawr.

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