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 and. Between 1770 and 1800 this resulted in the creation of about 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 earthen bank from Prenteg to Clog-y-Berth. 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.
Completion
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 to Boston Lodge in the Meirionnydd. The massive stone-lined earthwork was long, wide at the bottom, tapering to at the top, which was above the level of the river. The work completed the reclamation of of Traeth Mawr. 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, it eventually cost Madocks £60,000 to finish. At its seaward end, Traeth Mawr joins "Traeth Bach", the estuary of the Afon Dwyryd.
Transport
In 1836, the Ffestiniog Railway began using The Cob when it built a tramroad to transport slate from the quarries around the inland town of Blaenau Ffestiniog to Porthmadog where it was loaded onto ships. Empty wagons were hauled back up to the quarries by horses. Loaded dandy waggons used gravity to run downhill from Blaenau Ffestiniog to the port across The Cob. After a number of trials and some modifications, the line was upgraded to a narrow gauge railway in 1863. In 1927 the Cob was breached again, and took several months to repair. 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 toll. The collection of fees often caused traffic jams 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. On completion the original course of the A487 across The Cob was renumbered as the A4971. 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.
