Porthmadog Cob was built to carry the Ffestiniog Railway across the wide Afon Glaslyn estuary and to reclaim the large area of low-lying land to the north. Slate from the mines and quarries around Blaenau Ffestiniog was transported to Porthmadog, to be loaded onto ships. The beach is essentially a large sand-bank with a few muddy bits, turning to salt-marsh towards the north-eastern corner. On the west side is the artificial island of Cei Ballast - built up over the years from ballast dumped by the ships returning to Porthmadog. The sandy shore has very little gradient, and will flood quickly on a rising tide. As with most sandbanks, there are numerous channels which will fill early, so a close eye needs to be kept on the tide if you want to visit this beach and leave again without getting wet. Few people come here, and dogs are allowed at all times.
A footpath runs alongside the railway on the Cob, and access to the shore is by crossing the railway (look both ways first) and down the wall of boulders which protect it from the sea. Low-tide access is possible along the shore from Portmeirion.