Ancient Monuments

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Ogwen Fish Weir

A Scheduled Monument in Llandygai (Llandygái), Gwynedd

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Latitude: 53.2405 / 53°14'25"N

Longitude: -4.093 / 4°5'34"W

OS Eastings: 260421

OS Northings: 373543

OS Grid: SH604735

Mapcode National: GBR JN84.7VV

Mapcode Global: WH542.3T73

Entry Name: Ogwen Fish Weir

Scheduled Date: 23 July 2002

Source: Cadw

Source ID: 3895

Cadw Legacy ID: CN335

Schedule Class: Agriculture and Subsistence

Category: Fish weir

Period: Post Medieval/Modern

County: Gwynedd

Community: Llandygai (Llandygái)

Traditional County: Caernarfonshire


The monument consists of the remains of a very large, impressive and well-preserved fish weir, the construction of which has been dated by tree-ring analysis to 1556. A fish weir is usually characterised by a foundation of stone topped with a fence or row of stakes, often with nets attached forming an enclosure within a river or harbour and used for catching, or holding, fish. The Ogwen Fish Weir is rectilinear and, unusually, is largely defined by slate posts, set approximately 2m apart, with oak posts in between as a base for the wattle panels that once existed between them. Stonework is limited to a narrow stone bank that may have acted as protection against scouring at the base of the stakes. The outer 70m of both arms of the weir includes tubular steel posts and occasional lengths of rope and net survive, strung between the posts.

The inner arm of the weir runs northwards into the Menai Straits for some 800m at a right angle to the shore before curving towards the east. The outer wall turns to the south-east at a right angle and runs alongside the Ogwen channel for some 300m. A substantial spur, 150m in length, turns back to the shore at 45 degrees from the southern end of the outer arm.

There are the remains of a sluice at the apex of the trap. Erosion by the Ogwen channel has uncovered the remains of earlier phases of the weir and some 17 rows of posts are visible, some with remnants of wattle in-fill. A further row of posts runs off the outer arm into the Ogwen channel for approximately 20m. A well-preserved section of fallen wattle fence is preserved in the mud near the apex of the weir.

The monument is of national importance for its potential to enhance our knowledge of post-medieval maritime agricultural practices. It retains significant archaeological potential, including the presence of substantial amounts of preserved wood, with a strong probability of the presence of further associated archaeological features and deposits. The structure itself may be expected to contain archaeological information concerning chronology and building techniques.

The scheduled area comprises the remains described and areas around them within which related evidence may be expected to survive.

Source: Cadw

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