Ancient Monuments

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Rhaslas Pond South Dam

A Scheduled Monument in Darran Valley (Cwm Darran), Caerphilly (Caerffili)

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Latitude: 51.7543 / 51°45'15"N

Longitude: -3.3118 / 3°18'42"W

OS Eastings: 309548

OS Northings: 206993

OS Grid: SO095069

Mapcode National: GBR HR.0PPC

Mapcode Global: VH6CZ.K51H

Entry Name: Rhaslas Pond South Dam

Scheduled Date: 18 December 2017

Source: Cadw

Source ID: 4381

Cadw Legacy ID: GM624

Schedule Class: Water Supply and Drainage

Category: Dam

Period: Post Medieval

County: Caerphilly (Caerffili)

Community: Darran Valley (Cwm Darran)

Traditional County: Glamorgan


The monument comprises the remains of the south dam at Rhaslas Pond, which was the largest and most easterly of the ponds within the Dowlais Free Drainage System. First established in the late 18th century, this system gathered water from the hill slopes of Merthyr Common and Twyn-y-Waun, and transferred it by gravity through a network of leats and ponds to the Dowlais and Ivor Ironworks. It is thought that Rhaslas Pond was established by 1818, and the drainage system as a whole reached its maximum extent by 1868. Rhaslas Pond is impounded by a dam on its northern and southern sides and covers an area of c. 12.34ha. The south dam is the larger and more imposing of the two dams and is largely in its original condition. It forms a dog-leg, with a total length of 496m. It is made of earth and stone, and varies in height form 2-10m and has a maximum base of c.20m. The outer face is grass covered and has a maximum slope of 1:1. The dam is surmounted by a flat top 3.1m wide carrying a track. Concrete pads with metal stumps on the inner face are the remains of a later fence. The north slope of the dam is protected by a pitched, rubble stone, wave guard, which extends from the top of the slope to below the water level. In places this has been patched with concrete. In 2008, the outer face of the dam was reinforced with earth for a 70m length.

The monument is of national importance for its potential to enhance our knowledge of post-medieval industrial water management practices. It has been scheduled as the largest and best-preserved component of the Dowlais Free Drainage System and forms an important element within the wider industrial landscape. It is well preserved and retains considerable archaeological potential. There is a strong probability of the presence of evidence relating to building techniques and functional detail. It possesses important group value with the scheduled Sarn Howell Ponds and Leats (GM494), which is the most complete part of the system to survive. Together they show how important water was as a resource in the development of the iron industry in Merthyr Tydfil.

The scheduled area comprises of the remains described and an area around them within which related evidence may be expected to survive. It is a dog-leg polygon in shape and measures 505m in length by 25m transversely. The scheduled area is limited to the south dam structure and does not include the floor of the pond or the north dam structure.

Source: Cadw

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