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Grotto in Gnoll Estate

A Grade II Listed Building in Tonna, Neath Port Talbot

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Latitude: 51.6683 / 51°40'5"N

Longitude: -3.7649 / 3°45'53"W

OS Eastings: 278044

OS Northings: 198090

OS Grid: SS780980

Mapcode National: GBR H4.63QJ

Mapcode Global: VH5GN.PBJD

Entry Name: Grotto in Gnoll Estate

Listing Date: 25 February 2000

Last Amended: 25 February 2000

Grade: II

Source: Cadw

Source ID: 22860

Building Class: Gardens, Parks and Urban Spaces

Location: At the NE end of the Gnoll Estate on the hillside immediately W of the informal cascade.

County: Neath Port Talbot

Community: Tonna

Community: Tonna

Locality: Gnoll Estate

Traditional County: Glamorgan

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The extensive park and grounds at the Gnoll were laid out for Sir Humphrey Mackworth in 1724-7 and were centred upon fish ponds (that also supplied water to his copper works) and cascades. An informal upper cascade in Mosshouse Wood was added in the 1740s by an unknown designer, working for Herbert Mackworth. The grotto beside the cascade was also added at this time. Follies were later added in the 1780s by Sir Herbert Mackworth, which included a gazebo above the grotto and the Ivy Tower. The late C18 was the heyday of the Gnoll grounds, although it was revived in the C19 by the Grant family and Charles Evan Thomas. The latter came to an agreement with Neath Corporation for the building of Mosshouse Wood reservoir, which is dated 1889. The estate was acquired by the local authority in 1923. The house was demolished in 1957. Restoration of the grounds began in 1984-5 when the grotto was rediscovered.


Cut into bedrock beneath a rebuilt gazebo. The entrance has a large bedrock lintel and is flanked by drystone revetment walls, the whole composition intended to resemble a Cotswold-Severn style prehistoric chamber tomb. Inside, the single circular chamber is quarried out of bedrock and has a crude flattened domical stone vault, which was intended to drip lime and form stalactites. The floor has an outer circular pavement of random paving stones and a kerb, which probably defines a former pool in the centre.

Reasons for Listing

Listed as an unusual and well-preserved feature of an important landscape garden, and for group value with the adjacent cascade.

Other nearby listed buildings

  • II Mosshouse Wood Cascade
    On the NE side of the Gnoll Estate E of Mosshouse Wood reservoir.
  • II Ivy Tower
    Approximately 0.7km S of Tonna church, prominently sited on high ground to the E of Dan-y-lan Farm.
  • II Penlan-fach bridge
    On an unclassified road to Wenallt Farm approximately 1km ESE of Tonna church spanning a stream and the former Glyncorrwg Mineral Railway.
  • II Church of St Anne
    Set back on the S side of the main B4434 through the village.
  • II Nazareth Chapel
    On the S side of the main B4434 through the village on the E side of the junction with School Road, and set back from the main road behind a railed forecourt.
  • II Tyn yr Heol
    Set back from the main B4434 through the village 0.7km SW of Tonna church. In its own grounds with rubble boundary wall and stone gate-piers to the drive.
  • II ,12,Dulais Fach Road,Aberdulais,,,SA11 3JW
    On the E side of Dulais Fach.
  • II Dulais Fach
    Set back on the N side of Dulais Fach Road with Dulais Isaf House attached to the W and N sides.

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