Mapping Mountains – Significant Name Changes – The 500-Metre Tops of England and Wales – Deweys

Carn yr Hyrddod (SN 790 704) and Lan Ddu Fawr (SN 787 699) 

There has been a Significant Name Change due to a newly recognised twin top to a hill that is listed in the The 500-Metre Tops of England and Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey and LIDAR analysis conducted by Myrddyn Phillips and Aled Williams. 

Carn yr Hyrddod (SN 790 704)

The criteria for the list that this name change applies to are:

The 500-Metre Tops of England and Wales are affectionately known after their hill list compiler; Michael Dewey.  This list mixes metric and imperial height in its criteria to bookend up to the 2000ft height band and takes in all hills in England, Isle of Man and Wales that are 500m and above and below 2000ft (609.6m) in height that have 30m minimum drop. 

Mountain tables by Michael Dewey

Michael Dewey died on the 5^th November 2022.  His wife; Gillian wished for this list to be carried on and made provision for it to be co-authored by Myrddyn Phillips.  Michael will always be the compiler and originator, it’s just that he’s now got a co-author to safeguard and carry this list on.

The twin top hill is adjoined to the Carn yr Hyrddod group of hills, which are situated in the northern part of South Wales, and it is positioned with a minor road to its south and the B4343 road to its west, and has the village of Pontrhydfendigaid towards the west south-west. 

Lan Ddu Fawr (SN 787 699)

When the original list of Deweys appeared in the Mountain tables book published by Constable in 1995, this hill was listed under the name of Waun Claerddu with a summit height of 594m, based on the spot height positioned at SN 790 704 that appears on the Ordnance Survey 1:25,000 Explorer map. 

Extract from the Ordnance Survey 1:25,000 Explorer map

To the south of the originally listed Dewey is the summit of Lan Ddu Fawr which is given a 593m summit spot height adjoined to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.  As the northerly hill was given as 1m higher on Ordnance Survey maps it was this hill that was listed as the Dewey. 

Extract from the Ordnance Survey 1:50,000 Landranger map

However, these two summits cannot be split in height, with the difference produced by LIDAR analysis and a Trimble GeoXH 6000 survey being within the margin of uncertainty applicable to each method.  Therefore, these summits are now listed as a Twin Top Dewey.

When Michael originally compiled this list the priority hill was listed as Waun Claerddu.  This name relates to the large water basin/bog to the south-east of the southerly of these two summits.

The northerly hill has the name Carn yr Hyrddod applied to its ancient cairn on the contemporary Ordnance Survey 1:25,000 Explorer and 1:50,000 Landranger map and this is also the locally known name for this hill.

The southerly summit has the name Llan Ddu Fawr applied to it on the contemporary Ordnance Survey 1:25,000 Explorer map, whilst the Ordnance Survey Six-Inch map has Lan Ddu Fawr.  The word Lan can have the meaning of side/edge/up/hill and it is this word that is preferred, with Lan Ddu Fawr also the locally known name for the southerly of these summits.  Therefore, the word Lan is preferred to Llan, as per Ordnance Survey Six-Inch maps. 

Extract from the Ordnance Survey series of Six-Inch maps

Therefore, this hill is now listed as a Twin Top in the Deweys, with the name of the northerly twin top now listed as Carn yr Hyrddod and the name of the southerly twin top now listed as Lan Ddu Fawr, and this was derived from a combination of sources including contemporary Ordnance Survey maps and the series of Ordnance Survey Six-Inch maps, local enquiry and advice given by Aled Williams relating to the confines of the use of the words Lan and Llan in Wales. 

 

The full details for the hill are: 

Group:  Carn yr Hyrddod 

Name:  Carn yr Hyrddod and Lan Ddu Fawr 

Previously Listed Name:  Waun Claerddu

OS 1:50,000 map:  135, 147

OS 1:25,000:  213 

Summit Height:  592.7m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 79069 70421 (Trimble GeoXH 6000 for Carn yr Hyrddod) and SN 78759 69974 (Trimble GeoXH 6000 for Lan Ddu Fawr)  

Bwlch Height:  424.8m (LIDAR)

Bwlch Grid Reference:  SN 79547 66955 (LIDAR)

Drop:  168.0m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

My thanks to Aled Williams for advice relating to the listed name of this hill 

Myrddyn Phillips (August 2025)