Mapping Mountains – Hill Reclassifications – Marilyns

Mynydd Bwlch-y-Groes (SN 868 356) – Submarilyn deletion

This is one in a series of retrospective Hill Reclassification posts that detail hills whose status has altered in the listing of the Welsh Marilyns.  These reclassification posts will give details of hills where I have had direct association with their change of status, and they will tie in with a forthcoming Change Register giving detail to this list and its alterations since publication in the TACit Tables booklet

The listing of Welsh Marilyns was published in booklet format by TACit Tables in February 1997 and entitled The Hewitts and Marilyns of Wales.  The Marilyns were originally listed as an all British compilation and were published by Cicerone Press in 1992 in the book entitled The Relative Hills of Britain.  The list compiler for the Marilyns is Alan Dawson.

When the Welsh Marilyns list was published in booklet format there were 156 qualifying hills with a further 27 Welsh Submarilyns also included.  The criteria for Marilyn status being any hill that has a drop of at least 150m, irrespective of their height, with the criteria for Submarilyn being any hill that has 140m or more and below 150m of drop.

The Hewitts and Marilyns of Wales by Alan Dawson

The details for this deletion appear below:

The name of the hill is Mynydd Bwlch-y-Groes, and it is situated in the Mynydd Epynt group of hills in the southern part of mid Wales, and is positioned with the A483 road to its north-west and the A40 road to its south, and has the town of Llanymddyfri (Llandovery) towards the west.

Prior to the deletion of this hill from Submarilyn status it was listed with 140m of drop, based on the 442m summit height which is adjoined to a triangulation pillar and which appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and a bwlch height of 302m based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

Extract from the Ordnance Survey 1:25,000 Explorer map for the bwlch area of this hill

The details for this hill were examined during a re-assessment of the Y Pedwarau – The 400m Hills of Wales list and forwarded to the list author of the Marilyns; Alan Dawson.  The area of this hill’s bwlch was assessed using mapping on the OS Maps website.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.  This mapping has bwlch contours between 305m – 310m with the position of the 302m spot height that appears on the Ordnance Survey 1:25,000 Explorer map below the 305m contour line, indicating that it is not positioned at the critical point of this bwlch.

Extract from the OS Maps website for the bwlch area of this hill

These details were forwarded by Myrddyn Phillips via email to the list author of the Marilyns on the 8^th October 2019, and the notification that this hill had been deleted from the status of Submarilyn was received via email from Alan Dawson on the 16^th October 2019.

LIDAR summit image of Mynydd Bwlch-y-Groes (SN 868 356)

The hill was subsequently analysed by Myrddyn Phillips using latest available LIDAR, resulting in a minor summit relocation with a 441.9m summit height positioned at SN 86898 35691 and a 305.05m bwlch height positioned at SN 88817 38632, with these values confirming its deletion with 136.9m of drop. 

 

The full details for the hill are: 

Name:  Mynydd Bwlch-y-Groes 

OS 1:50,000 map:  160

OS 1:25,000 map:  12, 187

Summit Height:  441.9m (LIDAR) 

Summit Grid Reference:  SN 86898 35691 (LIDAR) 

Bwlch Height:  305.05m (LIDAR) 

Bwlch Grid Reference:  SN 88817 38632 (LIDAR) 

Drop:  136.9m (LIDAR) 

 

Myrddyn Phillips (March 2020)