Mapping Mountains – Hill Reclassifications – 600m Twmpau

Chwarel Graig Ddu (SH 725 455) – 600m Sub-Twmpau addition

There has been an addition to the listing of the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps. 

Chwarel Graig Ddu (SH 725 455)

The criteria for the list that this addition applies to are:

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 600m Twmpau by Myrddyn Phillips

The name the hill is listed by is Chwarel Graig Ddu, and it is adjoined to the Arenig Fach group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its west and the B4391 road to its south, and has the town of Blaenau Ffestiniog towards the west.

When the list that later became known as the 600m Twmpau was first compiled this hill was not included in the accompanying sub list, as because it is a part of a working quarry the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map do not show any spot heights or contours, therefore the hill seemed not to exist. 

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

Since the original compilation of this list there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the resources recently available online is the mapping on the OS Maps website.  This is the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the online Vector Map Local.  Although the mapping on the OS Maps website no longer has contours at 5m intervals, such contours are represented on other mapping available online and it was Aled Williams who scrutinised this mapping which has an uppermost 610m ring contour and bwlch contouring between 590m – 595m, with these contours giving this hill a drop value exceeding the minimum 15m to qualify for this list. 

Extract from the online map showing 5m contouring

The hill has subsequently been inspected from adjacent hillsides and its validity confirmed and as a quarry road passes over its summit area it is considered solid and stable.

Therefore, the addition of this hill to 600m Sub-Twmpau status is due to detail on contemporary online mapping with a subsequent inspection from adjacent hillsides undertaken to confirm its validity, resulting in a 613m summit height and a 592m bwlch height, with these values giving this hill 21m of drop, which is sufficient for it to be classified as a 600m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Arenig Fach

Name:  Chwarel Graig Ddu

OS 1:50,000 map:  115

Summit Height:  613m

Summit Grid Reference:  SH 72505 45533

Bwlch Height:  592m

Bwlch Grid Reference:  SH 72456 45744   

Drop:  21m 

 

Myrddyn Phillips (June 2022)