Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Mynydd Cerrig y Bleiddiau (SH 445 902) – 100m Sub-Twmpau deletion 

There has been a deletion to the list of 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Mynydd Cerrig y Bleiddiau (SH 445 902)

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Mynydd Cerrig y Bleiddiau and this was derived from the Tithe map, and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the B5111 road to its north-west and the A5025 road to its east, and has the village of Pen-y-sarn towards the east. 

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the Hills to be surveyed sub list with a 131m summit height. 

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 24m of drop, based on the 131m summit spot height and an estimated c 107m bwlch height based on interpolation of 5m contouring that was thought to be between 105m – 110m that appear on the Ordnance Survey 1:25,000 Explorer map.  With the estimated drop value later re-evaluated and listed as c 23m.  However, due to past quarrying activity the contours on the area of this hill’s bwlch are not continuous; therefore there was conjecture to its drop value. 

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

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

Therefore, the deletion of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 131.1m summit height and a 113.1m bwlch height, with these values giving this hill 18.0m of drop which is insufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Mynydd Cerrig y Bleiddiau 

OS 1:50,000 map:  114 

Summit Height:  131.1m (LIDAR) 

Summit Grid Reference:  SH 44539 90284 (LIDAR) 

Bwlch Height:  113.1m (LIDAR) 

Bwlch Grid Reference:  SH 44495 90291 (LIDAR) 

Drop:  18.0m (LIDAR) 

 

Myrddyn Phillips (March 2021)