Mapping Mountains – Hill Reclassifications – 30m–99m Twmpau

Ynys Fach (SM 822 327) - 30-99m Twmpau reclassified to 30-99m Sub-Twmpau


There has been a reclassification to the listing of the Twmpau (thirty welsh metre prominences and upward) hills due to a recent survey with the Trimble GeoXH 6000.  The hill is listed in the 30m – 99m height band of Twmpau hills and is situated on the northern Pembrokeshire coast close to the small communities of Porthgain to its west and Trefin (Trevine) to its east.

The hill is better described as an island, and even more so as a tidal island, and is connected to the mainland of Wales by a pebbled beach that according to maps is cut off from the mainland at high water.  It can be approached by boat, but an ascent of the island is feasible, albeit steep and potentially dangerous, from the mainland.  This approach starts from directly opposite the island and necessitates finding a route down bi-passing the rock and cliffs that predominate this part of the coastline.  If attempting this route please be careful.

The island was surveyed on 14^th May 2016 on a beautiful sunny evening in the company of Adrian Rayner, Ayako and Rob Woodall.  It was Rob’s suggestion to go and have a look at it, and he found the route down to the pebbled beach which constitutes this island’s connecting bwlch.

The name of the island is Ynys Fach and it first appeared in a hill list on Geoff Crowder’s v-g.me website as a part of the original listing of Welsh P30s taking in the 100m height bands below the height of 500m. 

Ynys Fach is given an uppermost contour ring of 30m on current Ordnance Survey maps with no adjoined spot height.  However, the Ordnance Survey Six Inch map revised in 1906 and published in 1908 gives a 99ft height for the island, this is when the composition of the name was presented as Ynys Fâch, as opposed to the current Ynys-fach.  The 99ft spot height was later given on the Ordnance Survey 1:25,000 Historical map, with 99ft being an approximate 30.2m in metric measurement.

Two points were surveyed for summit position and these are given below:

Summit 1:  30.8m (30.789m, converted to OSGM15) at SM 82209 32743

Summit 2:  30.7m (30.727m, converted to OSGM15) at SM 82217 32752

The data set for the bwlch was remarkably good when considering that the equipment was placed in what resembled an enclosed and walled canyon, with the result being 1.2m (1.225m, converted to OSGM15) at SM 82180 32693.  These values give Ynys Fach a drop of only 29.6m and therefore it is reclassified from a 30-99m Twmpau to a Sub-Twmpau.

The full details for the hill are:

Cardinal Hill:  Garn Fawr

Summit Height:  30.8m (converted to OSGM15)

Name:  Ynys Fach

OS 1:50,000 map:  157

Summit Grid Reference:  SM 822 327

Drop:  29.6m (converted to OSGM15)

Ynys Fach (SM 822 327) now reclassified from a Twmpau to a Sub-Twmpau

For details on the survey that reclassified this hill from P30 Twmpau to Sub-Twmpau status.

Myrddyn Phillips (May 2016)