Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Ynys Gron (SH 596 392) – 30-99m Twmpau reclassified to 30-99m Sub-Twmpau

There has been a reclassification to the Twmpau (thirty welsh metre prominences and upward) list with a 30-99m Twmpau being reclassified to a 30-99m Sub-Twmpau due to surveying with the Trimble GeoXH 6000.  The details relating to this hill’s reclassification are retrospective as the survey with the Trimble took place on the 21^th November 2014.

The criteria for 30-99m Twmpau status is all Welsh hills at or above 30m and below 100m in height with 30m minimum drop, whilst the criteria for 30-99m Sub-Twmpau status is all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop.

The hill had been listed in the original Welsh P30 lists when published on Geoff Crowder’s v-g.me website with a 96m summit height based on the Ordnance Survey 1:25,000 Outdoor Leisure spot height, with an accompanying note stating ‘98m on 1984 1:50,000 map.  Summit being reduced due to quarrying’.  When drop values were added to the P30 Master Lists this hill was given an estimated drop of c 50m.  The heights quoted appeared on maps of the day, although it was also known that the quarrying activities had already reduced the height of this hill.

The hill is situated in the Moelwynion group of hills and is placed in the Region of North Wales (Region A, Sub-Region A1) with its Cardinal Hill being Moelwyn Mawr (SH 658 448) and is situated between the towns of Porthmadog to the west and Penrhyndeudraeth to the east, and immediately above the busy A 487 road which is to its south.

The hill is not on open access land and the whole of its south-western section forms part of the Garth Quarry, which is operational, and because of this permission to visit should be sought.  However, the approach from the east is through woodland and would take a visitor direct to the remaining high point of the hill.

The name of the hill is Ynys Gron, which is a locally known name and it has been reclassified to a 30-99m Sub-Twmpau as the quarry has now obliterated the upper 30m of the hill and reduced it in height to 69.4m, and as its bwlch has been surveyed as 44.3m in height, it gives this hill only 25.1m of drop, which is insufficient for it to remain classified as a P30.

The full details for the hill are:

Cardinal Hill:  Moelwyn Mawr

Summit Height:  69.4m (converted to OSGM15)

Name:  Ynys Gron

OS 1:50,000 map:  124

Summit Grid Reference:  SH 59653 39268

Drop:  25.1m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the remaining high point of Ynys Gron

For details on the survey that reclassified this hill to a 30-99m Sub-Twmpau

Myrddyn Phillips (June 2017)