Rhinog Fach (SH 664 270) – New Marilyn

 

Rhinog Fach is positioned in the northern Rhinogydd, in the west of Wales between Abermaw (Barmouth) to its south and Trawsfynydd to its north and has awaited an accurate survey for many years.  It was listed with an estimated c 147m of drop in The Hewitts and Marilyns of Wales booklet published by TACit Tables in 1997, and has remained a high prominence sub (relative to P150) ever since.

Rhinog Fach (SH 664 270)

For the uninitiated a Marilyn is any hill in Britain that has a minimum drop of 150m, irrespective of its height.  The list is authored by Alan Dawson and was originally published by Cicerone Press in 1992 in the book entitled The Relative Hills of Britain. 

This hill had been on the surveying agenda for a number of years and with a 712m listed summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and bwlch contouring between 560m – 570m, it remained the most important high prominence Submarilyn yet to be accurately surveyed. 

I approached the hill from the east via forestry and Bwlch Drws Ardudwy, with a steep ascent following to its summit, which was easy to identify.  I imagined I’d have to take a number of data sets from various points, but the high point consists of a small protruding rock about 2 metres from the base of the cairn.  I had a good rummage in the top of the cairn, but couldn’t find anything natural and higher.  The end of the Trimble was placed on a rock taken from the cairn to give it horizontal alignment.

The summit of Rhinog Fach

1st summit survey - 711.632m

2nd summit survey - 711.642m

Two data sets were taken at the summit, one on the ground with the equipment’s internal antenna aligned to the high point and the second atop my rucksack with the measurement offset duly noted:

 

Rhinog Fach summit: 

711.632m at SH 66485 27017 (Trimble on ground)

711.642m at SH 66485 27017 (Trimble on rucksack)

Average = 711.637m 

 

Three data sets were taken from the area of the bwlch, which is positioned just to the east of the solid stone wall traversing these hills.  The ground on the west side of the wall and ground at the base on its eastern side is lower compared to the Trimble set-up positions.

 

Rhinog Fach bwlch:

560.618m at SH 66555 26531 (hill to hill traverse)

560.560m at SH 66557 26533 (hill to hill traverse)

560.599m at SH 66561 26533 (valley to valley traverse [and what turned out to also be the hill to hill traverse])

 

These values give Rhinog Fach 151.038m of drop.

 

1st bwlch survey - 560.618m

2nd bwlch survey - 560.560m

3rd bwlch survey - 560.599m

All surveys gave Estimated Accuracies as 5cm – 15cm: 100.00%, which is the norm for the Trimble GeoXH 6000 and the Standard Deviation ranged from 0.020m – 0.038m for the five surveys.

Alan Dawson is in receipt of the details relating to this survey and has confirmed Rhinog Fach as a new Marilyn, the first in Wales since the survey of Mynydd Anelog (SH 151 272) in July 2013, and the first in Britain since the survey of Beinn Dearg (NC 279 658) by Jon Metcalf in 2018.

 

Myrddyn Phillips