Sunday, 2 March 2014

Mapping Mountains – Trimble Surveys – Arenig

01.03.14  Y Garn (SH 758 375) and Pt. 477.0m (SH 758 378)

Pt. 477.0m (SH 758 378)
To the north and west of Llyn Celyn (SH 852 406) is a vast tract of empty moor with summits of heather and flat land of bog.  Much of this land is known as Y Migneint, this can be translated as ‘the marshy streams’, a rather apt name for the area.  Interspersed amongst the bog land are quiet summits which are predominantly heather bound and only occasionally visited.  The A4212 and B4391 roads bisect this land heading from Y Bala to Trawsfynydd and Ffestiniog respectively.

The hills we planned to visit are situated at the head of Cwm Prysor, this is the land that sweeps west toward the coast from Llyn Tryweryn (SH 788 385).  These hills have a chequered listing history which will be detailed shortly.  The southerly top has the name of Y Garn against it on the Ordnance Survey.

The hills were first published in a list by the blog author, they appeared in two sources; the first was on Geoff Crowder’s website and the second was in the RHB file database.  The second of these sources was then augmented by data from the late E.D. ‘Clem’ Clements.  Latterly they have appeared in the TuMPs listing by Mark Jackson and in the Europeaklist booklet entitled Y Pedwarau (Myrddyn Phillips and Aled Williams 2013).  Their listing history appears below:

Geoff Crowder’s Backpacking website (    Year: 2002    Author:  Myrddyn Phillips    List:  The Welsh 400 Metre Peaks

Y Garn                      SH 759 375          474m 
Pen Llyn y Garn      SH 759 378      c 470

The name of Pen Llyn y Garn is an invented name and has no historical or local credence

RHB File Database      Year:  2002 (Nov)      Author:  Myrddyn Phillips (with additional data later added by the late E.D. ‘Clem’ Clements)      List:  400m Hills of England, Isle of Man and Wales

Y Garn (S Top)                                      SH 759 375      474m          c 38m drop

Y Garn: N Top (Pen Llyn y Garn)      SH 759 379     476sd            33m drop

RHB File Database (latterly Hill Bagging website)      Year:  2009      Author:  Mark Jackson      List:  Tumps

Y Garn      SH 759 379      476m      33m drop

Europeaklist      Year:    2013      Authors:  Myrddyn Phillips and Aled Williams      List:  Y Pedwarau

Pt. 479m      SH 758 379      479m      34m drop

Y Garn          SH 758 375      474m      29m drop

I first visited these hills on the 2nd August 2003 and used my measuring staff to survey the drop from the connecting bwlch between the two hills (SH 758 378) to the summit of the northern peak (SH 758 379).  My brief comment relating to this survey was ‘Used lots of rock amongst lots of heather to measure on to, good wild area around Llyn y Garn’.  The measure came to 110ft (33.5m), therefore the hill was promoted from the ‘Hills to Survey’ Sub-List into the main 400m P30 list.  The southerly peak was not surveyed from the connecting bwlch as the distance and ground was deemed to be sufficiently difficult to ascertain an accurate result.  Although visiting the two hills on that day I unfortunately did not sight from one summit to the other, and as the northerly top did not have a spot height on the maps of the day it has always been difficult to determine which hill is the higher and if the southerly top has over 30m of prominence.

When re-evaluating the Welsh 400m hill data for Y Pedwarau I examined all available on-line mapping and found a 1461ft (445.3m) levelled height on the Ordnance Survey 1:2,500 map for the height of the connecting bwlch, this would give the northern peak an approximate height of 478.8m.  This levelled bwlch height would also give the southerly peak an approximate drop of 28.7m.  Some of these figures are based on my basic levelling survey having a margin of uncertainly over such distance and terrain of around + / - 2m.

It was our intention to take the Trimble to each summit and the connecting bwlch to determine an accurate height for each point and by doing so compare the data to that listed above.

I met Aled at a gravelled parking area beside the A 4212 (SH 784 386), we then walked west, south-west on the remains of the railway that used to connect Llan Ffestiniog to Y Bala.  The line closed to passengers in 1960 and to freight in 1961.  When the reservoir at Llyn Celyn was built the line was flooded and although a diversion was considered it never materialised.  The summit of the line was at the head of Cwm Prysor at 1278ft / 390m.  This is where the gravelled parking area is situated.

The highlight of walking this section of the old railway line is an impressive viaduct that spans the Afon Prysor as it flows southward from the morass of bog around Llyn Cors-y-barcud (SH 761 394).  The old railway line gives good walking, albeit today it was a little muddy in places.

The viaduct on the old railway line that used to connect Llan Ffestiniog with Y Bala
This approach to the hills was new for me, but Aled had been this way before and suggested that a steep rock strewn slope would give us access to the outflow from Llyn y Garn (SH 761 376).  This slope is just after the rock named as Craig Aderyn on the map and it is as Aled described; steep and full of rock.  Not a bad way up though and it bi-passed miles of heather.

The route up the steep rock strewn slope to the outflow of Llyn y Garn
By now the sun was making an appearance and the slight warmth of spring’s entrance could just be felt.  However, it didn’t last very long and as we crested the final bit of rock we were greeted by the expanse of Llyn y Garn, so close to an A road but wildly situated nonetheless.

Llyn y Garn
Only a short distance above us was the small summit cairn on the 474m high top of Y Garn.  Soon the Trimble was set up and gathering data.  Away to the west the Rhinogydd were quickly disappearing under one of the afternoon’s predicted showers.  We hoped the coastal bulk of this mountain range would keep the rain from heading inland.

The Trimble on the summit of the southerly top of Y Garn, with Arennig Fach (L) and Arennig Fawr (R) in the background
Once 11 minutes of data had been collected we headed down through the heather to the connecting bwlch, this consists of a reed infested runnel that feeds in to Llyn y Garn.  We judged the placement of the critical bwlch from various angles and left the Trimble to gather more data; by the time we had chatted about all manner of things it had been logging its points for 15 minutes.

Aled spotted this little creature as we made our way down to the connecting bwlch - a Common Lizard
Arennig Fawr (SH 827 369) towering over the expanse of Llyn y Garn
The ascent to the northern top consists of more heather, none of it too difficult and none of it knee high as in some places hereabouts.  We found the high point and set the Trimble up just as the first few rain drops were carried in the breeze toward us, the sky to the west had turned a dull grey heralding the end of the sunshine we had all too briefly enjoyed.

Aled on the high point of the northern peak (SH 758 378)
The Trimble GeoXH 6000 gathering data at the summit of the northern peak
Another 11 minutes of data was collected from the northern top we then headed east on a direct course toward the edge of the conifer plantation at SH 774 384.  Part of this ground is relatively easy as not all is on heather, but there are bogs and a few hidden rocks to make the descent entertaining.

Once down beside the edge of the conifer plantation we re-joined our inward route and followed the old railway line back to the awaiting cars. 

Survey Result

Y Garn

Summit Height:  474.2m (converted to OSGM15)

Summit Grid Reference:  SH 75874 37543

Bwlch Height:  443.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 75794 37853

Drop:  31.2m (400m Sub-Pedwar reclassified to Pedwar))

Dominance:  6.57%

Pt. 477.0m

Summit Height:  477.0m (converted to OSGM15)

Summit Grid Reference:  SH 75906 37969

Drop:  c 41m

Dominance:  8.60%

This result is interesting as the 443.1m bwlch height (+/- 0.3m margin of uncertainty for placement) does not compliment the 1461ft (445.3m) levelled height on the OS 1:2,500 map.  However, the position of the levelled height is not necessarily at the critical bwlch.  The drop value of 110ft (33.5m) from the basic levelling survey I conducted in August 2003 is in good accordance with the 33.9m drop value ascertained from the Trimble survey, taken from the 477.008m (converted to OSGM15) northerly summit to the 443.062m (converted to OSGM15) connecting bwlch.  This is the first new Pedwar that has been found from surveying with the Trimble GeoXH 6000. 

The new Pedwar of Y Garn (SH 758 375)
For further details please consult the Trimble survey spreadsheet click {here}

High accuracy surveying at work on the northern summit!

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