Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Hidden Slabs Stack (SS 119 982) – 30-99m Twmpau deletion

There has been confirmation of a deletion to the 30-99m Twmpau list due to LIDAR analysis conducted by Myrddyn Phillips, with the criteria for this list being:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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.

This hill was included in the original Welsh P30 lists that were published on Geoff Crowder’s v-g.me website and given an estimated summit height of c 30m.  When compiling these lists I contacted Dave Viggers in relation to sea stacks positioned around the Pembrokeshire coast, regarding their height, location and name.  Dave is now a Vice President of the Climber’s Club and gave me information including the location of this hill, its approximate height and its name, which he advised was known as Hidden Slabs Stack.  However, site visits to this location by a number of people including Jon Glew, Douglas Law, Rob Woodall and Adrian Rayner have brought in to doubt the existence of a sea stack at this point, and if anything did exist its height was estimated to be far lower than the listed c 30m.  

The name of this hill is Hidden Slabs Stack, and as its name and LIDAR analysis implies it is a sea stack.  The hill is adjoined to the Brandy Hill group of hills, which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), and is positioned with the coast to its immediate south and has the A 4139 road and the small community of Penally to its north-west.

If wanting to visit this hill permission to do so should be sought as it is not a part of designated open access land, for those wishing to do so the Pembrokeshire Coast Path is positioned to the north of the hill, but caution is advised as the easiest approach may necessitate some form of climbing.

The confirmation of the deletion of Hidden Slabs Stack from 30-99m Twmpau status is due to LIDAR analysis conducted by Myrddyn Phillips.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

2m DTM LIDAR image of Hidden Slabs Stack

1m DSM LIDAR image of Hidden Slabs Stack

The 1m DSM LIDAR analysis gives the hill the following details:

Name:  Hidden Slabs Stack

Summit Height:  6.5m

Summit Grid Reference:  SS 11902 98247

Bwlch Height:  N/A

Bwlch Grid Reference:  N/A

Drop:  6.5m

Therefore, the 6.5m LIDAR analysis for the summit position at SS 11902 98247 gives this hill 6.5m of drop, which is insufficient for continued 30-99m Twmpau status.

LIDAR image of the summit of Hidden Slabs Stack

The full details for the hill are:

Group:  Brandy Hill

Name:  Hidden Slabs Stack

OS 1:50,000 map:  158

Summit Height:  6.5m (LIDAR)

Summit Grid Reference:  SS 11902 98247 (LIDAR)

Bwlch Height:  N/A

Bwlch Grid Reference:  N/A

Drop:  6.5m (LIDAR)

Myrddyn Phillips (October 2018)