This is the RIGS citation for the site, which has now been formally designated. The process has taken well over a year, and it is certain that there has been heavy political pressure behind the scenes to prevent the designation from happening at all, or to replace it with something more in line with what the archaeologists and the National Park would like to see. Part of the problem is that there is no RIGS committee for West Wales, with decisions on designation now being taken at an all-Wales level, by a new body called Geoconservation Wales. That means that the "local knowledge" component has been lost. So there was an argument that this should be designated as a "dual" (geomorphological / archaeological) site. We stuck to our guns on this and argued that there is a separate system for designating archaeological and heritage sites, and that if the archaeologists want to go down that route, that's fine by us. We have made the citation less confusing by dropping a long para on the geological context and by concentrating on the Quaternary. We have also agreed the incorporation of a statement explaining that there is a disagreement between geomorphologists and archaeologists as to what the significance of this site really is!
Much ado about not very much, and thanks to John Downes for doing all the hard work. In any case, the citation is not set in stone, and can easily be modified in the future if new discoveries are made. But this is a designation with planning weight, which means a degree of protection for Rhosyfelin from future development.
Site Name: RhosyfelinRIGS (Regionally Important Geodiversity Sites) Number: 564
Grid Reference: SN117362
RIGS Statement of Interest:
Craig Rhosyfelin is a craggy outcrop of Ordovician rhyolite in the valley of the Afon Brynberian. The rhyolite belongs to the Fishguard Volcanic Group which outcrops along the northern margin of Mynydd Preseli. This site is of particular interest since the rocks are exposed on a series of fracture planes and rhyolite samples from the rock face have recently been matched to “bluestone” fragments in the "debitage" at Stonehenge (See also Carn Menyn RIGS 555). Rhosyfelin is also significant in that it offers an opportunity to examine some of the geomorphological processes and landforms typical of the Pleistocene period in the area. There are examples of scoured surfaces, frost shattered crags and scree, glacial till, fluvioglacial gravels and solifluction deposits. With regard to the rock face, from a geomorphological perspective there is ample evidence that glacial, periglacial and biological processes have all contributed to the widening of joints and the accumulation of rock debris at the foot of the Rhosyfelin crag.
Recent archaeological excavations (Parker Pearson et al, 2015), however, have led to the assertion that part of the Craig Rhosyfelin outcrop and some of the stone debris at its base (including a large, roughly rectangular block) are the result of prehistoric quarrying. This suggestion has been strongly refuted by the current authors (John, Elis-Gruffydd & Downes, 2015a, 2015b) who have argued that the features of the site constitute an association of natural geomorphological landforms and Quaternary sediments. Continued research at the site, both geomorphological and archaeological, including the possible application of Terrestrial Cosmogenic Nuclide (surface exposure) dating, may help resolve the relative contributions of natural and anthropogenic processes at the site.
The rhyolitic crags at Rhosyfelin in the Brynberian valley appear to be an erosional remnant of the main outcrop of the Fishguard Volcanic Group as mapped by the BGS on the 1:50,000 map ( Sheet 210 Fishguard). These rhyolitic lavas are Mid Ordovician in age (Llanvirn stage). They dip steeply northwards (75º according to the BGS) on the north flank of Mynydd Preseli. Field observations at Rhosyfelin reveal deep almost vertical fractures with numerous horizontal cross fractures. The rhyolite is a splintery dark blue rock which weathers to a light grey colour. A considerable amount of rock debris, accumulated at the foot of the steep rock face, has been uncovered during a recent dig by archaeologists.
The geomorphological history of Rhosyfelin is complex since the site has been overridden by the ice of the Irish Sea Glacier on at least two occasions during the last half million years. As on many of the other tors and crags in the area, there are signs of substantial ice smoothing and block removal; it is likely that Craig Rhosyfelin was once higher and more prominent than it is today. Glacial erratics from the north have also been transported into the locality (John, Elis-Gruffydd and Downes, 2015a, 2015b).
Fluvioglacial erosion has occurred at the site on a substantial scale. The main Brynberian river valley is steep sided and has clearly carried great volumes of meltwater. Subglacial meltwater flow is suggested by the small steep channel adjacent to the exposed rock face. It is possible that meltwater has flowed up and over a col under hydrostatic pressure before descending to join the main discharge route again at the end of the rocky spur. It is likely that these features are inherited from the Anglian glacial episode.
Apart from the broken rockfall debris and scree that has accumulated beneath the rock face on a smoothed and undulating bedrock surface, there is an extensive exposure of Devensian glacial till containing many boulders and smaller clasts of dolerite and other erratics. This has been exposed during the archaeological dig. Near the end of the spur this till grades into a fluvioglacial deposit of gravels incorporating many dolerite boulders and other rounded stones. Both bedrock and detached rock slabs in this vicinity are heavily abraded. Recent excavations have exposed a clay-rich horizon beneath the fluvioglacial gravels that may represent a temporary pro-glacial lake on the floor of the valley. There are also periglacial and colluvial slope deposits up to 2m thick, with some internal variation, possibly representing climatic oscillations during the Holocene.
On the valley floor there is a well-developed flood plain where river gravels are currently being reworked during swings by the river. The process of crag diminution by rockfalls is ongoing, with biological processes (root expansion in joints and fissures) currently prominent.
Network context of the site:
Quaternary and Geomorphological RIGS in S.W. Wales are assigned to one or more of the following networks.
1. Pre-Quaternary landscape evolution.
2. Glacial Geomorphology. This network includes such landforms as moraines, cirques, protalus ramparts and kettleholes, and deposits composed of till, moraine and scree, for example.
3. Periglacial Geomorphology. Sub-networks are landforms and deposits formed in environments around the fringes of glacial terrains. They include pingos, patterned ground, ice wedges and solifluction features.
4. Fluvio-glacial Geomorphology. Sub-networks are landforms that include alluvial fans, patterned ground and meltwater channels, together with their associated deposits.
5. Fluvial Geomorphology. Landforms are terraces, meanders, bars, waterfalls, gorges and palaeochannels, for example, which are associated with a variety of deposits.
6. Holocene Geomorphology. The landforms include raised bogs and screes, whilst the deposits comprise peat and gravel, for example.
7. Coastal Geomorphology. Landforms are diverse, comprising spits, dune fields, beaches and cusps, among several others. Associated deposits include dune sands and shingle.
This RIGS illustrates Glacial Geomorphology including such deposits as glacial till and ice scoured surfaces (Net 2) Periglacial Geomorphological features such as solifluction deposits (head) and scree (Net. 3). Features of Fluvioglacial Geomorphology (Net. 4) include meltwater channels and possible lacustrine deposits.
Potential use (general):
Although the geomorphology of the area is highly specialised, the significance of the rocks as a possible source of some of the rhyolitic fragments at Stonehenge suggests that a public awareness initiative may be appropriate.
Potential use (educational):
The rocks are of interest to geologists studying the Lower Palaeozoic igneous rocks of SW Wales, to geomorphologists studying the effects of Pleistocene glacial episodes on the flanks of Preseli, and also to archaeologists interested in the origin of some of the rhyolitic debris at Stonehenge.
The archaeological dig site has now been infilled and the site is accessible via a public footpath.
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JOHN, B.S. (2008) The Bluestone Enigma. Greencroft Books, Newport, Pembrokeshire, 160 pp.
JOHN,B,S. (2013) A Long History of Rhosyfelin. Scribd.com
JOHN, B.S, ELIS-GRUFFYDD, D & DOWNES, J (2015a). "Quaternary Events at Craig Rhosyfelin, Pembrokeshire." Quaternary Newsletter, October 2015. No 137, pp 16-32.
JOHN, B.S., ELIS-GRUFFYDD, D & DOWNES, J (2015b). Observations on the supposed ‘Neolithic Bluestone Quarry’ at Craig Rhosyfelin, Pembrokeshire. Archaeology in Wales 54, pp 139-148.
McCARROLL, D. (2001). The glacial geomorphology of West Wales. In WALKER, M.J.C. & McCARROLL, D. The Quaternary of West Wales: Field Guide. Quaternary Research Association, London, 9-16.
PARKER-PEARSON, M., BEVINS, R, E., IXER, R. A.; POLLARD, J.; RICHARDS, C.; WELHAM, K.; CHAN, B.; EDINBOROUGH, K.; HAMILTON, D.; MACPHAIL, R.; SCHLEE, D.; SCHWENNINGER, J.; SIMMONS, E. & SMITH, M. (2015). Craig Rhos-y-felin: a Welsh bluestone megalith quarry for Stonehenge. Antiquity, 89 (348) Dec 2015, pp 1331-1352
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