Thanks to Geraint Owen for further information on the Senni Beds. He has studied the Craig Ddu section in detail, and from his descriptions, and the work of Ixer, Bevins and Turner, I am now more than ever convinced that the Altar Stone might well have come from Craig Ddu near Llansteffan. Below I reproduce some of the key info on the Senni Beds from the big report by Barclay et al 2015.
It's clear from the published work that the Altar Stone has significant differences from the Cosheston Sandstones around Mill Bay (Milford Haven) and significant similarities with the sandstones described from Craig Ddu. The samples are not identical, but that is not surprising; we do not know the precise positions from which all the samples have been taken, along a rock exposure c 400m long and 20m high.
For the moment, until further work is done, I think we can assume that the Altar Stone has probably come from the Llansteffan Peninsula and maybe even from the Craig Ddu exposures. That makes perfect sense, since the eastern Preseli hills and the Llansteffan Peninsula are on pretty well the same Irish Sea Glacier flowline. All of the speculation about the Altar Stone having come from somewhere along the A40 road between Llandeilo and Sennybridge becomes redundant.
Big BGS report on ORS stratigraphy. It provides a more formal correlation of Cosheston Group and Senni Formation:
BARCLAY, W J, DAVIES, J R, HILLIER, R D, AND WATERS, R A. 2015. Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin. British Geological Survey Research Report, RR/14/02. 96pp.
It's a free download from the BGS website.
4.2.5 Senni FormationName
Derived from the Senni Valley (Glyn Senni), Powys, south-central Wales. The name Senni Formation supersedes the traditional Geological Survey name Senni Beds (Cantrill in Strahan et al., 1904). In the Clee Hills, the formation was previously named the Clee Sandstone Formation.
Senni Valley [SN 930 209]
Partial type section
Waterfall exposures [SN 930 209] in a tributary of the Afon Senni (Nant Ystwyth) above Tyleglas.
1. Heol Senni Quarry [SN 9154 2210], Powys provides a well documented reference section. It exposes about 40 m of grey-green sandstones, with minor siltstones, mudstones and intraformational conglomerates at the top of the formation (e.g. Edwards et al., 1978; Loeffler and Thomas, 1980; Dineley, 1999b; Barclay, 2005e).
2. Cliffs at Craig Ddu on the western side of the Llansteffan peninsula, Carmarthenshire [SN 32441015](Owen, 1995). These reach a height of 20 m and have continuous exposure for 400 m of beds near the top of the formation.
3. The stream draining north-westwards across Clee Liberty [SO 583 848–587 844] in the Clee Hills exposes about 140m of the formation (Allen, 1961) and provides a reference section for this area.
Mainly of green and green-grey (locally red-brown and purplish green), very fine to medium- grained, micaceous sandstones, mainly channelised, cross-bedded and parallel-laminated, with green and red-brown siltstone and mudstone interbeds, some calcretes and intraformational conglomerates; the formation is characterised by the presence of vascular fossil plant remains and some soft sediment deformation is also present. In the Clee Hills, pale green sandstones are mainly arranged in fining-upwards conglomerate–sandstone–siltstone cycles, with subordinate red or green, sporadically calcretised mudstone/siltstone interbeds (Ball and Dineley, 1952, 1961; Allen, 1961; Greig et al., 1968).
Lower and upper boundaries
The lower boundary is placed at the base of green sandstones, which overlie red-brown sandstones and mudstones of the underlying Freshwater West (formerly St Maughans) Formation. Where mature calcretes at the top of the Freshwater West Formation are present (the Ffynnon/Abdon Limestones), the base of the formation is placed at the top of the uppermost calcrete. The upper boundary is placed where red-brown sandstones of the Brownstones Formation overlie the green sandstones of the Senni Formation, the junction being gradational. In the Clee Hills, Here, the lower boundary of the formation is placed at an erosion surface cut in the uppermost calcrete of the Upper Abdon Limestone, where it is sharply overlain by the basal green sandstones. The upper boundary is a gradational passage into generally coarser-grained
strata lacking in argillaceous beds and in which the sandstones are more variably coloured, these being assigned to the Brownstones (previously Monkeys Fold) Formation.
300 to 450 m in the Brecon Beacons and Black Mountain, 150 to 200 m in the Black Mountains and 152 to 167 m in the Clee Hills.
From Carmarthen Bay eastwards to the Black Mountain, Brecon Beacons and Black Mountains, and from there southwards to Abergavenny, wedging out north of Pontypool.
High-discharge, mixed-bedload, sand-dominated, braided stream systems with relatively high sedimentation rates and water-table levels; although the formation is dominated by in-channel deposits, overbank floodplain silt and mud deposition also occurred.
Latest Lochkovian to latest Pragian or Emsian in age.
OWEN, G. 1995. Senni Beds of the Devonian Old Red Sandstone, Dyfed, Wales: anatomy of a semi-arid floodplain. Sedimentary Geology, Vol. 95, 221–235.
Also, RG Thomas's opus magnus on the Cosheston Group, which includes petrographic data:
THOMAS, R G, BARCLAY, W J, MORRISSEY, L, WILLIAMS, B P J, and ALLEN, K C. 2006. Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south-central Pembrokeshire, Wales. Geological Journal, Vol. 41, 481-536.