The Brian John Boulder

The Brian John Boulder: Picture Dr Brian John

Other debris and bullet shaped clasts: Picture: Dr Brian John

 Dr Brian John brings to our attention a bullet shaped clast of Rhyolite in the debris at Craig Rhosyfelin.

At Craig Rhosyfelin (also spelt Rhos-y-felin), a rocky outcrop of Ordovician rhyolite in north Pembrokeshire, Wales, the geological context reveals a site shaped predominantly by local glacial, periglacial, and fluvioglacial actions during the Late Devensian (last glacial episode, circa 20,000–11,000 years ago). The outcrop lies within a meltwater channel incised by glacial activity, where ice sheets from the Irish Sea Glacier overrode the area, causing direct abrasion and plucking without necessitating long-distance clast movement. Features previously interpreted as indicators of far-field glacial transport—such as the bullet shape, abraded surfaces, and weathering crusts—are re-evaluated here as products of localised, in-place modification. This aligns with geomorphological critiques that emphasise natural processes over anthropogenic quarrying or extensive ice entrainment, viewing the site's chaotic boulder litter as resulting from rockface disintegration, frost shattering, and short-range meltwater reworking.

1. Overall Shape and Morphology

The clast exhibits a bullet-like morphology: elongated (approximately 50–70 cm long), tapering to a pointed "nose" at one end while broadening to a blunter base at the other. Its contours follow the inherent foliation planes of the rhyolite, with subtle ridges and depressions aligned parallel to the rock's layered structure. In the image, it appears partially embedded in the sediment, amidst a scatter of similar fragments, suggesting detachment from the nearby outcrop.

This shape arises from natural fracturing along the rhyolite's prominent millimetre-scale foliation, which creates pillar-like forms prone to breaking into tapered segments. At Craig Rhosyfelin, the outcrop's vertical joints and planar banding facilitate such breakage under periglacial conditions, where repeated freeze-thaw cycles wedge apart weaknesses, producing rounded, bullet-like tips without any need for transport. Excavations have revealed numerous such detached "bullet stones" still at the site, confirming they result from in-place disintegration rather than ice flow dynamics. Local glacial overriding may have enhanced this by plucking blocks from the bedrock, but with minimal displacement—often mere metres—into adjacent meltwater channels. The asymmetry mimics streamlined forms but reflects differential weathering: exposed ends erode more via frost action, while basal connections to bedrock preserve blunter profiles.

2. Surface Characteristics and Abrasion Features

The surface is variably textured, featuring abraded facets (flat or curved planes), faint linear scratches, grooves aligned with the long axis, and minor pits or projections tied to the rock's lithology. The light grey-whitish tone in the image highlights these under ambient light, with potential microfeatures like arcuate indentations visible on closer inspection.

These abrasion traits stem from localised glacial and periglacial erosion at the outcrop itself. As Irish Sea Ice overrode the Preseli region, basal grinding against the bedrock—laden with embedded debris—created facets and striations without moving the clast far. Grooves and scratches likely formed from in situ pressure and shear along foliation planes, amplified by freeze-thaw expansion. Post-glacial exposure has added chemical etching and minor fluvial polishing from seasonal meltwater flows in the channel, but all within a few metres of origin. Such features are ubiquitous on frost-shattered rhyolite faces, where heavy abrasion occurs naturally from rockfall and colluvial movement, negating transport-related explanations.

3. Weathering Crust and Patina Development

A whitish weathering crust, up to several millimetres thick, coats the exposed surfaces unevenly, appearing chalky and flaking in places to reveal darker underlying rock. Distribution is patchy, thicker on upward-facing areas and thinner where sediment contact has offered protection.

This crust develops through prolonged post-glacial chemical weathering in place, via hydrolysis and oxidation in a humid, periglacial climate. Rhyolite's composition—rich in feldspar, chlorite, and quartz—promotes kaolinite rind formation, whitening exposed faces over millennia. At Craig Rhosyfelin, differential patina reflects partial burial in slope deposits after initial detachment, with no evidence of pre-weathering erasure by transport. Instead, it records static exposure since the Holocene, enhanced by local acidic soils and rainfall. Geomorphological studies attribute this to in situ periglacial action, where frost heaving and solifluction minimally shift clasts while accelerating surface degradation.

4. Clastic Fragments and Associated Debitage

Surrounding the clast are angular shards, rounded cobbles, and finer gravels of similar rhyolite, some with abraded edges or conchoidal fractures, forming an unsorted matrix in the fluvioglacial sediments.

These fragments result from localised rockface comminution via frost shattering and minor meltwater reworking. Pressure fractures occur along natural joints under ice load or thermal stress, with abrasion from mutual clast contact during short-range tumbling in seasonal streams. The chaotic litter represents accumulated rockfall and quarrying debris at the crag base, overlain by Holocene colluvium, without far-travelled erratics. Striated cobbles noted in exposures indicate in-place glacial polishing, but the assemblage's poor sorting points to periglacial slope processes rather than extensive transport.

5. Broader Contextual Erosion and Landscape Integration

The clast integrates into a site marked by jointed rhyolite crags, rockfall banks, and reddish sediments in a meltwater channel, with evidence of heavy abrasion on exposed surfaces.

The landscape records overriding by thin ice sheets, causing plucking and abrasion directly on the outcrop, followed by periglacial frost wedging that detaches pillar-like blocks. Fluvioglacial deposits suggest brief, high-energy meltwater episodes moving material only metres downslope. Disputed "engineering" features (e.g., platforms) may be reinterpreted as natural ledges from foliation-controlled erosion, with radiocarbon dates supporting intermittent occupation and quarrying.

Summary Table of Characteristics

Characteristic	Description in This Clast	How It Results from In Situ Events
Bullet Shape	Elongated taper along foliation, rounded tip	Frost shattering and plucking along natural planes; minimal displacement.
Abraded Facets & Scratches	Flat planes with grooves from pressure and shear	Local glacial overriding and periglacial grinding on bedrock.
Weathering Crust	Whitish rind on exposed faces, uneven	Post-glacial chemical alteration in place, protected by burial.
Clastic Fragments	Angular shards with fractures in unsorted matrix	Rockfall comminution and short-range meltwater abrasion.
Landscape Context	Chaotic litter in meltwater channel with jointed crags	Periglacial disintegration and fluvioglacial reworking locally.

In conclusion, the apparent glacial forms of this bullet-shaped clast are the result of in situ events at Craig Rhosyfelin. Overridden by ice during the Devensian, the outcrop underwent direct abrasion, plucking, and fracturing, with subsequent periglacial weathering and minor fluvioglacial adjustment detaching and modifying the clast within metres of its origin. This interpretation dismisses long-distance ice transport, viewing the features as natural outcomes of localised glacial and post-glacial processes.