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Monday, 14 September 2015

Solifluction sediments and colluvium at Rhosyfelin

This photo from 2011 shows the stratified solifluction deposits in which the "proto-orthostat" was embedded when it was discovered.
 

Colluvial and other deposits on the valley floor near the outer tip of the spur. At the base, fluvioglacial and glacial deposits (light grey). Above, dark grey finer-grained sediments, grading upwards into buff / cream coloured colluvium with iron-staining increasing upwards. Above that, an irregular junction with dark brown colluvium and modern soil.

Dr Simon Carr taking samples from the stained layer during a recent visit.

Interbedded with the rockfall debris and overlying the till and fluvioglacial deposits at Rhosyfelin, there is a layer of slope deposits up to 2.5 m thick. It's quite variable and very interesting indeed, and is difficult to interpret both with respect to age and origins.

The material exposed adjacent to the large slab is reminiscent of the “upper head” or stony solifluction layer found above Devensian glacial and fluvioglacial deposits in West Wales coastal sections. Exposures cut in the sediments in 2012 showed that more than 5m from the rock face there is a clear contact between the till and the overlying pseudo-stratified slope deposits. Within the latter there are at least five different but discontinuous layers, with a c 10 cm sandy/silty layer at the base. Above that, there are some layers of fine-grained sediments and others made up predominantly of elongated stones and flakes less than 10 cm in length. In addition to an abundance of sharp-edged local rhyolite fragments there are some large slabs and boulders and also, in the lower layers, rounded and sub-rounded erratics derived from upslope glacial materials. There are many signs of root penetration through this sequence, and the fine-grained layers contain many streaks of peaty organic debris. These materials have moved downslope predominantly from the NW, W and SW. The big flat-topped "proto-orthostat was embedded in this sediment layer when it was discovered. There do not appear to be any ice wedge casts which might suggest the presence of permafrost at the time of accumulation or at a later date, but in lower horizons there are some signs of bedding disturbances possibly attributable to frost-heave processes.

Further downslope, where the surface gradient decreases, the stones in this layer become less abundant, and it is predominantly made up of colluvial gravels, sands, silts and clays. Resting directly on the fluvioglacial deposits on the edge of the Afon Brynberian floodplain, there are at least three bands which are difficult to correlate precisely with the layers at the upslope end of the dig site. That having been said, there do not appear to be any discontinuities or unconformities that might show a long break in sedimentation.

At the base (close to the valley floor water table) there is a blue-grey layer at least 30 cm thick. It is clay-rich but incorporates bands of gravels which appear in places to have been deformed either by loading or frost-heave processes.

Above that is a layer up to 80 cm thick made up of sands, silts and clays but with some gravel and stone inclusions. There is no sharp junction between this and the underlying grey-blue sediment; texturally the two bands appear to be related. In this deposit there are occasional fragments of charcoal, suggestive of either natural / accidental burning of woodland or scrub in the vicinity, or else human occupation. This layer is buff- or cream-coloured at its base, but passing upwards iron-staining becomes more and more prominent, until the sediment has a distinct foxy-red colour similar to that on the till surface elsewhere on the site. It is noteworthy that the iron-enriched band transgresses the junctions between stratigraphic layers, suggesting that it is a pedogenic feature related more to water table oscillations than to age. (Iron "pans" are common in podzol soil horizons across north Pembrokeshire.)

Finally there is a grey-brown surface layer made of accumulated fine-grained slope materials, passing upwards into modern soil. There is a high content of organic matter, and more fragments of charcoal. This layer is up to 80 cm thick on the lower part of the site.

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This layer with many internal variations has to be post-glacial because it sits directly on till and fluvio-glacial gravels. But how old is it? Does it represent the whole period between c 18,000 years ago (when the last of the ice may be assumed to have melted away) and the present day? Or could there be a lot of sediments missing, and could it be of much more recent date?

Radiocarbon dates from some of the charcoal inclusions might help with the dating process: but one technique that might be particularly useful is OSL dating (optical thermoluminescence dating) which is best used on sediments that at the time of formation were exposed to daylight. The solifluction deposits might not be very suitable for sampling, but the colluvial deposits might be;   advice is needed on this.

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