Solution Subsidence Mechanisms and End-Products in South-East Breconshire

Low-dipping Carboniferous Limestone and Basal Grit (Millstone Grit) provide the structural framework for stepped scarp faces and intervening elevated dip-slopes on this segment of the North Crop of the South Wales coalfield. Fully half the extent of the limestone country is veneered with the jumbled and highly-broken remnants of a former grit cover. This has been largely derived through solution-subsidence agencies, but has been appreciably modified by periglacial influences. It is considered that concentrated solution along or directly beneath a former up-dip extension of the irregular plane of unconformity separating the two main formations of this area was, in the first place, largely responsible for the collapse of the overlying beds. Accelerated rates of solution directed from the base of depressions within this subgrit surface probably initiated the production of most of the numerous 'pockets' of collapsed grit blocks, while a notable proportion of the spectacular collapse dolines pitting the present in situ Basal Grit exposures are likely to reflect roof falls in underlying major caverns developed along or immediately below the down-dip continuation of this same plane. THE relatively narrow outcrops of the Carboniferous Limestone and Basal Grit (Millstone Grit) which fringe the North Crop of the South Wales coalfield show specific karstic landforms on more impressive scales than are recorded elsewhere in Britain.1 Foremost amongst such landforms are the slumped masses of Basal Grit let down throughout a long history of solution subsidence so as to lie at present at highly anomalous levels well within the Carboniferous Limestone outcrop and in extreme cases up to 250 m below their original in situ position. They vividly reflect the potency of solution-collapse processes as erosive agents where calcareous beds are present, but on the other hand when closely spaced over wide areas they have the effect of creating a form of derived scenery which can be differentiated from that featuring drift-covered Carboniferous Limestone terrain. The mapping of these collapsed Millstone Grit masses has previously been recorded for the western half of south Breconshire and the adjoining sectors of south-east Carmarthenshire.2 Recent re-surveys of the Ammanford (230)3 and Merthyr Tydfil (23 )4 One-Inch Sheets by the Institute of Geological Sciences have also confirmed that slumped Basal Grit derived by this means is much more prevalent than was indicated by the earlier surveys. Within the 20-km stretch of high moorland country lying between the Taf Fechan valley to the north of Merthyr Tydfil and the northern slopes of the Clydach valley to the east of Brynmawr (Figs. I and 2) there are significant differences between the collapsed Basal Grit deposits and those occurring in the areas located farther west. In these western areas the extraction of siliceous materials from a number of open pits has revealed that bedding planes are still discernible in the collapsed grit and beds of excessively-rotten, quartzose sandstone or seams of loose quartz sand make up a notable proportion of the stratigraphic sequence which may attain a vertical thickness in excess of 25 m. In contrast, in the area under review the deposits invariably comprise a jumble of large to medium-sized blocks of quartzitic sandstone or quartz conglomerate with little or no interstitial material. Pocket deposits of Millstone Grit blocks are also more prevalent in this eastern

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  Open Access•Harry Timmermans•Urban Studies•1986

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