Swallow Holes on the Millstone Grit and Carboniferous Limestone of the South Wales Coalfield

Abstract

SWALLOW holes or depressions caused by the collapse of underlying caverns are characteristic f ar as underlain by calcareous beds. The junctions of essentially calcareous formations and overlying systems of sharply contrasting lithological character have frequently been described as showing a profusion, or concentrated alignment, of swallow holes or allied forms.1 These generally mark the disappearance points of surface streams into underground courses so that the holes themselves are more attributable to the solution and enlargement of joint planes than to the collapse of pre-existing caverns. Outcrops of looselycompacted arenaceous beds forming cappings to underlying massive calcareous beds are liable to show surface reflections of the caving in of cavern roofs. It is thus normal, especially in areas with horizontal or gently-tilted strata, to find a fair scattering of these depressed features even though the top of the calcareous, cavern-bearing beds is at depths up to 30 or 40 feet. On the North Crop of the South Wales Coalfield, where comparatively narrow and roughly concentric outcrops of Millstone Grit and Carboniferous Limestone consecutively fringe the productive Coal Measures, an apparently anomalous state of affairs exists which appears to be unique. Here the most conspicuous and perfectly developed collapse features, in the form of large swallow holes, are decidedly more numer? ous on the outcrop of the Basal Grit, the lowest sub-division of the Millstone Grit, than on the outcrop of the underlying Carboniferous Limestone within whose beds such features were initiated. At certain points some of these swallow holes are located at distances of more than a mile south of the junction with the Carboniferous Limestone and underlain by a maximum thickness of about 250 feet of Grit beds. To illustrate fully this abnormal development a detailed six-inch survey was undertaken of the distribution of swallow holes and other solution sub? sidence phenomena on the Carboniferous Limestone and Millstone Grit out? crops ranging over a west-east length of 30 miles from the Twrch valley in south-east Carmarthenshire to Blorenge Mountain in north-west Monmouth shire and with a maximum north-south width of 5 miles. The results of this survey have been compounded (Fig. 1) so as to provide in one diagram a distribution picture for the region as a whole. It has revealed a wider acreage more closely pocked with swallow holes than in any other area of the British Isles. Certain sections of this area of survey, where they portray typical or special features of the distribution pattern, have been retained on a larger scale (Figs. 2 and 3). (For Fig. 1 see facing p. 474.) For the most part the Carboniferous Limestone within the limits defined has a vertical thickness of 500 to 700 feet, but in the extreme east there is a rapid diminution so that on Blorenge Mountain it is rather less than 100 feet thick.2