187497b0Nature1874736196008064974980028-0836196010.1038/187497b0ukNatureNatureNATUREnatureNature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public./nature/journal/v187/n4736issueJournal homeArchiveCurrent issueAdvance online publicationPrivacy policySubscribeNature Publishing GroupCurrent issue187497b0Relic-Soil on Limestone in South Wales
AU  - BALL, D. F.The Nature Conservancy, Headquarters for Wales and Bangor Research Station, Penrhos Road, Bangor, Caernarvonshire.THE Worm's Head, part of the Nature Conservancy Gower Coast National Nature Reserve, lies at the south-western tip of the Gower Peninsula of Glamorganshire, South Wales. It is a Carboniferous limestone headland accessible only at low tide across a wave-cut limestone platform. On the southern side of the Inner Head (Ord. Surv. Grid Ref. SS 395873) is a section in Pleistocene deposits of a type recorded widely in Gower1-3, and elsewhere in southern Britain. This can be summarized from the base upwards as follows : (1) Wave-cut limestone platform; (2) raised beach of calcite-cemented limestone pebbles banked against solid limestone; the Patella beach; (3) remnants of bright red sandy clay loam, 10-18 in. thick, containing rare limestone pebbles with red weathered outer zones; (4) brown loam containing abundant angular limestone, a periglacial head deposit. The thickness of this is variable from 0 to 10 ft.; (5) dull grey-brown stony sandy loam, glacial drift containing abundant Old Red Sandstone and Carboniferous grits and sandstones; where this is preserved on the flanks of the Worm's Head, the depth averages 4-6 ft.; (6) dull, grey-brown, sandy loam, almost stoneless, similar to the matrix of horizon 5, resting variously on horizons 2, 3 or 6.The Patella beach (Horizon 2) includes fauna characteristic of a cold climate, and deposits overlying this beach elsewhere in Oower have been found to have included a fauna indicating "a climate more genial than that of the present day"2. Horizon 3 is interpreted here as relics of a soil which had developed on Carboniferous Limestone prior to the return of periglacial and glacial conditions responsible for Horizons 4-6.
The colour of Horizon 3 is red (Munsell colour 2.5FJS4/6-5/8). It is sticky and mouldable when wet, moderately massive when dry. Field texture, silty loam to silty clay loam, is heavier than that given by laboratory analysis. This, by the hydrometer method, is sandy clay loam (5-5 per cent American silt, 28 per cent clay). Organic carbon content is 4 per cent and free iron oxides (by Deb's method) 3 per cent. The clay fraction minerals are dominantly illite with a minor proportion of chlorite. Thin sections reveal a skeleton of clean sub-angular quartz of mainly fine sand size in a matrix of yellow-brown colour. There are rare, small, round concretionary flecks of calcite and roughly rounded dark red-brown segregation patches of iron oxides, the latter translucent with included quartz grains. Small, round or ovoid black opaque areas, reddish-brown in reflected light, are apparently also iron oxides. Between crossed polars the clay is markedly birefringent. There are moderately well and strongly oriented coatings around sand-grains and the general clay areas are seen as fine aggregates, in the terminology of Brewer and Haldane4. The fabric seen in sections of this material is similar to that described in rotlehm from Cyprus5 and in material discussed by Dalrymple6, notably a red clay from head deposit at Slindon, Sussex. Examination of slides, including the Dalrymple material, at the Institute of Archaeology, University of London (by permission of Dr. I. Cornwall), showed comparable fabric also in rotlehm from Morocco. The oriented clay is not concentrated in channels or pores, but is uniformly distributed as coatings to grains throughout the sections examined. The orientation apparently results from alternate wetting and drying of the soil-clay in situ rather than from illuviation of mobile clay7. This fabric is closely comparable to that described for terra fusca8, a soil of braunlehm type occurring on limestone in climates of humid conditions, slightly warmer than most of Britain. The occurrence of iron segregations and of small concretions is considered a stage in the process of rubefaction9 leading in hotter climates to a development of terra rossa from terra fusca, and rotlehm from braunlehm.
The interpretation of this material, in field morphology close to siallitic terra rossa and in micro-morphology close to terra fusca, is that it is a soil of transitional character between these two sub-types. The climate for its production is likely to have been essentially similar to that of south-western Britain to-day, but with warmer summers. The shallow soils developed on limestone on the mainland cliffs adjacent to the Worm's Head show in thin section weakly developed moderately oriented coatings to sand grains, but have a higher organic matter content and well-developed crumb or granular structure. It seems that only the first stages of transition from brown earth to terra fusca have been produced as a result of post-glacial conditions in this area. A detailed study of a range of soils on limestone in Glamorgan is likely to be interesting. In particular, it is of geological and pedological significance whether the red silty clay frequently preserved over limestone in the Vale of Glamorgan and Gower is a relict material or a result of post-glacial weathering. It is suggested that it may well prove to be an inter-glacial relic occurring locally south of the deposits of the last glaciation.
I am grateful to Dr. D. A. Osmond for helpful discussion on this topic.Strahan, , A., [ldquo]The Geology of the South Wales Coalfield[rdquo]. Pt. 9 (H.M.S.O., 1907).George, , T. N., Proc. Geol. Assoc., 43, 4, 291 (1932).George, , T. N., Geol. Mag., 70, 208 (1933).Brewer, , R., and Haldane, , A. D., Soil Sci., 84, 301 (1957).ChemPortOsmond, , D. A., and Stephen, , I., J. Soil Sci., 8, 1, 19 (1957).Dalrymple, , J. B., J. Soil Sci., 9, 2, 199 (1958).Stephen, , I., Science Prog., 48, 322 (1960).Kubiena, , W. L., [ldquo]The Soils of Europe[rdquo] (Murby, 1953).Kubiena, , W. L., Proc. Sixth Int. Congr. Soil Sci., E, 247 (1956).