Aspects of the English silcretes and comparison with some Australian occurrences

Abstract

The silcretes of southern England are fragments of larger, but discontinuous rock bodies. They are comparable with those of the upwarped palaeoplain of the Australian eastern Highlands. Growth structures, common in Australia, are recognized in the English silcretes and can be seen, for example, in the megalithic monuments such as the Avebury Stone Circle. There may have been much anthropogeomorphological activity in stripping land surfaces of their previous silcrete cover. The English silcretes are probably of two ages and reflect differing depositional conditions of the host rock which was subsequently silicified. The conglomeratic ‘Hertfordshire Puddingstone’ occurs sporadically in the Palaeocene Reading Beds; and the younger ‘sarsen sandstone’, of post-Barton (Eocene) age, oversteps it extensively. Both formations show transitions from one rock type to the other. On the dip-slope of the Chiltern Hills mixing of the two rock types (often within the Clay-with-flints) may have been caused by progressive pediplanation, activated along a structural line. This is offered as an alternative explanation to the ‘degraded cliff line’ caused by ‘Calabrian’ marine transgression. The sarsens, used as geomorphological markers, authenticate the previously existing palaeosurfaces, although they now survive only sparsely on the higher hills and beneath the Plateau Gravels. Apparent increases downslope and in the valleys (Clatford Bottom) is not attributed primarily to periglaciation.