The Oligocene and Miocene Outliers of West Cornwall and their Bearing on the Geomorphological Evolution of Oldland Britain

Abstract

The Tertiary sediments preserved at St Agnes, Crousa Downs and Polcrebo Downs, Cornwall, are described in detail. The deposits at St Agnes have yielded two contrasting floras, one mid-Oligocene, the other Miocene in age. The latter appears to be the first recorded proven Miocene extant in the British landscape. Accordingly, it has become necessary to distinguish two discrete outliers at St Agnes: that termed the St Agnes Formation sensu stricto (Miocene) and the Beacon Cottage Farm Formation (Oligocene, the local equivalent of the Bovey Formation). A tripartite stratigraphy is proposed for the former, the upper and lower Members comprising wind-blown sands and the middle Member a colluvial slope-wash deposit; all were apparently formed under warm-temperate conditions. The sediments at Crousa and Polcrebo remain undated; both preserve deep weathering profiles of tropical or subtropical type in the Palaeozoic floor below and are themselves much affected by in situ deep weathering. Structurally, all four outliers appear to be simple erosional relics of once more widespread and, possibly, thicker sheets. No secondary structures have been identified. All four apparently survived as a result of their locations on prominent watersheds originating in at least mid- or even early Tertiary times. The geomorphological relations of the four outliers are considered in detail. The sub-St Agnes Surface is shown to be the homologue of a planation surface that is widely developed in west Cornwall at an elevation of between ca. 75-131 m above sea level (ASL). The latter is redefined and termed the Reskajeage Surface. Its age is shown to be pre-Upper Miocene rather than Pleistocene, as was previously widely held. There is indirect evidence that it formed subaerially over a long period during mid-Tertiary times; essentially, it appears to be an etchplain of tropical and subtropical origins, now largely stripped of its former saprolitic cover. It is demonstrated that the grosser physiography of west Cornwall was established by the end of the Palaeogene at the latest and, also, that the area has probably never been inundated by Tertiary seas, except shallowly near to the end of that period (i.e. by the St Erth transgression). The prolonged morphodynamic stability of the region, as established in this paper, has important consequences for the interpretation of landscape evolution elsewhere in areas of Oldland (i.e. post-Armorican) western Europe, especially where datable Tertiary deposits have yet to be recognized.