Publications
The Ashdown and Wadhurst series of the Weald of Kent, Surrey and Sussex, by Percival Allen, 1843 at University of Reading (Ph.D. thesis)
A Wealden soil bed with Equisetites lyelli (Mantell), by P. Allen, B.Sc., published 1941 in The Proceedings of the Geologists' Association London (no. 52 issue 4, article, pp.362-374) View Online
Abstract:This modern-looking horsetail was first described and figured by Mantell in 1833. Since then the only important contributions to our knowledge of the plant have been made by Seward [11, pp. 24-27]. Until recently Equisetites lyelli was known only from stem fragments whose relations to the whole plant were uncertain, and from apical buds and leaf sheathes from different-sized and differently-proportioned stems. In 1938 the author located near the base of the Wadhurst Clay in the Weald, an extensive fossil soil bed containing E. lyelli. The subterranean parts of the plants are there preserved as casts in their original position of growth, and the detached aerial portions of the erect stems occur as fragments in the 1-2 ft. of overlying sediment.
It has thus been possible to obtain for the first time a fairly complete idea of the main features of the plant as a whole. In this paper a description of these features is given and certain geological aspects of the new evidence are considered.
It has thus been possible to obtain for the first time a fairly complete idea of the main features of the plant as a whole. In this paper a description of these features is given and certain geological aspects of the new evidence are considered.
Notes on Wealden fossil soil-beds, by P. Allen, M.A., B.Sc., Ph.D., F.G.S., published 1946 in The Proceedings of the Geologists' Association London (no. 57 issue 4, article, pp.303-314) View Online
Abstract:The following appear to be the more significant of the new facts:
- Both soil- and rootlet-beds are frequent in the strata examined. It follows that the basin of deposition must have been shallow during much of Lower Wealden time.
- All but one of the soil-beds occur in transitional strata of very similar aspect. The upward change involved may be from dominantly arenaceous to dominantly argillaceous facies, or vice versa - which, appears to be irrelevant. Evidently, the conditions necessary for colonisation (by horsetails, at least) were strictly limited. The nature of the strata lead to the conclusion that colonisation was controlled by one or more of the following factors: depth of water, amount of movement of the water, size-distribution of the sedimentary particles, chemical nature of the sediment, rate of accumulation of the sediment. That other factors were sometimes limiting is shown by the occasional presence of similar transitional strata without horsetails.
- Though the data are scanty, there are indications of a positive correlation between the lithological similarity of the soil-beds and the taxonomic similarity of the plants they contain.
- The bulk of the recognisable plant-tissues are preserved as ferruginous substances. Carbonised and 'coalified' remains are unusual at all the horizons. Doubtless a phase of oxidising conditions formed part of the history of each.
- Contemporaneous erosion was not normally severe. Where considerable, it was confined to the frontal (S.E.) margin of the delta-complex
Wealden petrology: The Top Ashdown pebble bed and the Top Ashdown sandstone, by Percival Allen, published February 1948 in Quarterly Journal of the Geological Society (vol. 104, issue 1-4, article, pp.257-321) View Online
Abstract:The Top Ashdown Pebble Bed occurs immediately below the Wadhurst Clay, averages three to four inches in thickness and is well exposed over an area of about 700 square miles in Sussex, Kent and Surrey. It is thus well suited to areal petrological study, being free from the invalidating influences of vertical variation. The "Top Ashdown Sandstone" is an arbitrary horizon situated four inches beneath the pebble bed, and designed primarily as a check. The Wealden strata of the Weald contain thin, yet widespread, horizons similar to the Top Pebble Bed. Detailed petrological studies of them have been undertaken. Where possible, all information, petrological and other, is expressed quantitatively. In no case is the assumption made that sampling information necessarily represents geological reality. Sampling data are linked with the unknowable truth, which they estimate, only through probability statements based on objective statistical analysis. Throughout the paper, the chances that the author may be talking nonsense are given for most alleged facts and conclusions.
Wealden petrology: The Top Ashdown pebble bed and the Top Ashdown sandstone, by Percival Allen, published January 1949 in Quarterly Journal of the Geological Society (vol. 105, issue 1-4, article, pp.535-537) View Online
Abstract:After the publication of the above paper, the author was asked if he would demonstrate, over a selected route, the field evidence for his fundamental conclusion that the Top Ashdown Pebble Bed is a single Weald-wide stratigraphic unit, and not a series of lenticular pebble-beds at varying horizons. On this conclusion, of course, the validity of his whole argument depends.
He very kindly agreed to do so, and a route embracing Cowden, Penshurst, Wadhurst, Rotherfield, Burwash, Dallington, Battle, Crowhurst, Westfield, Brede, Robertsbridge and Hurst Green (traceable by reference to fig. 2, p. 268 of the paper) was chosen.
Numerous sections- at least one in each of these localities -were examined, all at the junction of the Wadhurst Clay and Ashdown Sand as shown on the one-inch scale Old Series Geological Survey maps. Without exception, Dr. Allen was able to demonstrate the presence of a well-developed pebble bed, varying in thickness from an inch to nine inches and in most instances associated with the swamp soil-bed, containing many rhizomes and stalks of Equisetites lyelli, as described in his paper.
Dr. Allen's evidence is clear, for all to see; and his correlation of the various sections?in making which much other field evidence was taken into count, and by virtue of which he concludes the Top Pebble Bed to be everywhere one and the same?is, in the writer's view, fully justified.
He very kindly agreed to do so, and a route embracing Cowden, Penshurst, Wadhurst, Rotherfield, Burwash, Dallington, Battle, Crowhurst, Westfield, Brede, Robertsbridge and Hurst Green (traceable by reference to fig. 2, p. 268 of the paper) was chosen.
Numerous sections- at least one in each of these localities -were examined, all at the junction of the Wadhurst Clay and Ashdown Sand as shown on the one-inch scale Old Series Geological Survey maps. Without exception, Dr. Allen was able to demonstrate the presence of a well-developed pebble bed, varying in thickness from an inch to nine inches and in most instances associated with the swamp soil-bed, containing many rhizomes and stalks of Equisetites lyelli, as described in his paper.
Dr. Allen's evidence is clear, for all to see; and his correlation of the various sections?in making which much other field evidence was taken into count, and by virtue of which he concludes the Top Pebble Bed to be everywhere one and the same?is, in the writer's view, fully justified.
Geology of the Central Weald: The Hastings Beds, by Percival Allen, published 1958 (Geologists' Association Guide no. 24, Geologist's Association)
Wealden of the Weald: a new model, by P. Allen, published 1975 in The Proceedings of the Geologists' Association London (no. 86 issue 4, article, pp.389-437) View Online
Abstract:An alternative to the deltaic model is presented. The Weald is seen as a subsiding graben-basin, with 'Macaraibo' sedimentary features, spasmodically open to the sea and margined by active horsts.
Channel-facies are commoner in the Wealden sand and clay formations than previously suspected. The 'normal' environment was a variable-salinity coastal mudplain with lagoons and sandy water-courses loosely connected north-westwards with the East Anglian Sea. Channel patterns fluctuated across the low ? high sinuosity transition. Periodically, increased riverflow transformed parts of the basin into sandy braidplains, culminating sometimes in coalescent alluvial fans. These interruptions were brief and generated by marginal upfaulting of surrounding blocks combined with attendant climatic changes. Large expanses of alluvial plain were bare of trees and bushes, but supported rich growths of herbaceous pteridophytes where deposition and erosion became inactive temporarily. Herds of dinosaur travelled freely across the basin and maintained themselves in it. The climate was warm, with marked wet and dry seasons and, possibly, diurnal rhythms in precipitation. Its general trend was towards 'amelioration', as though Britain was leaving the Purbeck semiarid zone and moving across the warm temperate belt.
Following mid-Purbeck earth movements and the final Cinder Bed transgression, the London-Kent horsts dominated Hastings times, being the main suppliers of arenaceous sediment and controlling water-salinity by acting as imperfect barriers against further inroads by the northern sea. On two occasions (Ashdown and Lower Tunbridge Wells formations) the channel-networks merged to build up a single braidplain spanning the basin. Bedload from Wessex and the Isle of Wight stopped short at the Hampshire-Sussex border, though some Norman sand may have reached south-east Sussex. During Weald Clay times the London and Kent blocks ceased to be important sources of sediment, the former letting in the muddy 'Snettisham' Sea voluminously. Several of the brackish-marine inundations sprinkled East Anglian sand across the north-west Weald, mixing it with 'London' gravel gathered up in passing. But most of the sand, now sparse, was generated by jolts in distant Cornubia and Armorica, rejuvenating the rivers and causing the Wessex alluvia to probe the western Weald. After their Horsham première the movements weakened, the younger sands reaching less far eastwards and becoming more restricted to their channel systems. Least affected by tectonic and marine influences were the eastern parts of the basin. The regional palaeoslope in the Anglo-French area seems to have tilted northeast, away from the ruptured continental margin, preluding increased spreading rates in the mid-Cretaceous Atlantic.
There is no room in the new model for large-scale classical deltaic processes or for the traditional derivation of the immature 'western' detritus from Hercynian granites (or any other granites). The model removes some previous difficulties, e.g. the small sizes of the London catchments, the rapidity of many sedimentological changes and the paradox that both marine and fluviatile invasions appear to come from the same general direction.
In the field, the well-known 'Rocks' (massive sandstone members) become the merged multistorey networks of braided channel-fills formed during climactic phases of uplift and river-rejuvenation on the basin margins.
Channel-facies are commoner in the Wealden sand and clay formations than previously suspected. The 'normal' environment was a variable-salinity coastal mudplain with lagoons and sandy water-courses loosely connected north-westwards with the East Anglian Sea. Channel patterns fluctuated across the low ? high sinuosity transition. Periodically, increased riverflow transformed parts of the basin into sandy braidplains, culminating sometimes in coalescent alluvial fans. These interruptions were brief and generated by marginal upfaulting of surrounding blocks combined with attendant climatic changes. Large expanses of alluvial plain were bare of trees and bushes, but supported rich growths of herbaceous pteridophytes where deposition and erosion became inactive temporarily. Herds of dinosaur travelled freely across the basin and maintained themselves in it. The climate was warm, with marked wet and dry seasons and, possibly, diurnal rhythms in precipitation. Its general trend was towards 'amelioration', as though Britain was leaving the Purbeck semiarid zone and moving across the warm temperate belt.
Following mid-Purbeck earth movements and the final Cinder Bed transgression, the London-Kent horsts dominated Hastings times, being the main suppliers of arenaceous sediment and controlling water-salinity by acting as imperfect barriers against further inroads by the northern sea. On two occasions (Ashdown and Lower Tunbridge Wells formations) the channel-networks merged to build up a single braidplain spanning the basin. Bedload from Wessex and the Isle of Wight stopped short at the Hampshire-Sussex border, though some Norman sand may have reached south-east Sussex. During Weald Clay times the London and Kent blocks ceased to be important sources of sediment, the former letting in the muddy 'Snettisham' Sea voluminously. Several of the brackish-marine inundations sprinkled East Anglian sand across the north-west Weald, mixing it with 'London' gravel gathered up in passing. But most of the sand, now sparse, was generated by jolts in distant Cornubia and Armorica, rejuvenating the rivers and causing the Wessex alluvia to probe the western Weald. After their Horsham première the movements weakened, the younger sands reaching less far eastwards and becoming more restricted to their channel systems. Least affected by tectonic and marine influences were the eastern parts of the basin. The regional palaeoslope in the Anglo-French area seems to have tilted northeast, away from the ruptured continental margin, preluding increased spreading rates in the mid-Cretaceous Atlantic.
There is no room in the new model for large-scale classical deltaic processes or for the traditional derivation of the immature 'western' detritus from Hercynian granites (or any other granites). The model removes some previous difficulties, e.g. the small sizes of the London catchments, the rapidity of many sedimentological changes and the paradox that both marine and fluviatile invasions appear to come from the same general direction.
In the field, the well-known 'Rocks' (massive sandstone members) become the merged multistorey networks of braided channel-fills formed during climactic phases of uplift and river-rejuvenation on the basin margins.
The non-marine Lower Cretaceous Wealden strata of southern England, by Jonathan D. Radley and Percival Allen, published 2012 in The Proceedings of the Geologists' Association London (no. 123 issue 2, article, pp.235-244) View Online
Abstract:The non-marine Lower Cretaceous Wealden strata of the Wessex-Weald Basin (southern England) are introduced, with reference to the depositional model developed by Professor Percival Allen FRS (Allen, 1975). To demonstrate this model and the development of Wealden palaeoenvironments through time, Wealden sites have been selected for the Geological Conservation Review programme. Site selection rationale is briefly outlined.
The Wealden (non-marine Lower Cretaceous) of the Weald Sub-basin, southern England, by Jonathan D. Radley and Percival Allen, published 2012 in The Proceedings of the Geologists' Association London (no. 123 issue 2, article, pp.245-318) View Online
Abstract:The Wealden strata (non-marine Lower Cretaceous) of the Weald Sub-basin outcrop in the Weald district of south-east England; the Wealden type-area. The succession is made up of the mixed alluvial-lacustrine-lagoonal Hastings Beds Group below and the predominantly lacustrine-lagoonal Weald Clay Group above. Deposition was strongly influenced by tectonism amongst surrounding massifs, and the warm to hot, periodically wet Wealden climate. Geological Conservation Review sites within the Weald district are dominated by inland sites, but also include extensive coastal cliff and foreshore exposures near Hastings, East Sussex. The Wealden strata have been documented and interpreted since the earliest days of geological enquiry in Great Britain. Collectively, the selected sites demonstrate the key elements of a depositional model for the Wealden of the Weald, developed and published by Professor Percival Allen FRS (1917-2008) in these Proceedings (Allen, 1975). The sites are documented and interpreted, with special reference to research history, chronostratigraphy, structural context, palaeoenvironments, palaeobiology and palaeoclimatology. New directions for research are proposed, as applicable.