Quaternary events in the Lower Severn and Avon valleys
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Events in the Lower Severn and Avon valleys
The Quaternary history of the lower Severn valley is one in which erosion has dominated over deposition. Nevertheless, sufficient remnants of drift deposits, mainly in terraces of the rivers Severn and Warwickshire Avon and their tributaries, remain to give an indication of the later Pleistocene history of the area.
In pre-Anglian times, the greater part of the present Warwickshire Avon drainage was to the north-east, joining what is now the River Soar. The Avon, at the most, would have been a minor tributary of the Severn. The headwaters of the Severn lay south of Ironbridge, and the lower Severn drainage system was appreciably above present-day sea level. At the height of the Anglian glaciation the lower Severn valley was blocked at least as far south as Gloucester by a glacier deriving from Wales, and the Bristol Channel was occupied by an eastward-moving glacier. Meanwhile, the drainage into the lower ‘proto-Soar’ valley was blocked by glaciers moving into the area from the north and north-east. These three ice-masses and the main Jurassic escarpments to the south-east confined the drainage into a vast glacial lake that has been named Lake Harrison. Drainage from this lake took place through overflow gaps at about 125 m above OD in the Jurassic escarpment. The Moreton Gap, which occurs in the present district, spilled over into the Evenlode valley, in the Thames catchment.
The oldest terrace deposits of the River Severn represent either fluvioglacial outwash gravels from the retreating glaciers or material redeposited from them. The upper part of the ‘proto-Soar’ valley was choked with the glacial deposits of the Anglian Stage and, as the ice waned, the River Avon, as we know it, was born by a reversal of the drainage into the Severn basin. The Fifth Terrace of the River Avon has recently yielded a temperate fauna including red deer.
The deposits of the Fourth Terrace of the Avon and, farther upstream to the north of the region, its tributary the Stour at Stratford provide evidence for warm and cold periods between the Anglian and the Ipswichian stages. At Twyning, the Avon Fourth Terrace deposits comprise a basal Jurassic-rich gravel member with a full glacial fauna including mammoth and reindeer, with overlying Triassic-rich sands and gravels including the ‘warm’ bivalve Corbiculafluminalis. The presence of ice-wedge casts in the uppermost part is puzzling and apparently represents a further climatic change (Whitehead, 1988). The Fourth Stour Terrace at Ailstone contains an undoubted interglacial fauna with C. fluminalis, and on this basis was long correlated with the Avon Third Terrace in spite of the altitudinal problems involved. It is now known that the Fourth Terrace is both higher and older than the Avon Third Terrace and hence an additional interglacial period is involved (Maddy et al., 1991).
The succeeding Ipswichian (interglacial) Stage is estimated to have lasted from about 130 000 to 75 000 years ago. The Third Terrace gravels of the Avon, which contain a ‘warm fauna’ including hippopotamus, the straight-tusked elephant Palaeoloxodon antiquus and Corbicula fluminalis, are assigned to this period. The corresponding Severn terraces appear to have been completely removed during subsequent erosional phases.
The Devensian Stage is divided into two cold parts separated by a somewhat milder period, including the sub-temperate Upton Warren Interstadial. The first of the cold parts corresponds with the Avon Second Terrace, which has yielded a cold fauna including mammoth, woolly rhinoceras, reindeer and various molluscs. After a relatively warm interstadial, the build-up of ice to the north of the region resumed and reached its maximum in the early part of the late Devensian. It resulted in the damming of a large river that had hitherto flowed north-westwards into Cheshire and caused it to divert into the Severn drainage basin, cutting the Ironbridge Gorge in the process. This great increase in the volume of the River Severn led to the intense erosional and depositional activity represented by its Main, Worcester and Power House terraces (Third to First). Recent radiocarbon dating results indicate that this period probably lasted between 25 000 and 10 000 years ago. The former Third Terrace in the main channel of the Severn, which corresponds to the Avon Second Terrace, was replaced by the Main Terrace of later date, following the Ironbridge breakthrough. The Main Terrace and later Severn deposits are distinguished from the earlier gravels by an influx of north-western material derived from the greatly enlarged catchment area, including detritus from the Irish Sea glacier. The great sheets of fan gravel, formed of locally derived Jurassic rocks, that spread downwards from the Cotswolds onto the Severn plain are witness to the rigorous periglacial environment south of the ice-sheets during this period.
Following the building up of the Worcester Terrace during this cold period, there was a fall in the level of the sea to possibly as much as 100 m below OD. The River Severn cut a deep canyon that extended upstream at least as far as Gloucester, and most of the Bristol Channel became dry land.
With the amelioration of the climate at the beginning of Holocene times, sea level rose relatively rapidly until about 6000 years ago, when it stood at about 6 m below OD. Since then, sea level rise has continued at a diminishing rate and sedimentation has reduced, so that in the last 3000 years there has been little change. The climate, as shown by pollen studies, reached an optimum of wet and warm weather around 7500 to 5000 years ago (Pollen Zone Vila).
- Maddy, D, Keen, D H, Bridgland, D R, and Green, C P. 1991. Revised model for the Pleistocene development of the River Avon, Warwickshire. Quarterly Journal of the Geological Society of London, Vol. 148, 473–484.