Quaternary, Bristol and Gloucester region
| Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.) |
The Quaternary Period, covering about the last two million years of earth history, is subdivided into the Pleistocene and Holocene. The latter, also known as Recent and corresponding to the Flandrian Stage, accounts for the last 10 000 years and equates approximately to the period since the end of the last glacial episode.
The deposits of the Quaternary Period comprise varied unconsolidated beds, collectively known as ‘Drift’, including glacial, fluviatile, littoral and estuarine deposits, and a mixed group of periglacial deposits known as ‘Head’, which grade into colluvium (or hillwash), mudflow and landslip.
Correlation of Quaternary deposits on land is hampered by scarce fossil evidence, considerable lateral variations, general absence of good marker horizons and the complexities of deposition in a glacial environment. Radiocarbon ages can be used, when available, to establish precise chronological dating within the last 40 000 years. The promise of relatively new dating methods such as amino-acid racemisation (AAR) has not yet been realised but, in combination, the various methods offer the prospect of eventually solving the outstanding stratigraphical problems.
The outstanding feature of the Quaternary was the fluctuation in climate between glacial and temperate. The oxygen isotope ratios in shells recovered from deep-sea cores provide good evidence of numerous temperature fluctuations in the oceans throughout the Quaternary, and the relationship between the oxygen isotope stages and the onshore geological successions in north-west Europe is providing an important stimulus for much new research.
The present British classification is largely land-based and relates stages to separate type localities, except for three of the oldest stages. There is broad agreement as to the sequence of events as far back as the last interglacial period (Ipswichian Stage); also an emerging consensus that the great bulk of the pre-Ipswichian glacial sediments in southern Britain can be assigned to one major glaciation (Anglian Stage). There is as yet no generally agreed stratigraphical scheme for the pre-Ipswichian succession.
A postulated sequence of events for the present region is summarised in P948999, which includes a tentative correlation with oxygen isotope stages, mainly following Bowen et al. (1986[1], 1988[2]). Several cold and/or glacial periods are represented, although only in the Anglian can an ice-sheet be proved to have entered the region. The deposits of the type area for the Wolstonian stage are now generally agreed to be of Anglian age, but the stage term is retained within quotes because there is evidence in the adjacent Birmingham and South Wales areas of post-Hoxnian glacial deposits, including till, at this stratigraphical level.
The presence of large ice sheets in the vicinity in the late Devensian created widespread periglacial climatic conditions. Local snow-caps were developed and, during the short seasonal thaws, torrents of meltwater carried away rock debris and frozen mud to be deposited in great spreads of fan gravel. Many of the present-day dry valleys in limestone terrain (P210756 and P006945) were probably formed under such conditions; the frozen subsoil did not allow the normal underground percolation of surface water which, as a result, flowed overground and excavated the valleys. Meltwaters from the large ice-fronts fed the ancestors of the present-day rivers, which transported masses of glacial drift, redepositing the material as terraced river gravels.
The sea level varied in proportion to the amount of water locked up in ice. During glacial periods it was appreciably lower than at present, and during warmer periods it was at the present or higher levels. The then prevailing sea levels are reflected in the heights of the ancient river terraces and sea beaches.
References
- ↑ Bowen, D Q, Rose, J, McCabe, A M, and Sutherland, D G. 1986. Correlation of Quaternary Glaciations in England, Ireland, Scotland and Wales. Quaternary Science Reviews, Vol. 5, 299–340.
- ↑ Bowen, D Q, and Sykes, G A. 1988. Correlation of marine events and glaciations on the north-east Atlantic margin. Philosophical Transactions of the Royal Society of London, (B), 318, 619–635.