Identification and subdivision of interglacial fluvial sequences
The record of Quaternary interglacial periods in lowland Britain is full, but fragmentary. The identification and differentiation of individual interglacial events, principally for stratigraphical purposes, relies on a range of approaches, but the need to compare these events on a generic basis requires a uniform subdivision scheme. The following discussion summarises the basis of this scheme.
In Britain, interglacial stages have been identified almost exclusively on the basis of their fossil content. The cyclic pattern of interglacial vegetational development that typifies all known temperate events was developed as a means of subdividing, comparing and therefore characterizing temperate events. This scheme was principally based on pollen and spore assemblages and is applicable throughout NW Europe. In this scheme temperate (interglacial) event sequences are subdivided into four substages as follows: pre-temperate, early temperate, late temperate and post-temperate, abbreviated to I-IV respectively (Table 1). Preceding and following glacial substages are referred to as late-glacial or early glacial, as appropriate.
Table 1. Example chronostratigraphical substage divisions of interglacial (temperate) stages and related cold (glacial) stages of the Middle and Late Pleistocene, modified after West (1968) and West & Turner (1968). For the Flandrian (Holocene), the correlations with the Godwin (1956) zones (sensu this paper) are also indicated. Comparable zonation schemes for additional events are not included, since these periods have yet to be formally-defined, although they would be expected to follow a similar pattern.
The four-fold substage scheme is effective at the broad scale for all the Pleistocene temperate (interglacial) events that have been studied. It is certainly possible that other temperate events may have occurred for which no record has yet been identified on the land. The scheme has also been applied to the Holocene (Flandrian = Fl) (Table 1). In the present interglacial the positioning of the boundaries is based on the same criteria as for the preceding interglacials, i.e. changes in forest tree composition. However, the post-glacial impact of humans has led to considerable modification of the vegetation, particularly over the last 5000 years, from forest clearance for agriculture . This makes the positioning of the early-temperate / late-temperate substage boundary somewhat difficult. In earlier interglacials this boundary is placed at the arrival of late-temperate trees, such as Carpinus or Fagus. However, in the post-glacial, West and Turner & West place their Fl II – III boundary at the ‘elm decline’. This decrease in Ulmus pollen frequencies is clearly marked in pollen sequences across the whole of southern Britain thus providing a useful marker at c. 4,500 y BP. Its cause remains debatable, but the close correspondence of the elm decline to the arrival of agricultural indicators, at the beginning of the Neolithic Period, implies that forest clearance was an important factor in the decline. Consequently, the Fl II – III boundary does not coincide with the arrival of typical late-temperate tree taxa, which, in spite of their low numbers, consistently arrive in SE England from c. 3000 y BP; very close to the Godwin zones VIIb / VIII boundary. The latter corresponds to the end of the ‘climatic optimum’ and therefore seems more appropriate for the Fl III basal boundary. Since compatibility with previous interglacial subdivisions is the primary purpose of this division, this younger datum has been adopted here.