Pollen analyses of late- and post-glacial deposits in the Western Forth Valley

Abstract

The objective of this thesis is the elucidation of the characteristics and chronology of vegetational sequences that have accompanied changes in sea -level and climate in the western Forth valley during the Late - Quaternery.

A major aim of the investigations was to construct e type diagram illustrative of regional Post -glacial vegetational development. In this diagram, from e site in the westernmost part of the Forth valley, eic;ht pollen zones (Forth Valley Zones I - VIII) each characterised by, and named after, certain pollen assemblages are distinguished. These are correlated with pollen zones of the English zonation in diagrams from other sites in the research area. Broad trends of Post -glacial woodland development are indicated in the type diagram and are divided into three chronozones. The earliest of these covers the immigration of trees into the region during the Pre -Boreal and Boreal; the second relates to the Atlantic period, the time of the climatic optimum when mixed broad -leaf deciduous forests attained their maximum development and raised bogs developed in the western Forth valley; the third covers the period of probable anthropogenic influence upon forest development.

Radiocarbon dates from Post -glacial deposits in the western Forth valley fall within dated Post -glacial pollen zones at a site at Scaleby Moss in northern England, suggesting that the pollen zones at C,caleby may be synchronous with Forth valley zones. On this basis and until more C¹⁴ dates of Forth valley pollen zones become available, the dated zonal divisions at Scaleby may be used as a temporary chronological index for dating major vegetational events during the Post -glacial. Until further radiocarbon evidence is forthcoming, the pollen assemblage zones and chronozones at the type site provide a broad chronological framework of major vegetational events with which other pollen diagrams from sites in the area can be compared and assessed.

At most of the sites investigated, some pollen spectra are from minerogenic deposits from buried raised beaches and marine clays laid down during former periods of high sea -level. Pollen frequencies from these deposits are indicative of regional vegetation that was contemporaneous with the formation of the buried beaches and carse clays. At one site, however, the deposition of river -borne pollens into the sea has resulted in the over -representation of alder in the carse clays in this locality. Other factors, such as pollen selectivity by moving water, especially by sea currents, and reworked pollen from older material (apart from Pre -Quaternary spores from Devonian and Carbon- iferous rocks in the region) are considered to be insignificant, as pollen frequencies from the buried beaches and carse clays are comparable to those from organic material at the type site and at other sites investigated. Thus pollen from ice -transported marine deposits of Zone II age in Zone III terminal moraines at Menteith in the western Forth valley and at Kinlochspelve in south -east Mull, are

also considered to be representative of the vegetation in these areas around 11,800 B.P. and 11,300 B.P. respectively, when sea -level was high, and this type of vegetation is comparable to that which characterises Zone II at other Scottish sites.

Changes in relative levels of land and sea in the area gave rise to variations in soil conditions. These were associated with changes in plant assemblages that have been interpreted from pollen and stratigraphic data. Falling sea -levels produced local vegetation successions from the halophytic species characteristic of saltmarshes to freshwater marsh and swamp communities; but this sequence was followed by retrogression as sea -level again rose.

Pollen evidence together with radiocarbon dates provides a chronological framework which supported by geomorphological evidence, can be used to date relative changes of sea -level in the western Forth valley during the Late -Quaternary. During the earliest part of the period under consideration, namely the Late -glacial Interstadial (Zone II), sea -level was high in the westernmost part of the Forth valley by about 11,800 B.P. The height of this sea -level in relation to the land is unknown. Around 10,800 B.P., ice of the Loch Lomond Readvance moved down the Forth valley to Iienteith where it culminated probably c. 10,300 B.P. This event corresponded with the culmination of the main Late - glacial marine transgression, represented by the Buried Gravel Shoreline in south -east Scotland and, farther west, by the High Buried Beach, when sea -level was between 10 - 12m OD.

Subsequently, ice disappeared from the Forth lowlands (and other parts of Scotland) as a result of climatic amelioration. Then during Zone F.V. i (the Pre- Boreal) the sea rose and the Main Buried Beach was formed. Pollen evidence indicates that the sea transgressed swiftly into the research area, reaching the western extremity of the Forth valley before the end of Zone F.V. 1, and that the influence of land uplift, consequent upon glacial unloading, upon the rise of this sea -level was insignificant.

Following the formation of the Main Buried Beach sea - level dropped to about 7m OD, when deposits of the Low Buried Beach accumulated. This event took place during Zone V (Zone F.V. 2). The level of the sea then fell further, reaching a minimal level c. 8,500 B.P. Shortly after this date, the sea began to rise rapidly in relation to the land and carse clays began to accumulate over the buried beaches and the peat deposits resting upon them. This marine transgression, the principal Post -glacial marine transgression in the Forth valley, was in response to a eustatic rise of sea -level resulting from the disintegration of the world's ice sheets.

Geomorphological data indicate that the land was rising as the main Post -glacial sea transgressed into the upper Forth valley. A major objective of the thesis was to determine whether the early rise of the sea at this time was retarded by land uplift. Pollen evidence signifies that the advance of the sea into the western Forth valley was relatively swift, beginning in Zone F.V.3 (Via) and reaching the western margin of the Forth valley by the beginning of Zone F.V.4 (Vlb). Thus the influence of isostatic recovery upon the early rise of the main Post -glacial sea in this region was insignificant.

However, the sea reached some sites at later times. This may be explained by local variables related to differences in site conditions and locations. These include the rapid development, in relation to the rising sea, of peat and the deposition of river -borne sands and gravels which resulted in the surfaces of these deposits being raised above sea -level until the sea rose and began to cover them with carse clays. At other locations carse clays, and possibly fine river -borne sediments, accumulated, resulting in the raising of the level of these clays above the sea. Subsequently, peat deposits were formed upon them until the sea rose and began to cover these minerogenic and organic layers with carse clay. At some sites sands and gravels were discharged into the sea by streams and were built up above sea -level. Later, these minerogenic deposits were buried beneath carse clays as the sea rose.

These local variables focus attention on some of the difficulties of attempting to correlate isolated fragments of relict marine features in the western Forth valley, but it is hoped that the data presented in this thesis may be of use as a basis for further research into problems relating to Post- glacial land and sea -level changes, particularly in the central Scottish lowlands.