<%@LANGUAGE="JAVASCRIPT" CODEPAGE="1252"%> Scottish Sea Levels : introduction to Scottish sea level change

Scottish Sea Levels

Introduction to Scottish Sea Level Change Benchmark Publications The Data Home Curriculum Vitae

 

Introduction to Scottish Sea Level Change

1. Background


.1 The Historical record: catastrophists and uniformitarians

Along the coastline of Scotland, many features formed by the action of the sea now stand above the reach of the waves. Raised terraces, shingle ridges, cliffs, stacks, caves, rock platforms and areas of sediment containing marine shells, seal skeletons and even whale skeletons, have been remarked on for many years. Interest in this evidence of higher sea levels has not been confined to scientists. The reports of clerics in the Statistical Accounts of the eighteenth and nineteenth centuries, early books on the landscape of the country and comments in newspapers in the early nineteenth century bear eloquent testimony to the place of these features in the public psyche.


James Hutton (1726-1797)

Given the widespread evidence that the sea once lay higher than today, it is not surprising that the thoughts of some would turn to explanation. Sea level changes were cited in the arguments between Catastrophists, who believed that these levels were somehow evidence of a great flood, perhaps as described in the Bible, and Uniformitarians, who, whilst not against religion, looked upon the Old Testament as largely allegorical and believed instead in a more ordered and rational progression of change. Such arguments raged in Edinburgh at the end of the eighteenth century, with James Hutton (writing in 1795) and John Playfair (1802) the proponents of gradual change.


John Playfair (1748-1819)

Both Hutton and Playfair believed that the level of the sea surface had remained essentially stable through time, but as it became clear that there had been at least one “Great Ice Age”, Charles Maclaren in 1842 deduced that with expanded glaciers and ice sheets, sufficient water would have been abstracted from the hydrological cycle to lower the level of the oceans and seas of the world, a view increasingly supported in subsequent years. Uniformitarianism had not been overturned, but an element of dynamic change had been introduced into the sea level story.


Charles MacLaren (1782-1866)


.2 Jamieson and isostasy

It was in the context of these developments that, a few years later, scientists developed their views on the origins of raised marine features and deposits in Scotland. This may have begun with a letter to The Scotsman in 1834. An anonymous writer, referring to a bed of shells which had been found above sea level near Barrowstowness, on the Forth estuary, theorised that they were evidence that the land had risen, rather than the sea had fallen, and it was also suggested that the land had risen unequally across the area involved.

Some years later, one of the most famous of Scottish geologists, Thomas Jamieson, developed what we now call the Theory of Isostasy and set the scene for a considerable advance in understanding Scottish sea levels. Jamieson was the Factor of the Ellon Castle estate in North-East Scotland, but had a strong interest in geology and became Fordyce Lecturer in Geology at the University of Aberdeen. In 1865, visiting the Blairdrummond estate near Stirling, he was struck by the sequence of deposits exposed in drainage ditches. Here, he saw, at the base stony clay, probably laid down when a glacier occupied the area during the Ice Age. Resting upon this stony clay was clay containing marine shells, which Jamieson thought had been laid down in the sea. Next, peat occurred above the clay with shells, indicating that the sea had left the area allowing a peat moss to accumulate. Above the peat, silty clay occurred, containing the bones of a whale, clearly indicating that the sea had returned, before once more receding to reveal the present land surface, upon which the great peat mosses of the area, now largely cleared, had developed. Jamieson theorised that the weight of the glaciers had lowered the land surface so that when they retreated the sea was able to flood in to low lying areas, but that the land, now free of ice, rose and carried the sediment to present levels. Later, Jamieson (1882; 1906) showed that the depression of the land beneath ice had probably been proportionate to the thickness of the ice, so that features of the former sea shore today decline in elevation away from the area where ice had been thickest. However, Jamieson never used the term “isostasy”. That was introduced by US geologist Charles Dutton, in 1889.


Thomas Jamieson (1829-1913)


.3 Wright and the isokinetic theory

Struck by the fact that both land and sea must have moved, in 1934 William Wright observed that raised shorelines in such areas as Scotland may have been produced during periods when the rise of the sea surface during and after the decay of the ice, together with the rise of the land, envisaged by Jamieson, may have resulted in periods when, in some coastal areas, no apparent change was taking place, and that such areas would migrate away from the centre of uplift over time as the uplift of the land and the rise of the sea both declined. Thus shorelines would be time-transgressive, or diachronous. He termed this concept the “isokinetic theory”. Although this term is not used today, the concept remains important in understanding Scottish sea levels. The concept explained how shorelines in Scotland could be “tilted”, declining in altitude away from the centre of uplift as Jamieson had previously observed, and showed how older shorelines might slope more steeply than younger ones.

The many advances in understanding sea level change in Scotland now ceased as the view that shorelines were actually horizontal at set levels, as misleadingly represented on Geological Survey maps showing levels at intervals of 25 feet, gained currency. The torch of discovery had now long passed to Scandinavia, where since the nineteenth century, scientists had been further developing the concepts first introduced by Jamieson.

 

2. The modern era


.1 The morphological approach

It was not until the 1960s that the modern era of sea level studies began in Scotland. Geography was by this time experiencing its “Quantitative Revolution”, with a move away from qualitative work and a stress on measurement and detailed observation. Applying this methodology to the raised marine features of Scotland, Sissons (1963; 1967), working around the Forth valley and estuary, demonstrated that a large number of different former sea levels could be recognised. Contour maps of land uplift, or isobase maps, based upon altitude measurement, began to be produced for Scotland as a whole, showing that the greatest land uplift had occurred in the South-East Grampian Highlands, with amounts of uplift declining away from that area.


Marine terraces, Raasay - view enlargement


.2 Morphology and stratigraphy: the buried shorelines

At about this time, it began to be recognised that evidence for former sea levels also lay buried beneath other sediments in several Scottish estuarine areas. In retrospect, this was not surprising, since if former shorelines were not level along their lengths, but sloped at different rates away from the area of greatest uplift, some might be buried beneath others. Thus, in addition to visible shorelines identified from raised marine features, “buried beaches” and a buried intertidal erosion surface, the “buried gravel layer”, were identified (Sissons, 1966; Sissons et al., 1966).


.3 Crustal dislocation: neotectonic effects

From studies of the buried beaches and the later shorelines which overlie them, the morphological and stratigraphical approach was employed by Sissons in the Forth valley to identify evidence that the uplift of the land involved a degree of dislocation, at least partly along pre-existing fault lines. These “neotectonic” effects had not been so conclusively demonstrated before, and indeed the Forth valley study details some of the clearest evidence for neotectonic effects in shorelines in any formerly glaciated area. Importantly, Sissons (1972) was able to show that glacio-isostatically uplifted areas of the crust may actually comprise small segments separated by faults. This observation remains one of the unsung discoveries of aspects of land uplift in Scotland.


.4 The rock platforms of the West coast

By the 1970s, the new approach to Scottish sea levels, which had previously focussed largely on eastern Scotland, was turning to studies of changes in western Scotland, where the raised rock platforms had been a source of interest for some time. Studies here (e.g. Sissons, 1974) disclosed at least three levels, although any of these could have been a composite level reached on one or more occasion. However, most interest centred on the Main Rock Platform, the altitude of which appeared to slope downward towards the west and south-west, and which was associated with the cold climate episode of the Loch Lomond Stadial (Younger Dryas). The Main Rock Platform was correlated with the buried gravel layer in eastern Scotland, and today these are referred to as the Main Lateglacial Shoreline.


High rock platform, Islay - view enlargement


.5 Morphology and stratigraphy: glacier margins and sea levels

Sea levels were quite early on seen as one method of correlating glacier margins, but this was later discounted as the complexities of sea level change became appreciated. However, there developed a number of detailed studies linking ice limits locally with sea levels (e.g. Cullingford and Smith, 1980; Sissons and Dawson, 1981; Firth, 1988). Two of these studies (Sissons and Smith, 1965; Sutherland, 1984) recognised evidence that the advance of glaciers had resulted in temporary re-depression of the uplifting crust in some areas, thus adding further detail to the pattern of glacio-isostatic uplift.


.6 Morphology and biostratigraphy: the carselands

Influenced by the impressive nature of the raised shore features of Scotland, most of the research undertaken had been based upon an essentially morphological approach, with little detailed dating of the shorelines identified. However this changed for sea levels reached during the Holocene (the last 11000 years) as detailed stratigraphies beneath the “carselands” of Scotland began to be studied in ever increasing detail. “Carse” is a name used throughout Scotland to denote a flat, former estuarine surface lying around the head of an estuary. Jamieson had first realised the potential of the carse in studies of sea level change on his visit to Blairdrummond, for the section he examined was cut in the carselands there.

Carseland stratigraphy had provided evidence for buried shorelines; by now it was providing the opportunity for studies of the detailed fluctuations of land and sea, for within the carse lay horizons of peat, which, having accumulated as sea level changed, could be examined using microfossil analyses (usually of pollen and diatoms) and dated by radiocarbon. Such work could provide a framework for the identification of sequences of sea level change, and in the ensuing decades a number of studies of different carseland areas on both East and West coasts gradually revealed the detail of sea level changes during this period, with many graphs produced showing very detailed changes in sea level as shorelines were reached and subsequently abandoned at different levels away from the centre of uplift (e.g. Smith et al., 2003). Eventually, studies disclosed at least one dated, time-transgressive shoreline, supporting the original view of Wright (e.g. Smith et al., 2002).


Forth Valley and Estuary shoreline diagram - view enlargement


.7 Biostratigraphy: molluscs and other macrofauna

Ever since the first observations on raised shell beds were made, there has developed an interest in what macrofauna, especially molluscs, can tell about the environmental context of the sea level changes taking place. Much of this work has concentrated upon the period during and shortly after the decay of ice, with the view being expressed that at these times sea temperatures were distinctly less clement on the east coast. Of particular interest are the “Errol Beds” of eastern Scotland and the “Clyde Beds” of western Scotland (e.g. Peacock, 1996; 1999). However, as investigations continue, the complexity of the evidence has become evident and relating the deposits studied to specific shorelines difficult.


.8 West coast “isolation basins”

In the last decade or so, the detailed stratigraphical studies in the carselands have been complemented in areas of the West coast where the coastal landscape is often one of small basins, many of which contain the sediments of former sea levels. These basins, termed “isolation basins”, contain marine sediments which accumulated during high sea levels, but then became isolated as sea level fell. Investigations of these features has been based upon an approach developed in Norway. Within many of these basins, more than one episode of marine flooding is recorded, the episodes separated by peat or other organic material, thus as with the carseland stratigraphies, a framework of radiocarbon dates can be obtained (e.g. Shennan et al., 1995; 2000). A particular value of these basins has been that much older events can be dated than was the case for studies in the carseland areas, with for the first time reliable dates for the last 17,000 years becoming available, although because these basins only provide evidence for sea level at “snapshots” in time, no continuous changes can be identified.


.9 The Holocene Storegga Slide tsunami

The accent on detailed study of the sediments laid down by the sea led to the realisation in the 1980s that the northern and eastern coasts of Scotland had been struck by a major tsunami at about 6000 BC. In many areas, especially in the carselands, a layer of fine sand, deposited during the tsunami, can be found. Such a layer was first found in 1965 ( Smith et al., 1985), but its significance was not realised until later (Dawson et al., 1988), and in subsequent years many places have been found where evidence for the tsunami exists (Smith et al., 2004). It is probably not too much of an exaggeration to say that discovery of this event galvanised tsunami studies in North-West Europe, and has led to improved understanding of these phenomena. The publicity it has provided, not always welcome to scientists, has nevertheless raised public awareness and reportedly may actually have saved lives amongst foreign tourists in Phuket during the recent Indian Ocean tsunami.


Cliff section showing the tsunami deposit near Montrose - view enlargement


.10 Understanding patterns of land uplift

Over the last decade, many scientists have sought to define and understand the patterns of land uplift in Scotland which the evidence for sea level change indicates. Theoretical studies, based upon the structure of the Earth’s interior, the weight of the ice, and the weight of the water offshore, on the one hand (e.g. Lambeck, 1995; Shennan et al., 2000; Peltier et al., 2002), and empirical studies, based upon measurement of the altitudes of former sea levels, on the other (e.g. Smith et al., 1969; 2000; Cullingford et al., 1991), produce similar results near the centre of land uplift, but the empirical studies produce much more accurate results farther away though within the area of observations. Improvements in both approaches are awaited, since not only will this be of value in understanding the Scottish coastline; it will contribute to an understanding of other areas of the world where the departure of ice sheets and glaciers has resulted in land uplift.


An isobase map of land uplift since the Holocene Storegga Slide tsunami - view enlargement


.11 Sea levels and climate change

Finally, some researchers are examining the effects on Scotland of the present rise in sea surface levels across much of the globe due to climate change (as summarised in Smith et al., 2000). Isobase maps of land uplift help to define the areas likely to be affected markedly by sea surface rise. Such areas are undoubtedly the Outer Hebrides and Northern Isles, far from the former main centres of ice, where land uplift today is small or absent and where the sea surface is gaining on the land. While Britain understandably worries about sea level rise in the heavily populated South-East, it is perhaps worth sparing a thought for the islands beyond the Scottish mainland, where the threat is real and continuing, and where communities, though small, may be equally affected in the future.


The Intergovernmental Panel on Climate Change Third Assessment Report estimates that the global average sea level will rise between 9cm and 88cm from 1990 to 2100, the rise accelerating after circa 2040 - view enlargement

 

3. Conclusion

Scottish scientists led the world in the nineteenth century in sea level studies and established concepts which endure today. After a period of little development in the early twentieth century, sea level studies in Scotland saw a revival of interest in the early 1960s. There is now the prospect of a new era in sea level research, in which the very detailed studies being undertaken in Scotland may help improve understanding of both the movement of the earth’s crust in the form of isostatic processes and the detailed changes of sea level at the coast. In a world of changing climates, such a prospect is timely.

 

 

Acknowledgement of Copyright:

Portrait of James Hutton: copyright National Galleries of Scotland, Edinburgh

Portrait of John Playfair: copyright National Portrait Gallery, London

Portrait of Charles Maclaren from Cox,R. and Nicol, J. 1869. Selected Writings of the late C. Maclaren. Edmonston and Douglas, Edinburgh.

Portrait of Thomas Jamieson from Smith,D.E. and Dawson, A.G. (eds.) 1983. Shorelines and Isostasy. Academic Press, London.