IN this edited extract from Set In Stone, Alan McKirdy sets off on a three billion year geological journey of Scotland
Scotland today is hugely influenced by its geological past. The location of mountain ranges, lochs and other iconic landscape features, the distribution of land suitable for growing crops and the places where great towns and cities were founded are matters largely pre-determined by its rocky heritage. Even the appearance of settlements is influenced by the building materials available to the architects, which in turn was determined by local geology. Compare, for example, the classical architecture of Edinburgh, Scotland’s Athens of the North, with the grey granite buildings of Aberdeen. The difference in style is largely determined by what lies beneath our feet. The wider landscape is also a product of the past. The Highlands were created when the Iapetus Ocean closed some 420 million years ago and have remained high ground since that time. The rocks produced by these geological processes are tough and have resisted erosion by the elements. In contrast, the softer, more douce countryside of central Scotland and parts of the Scottish Borders result from the bedrock having been worn down more easily by wind, water and ice. When the Industrial Revolution took hold in Scotland, new mining settlements were located in Fife, between Edinburgh and Glasgow and southwards into Lanarkshire and Ayrshire because the exploitable coal resources that powered these social changes were located in these areas. Although the coal mining industry in Scotland has been consigned to history, these areas remain some of the most heavily populated parts of the country to this day. Road and rail transportation routes which link communities followed the lead taken by the glaciers. The ice carved great pathways through the higher ground that engineers would be hard pressed to emulate. So they did the sensible thing and took advantage of the work already done by the glaciers.
On the jigsaw of assorted rock types of different ages, a rich and diverse community of plants and animals has developed since the ice melted. But, without a growing medium, none of this burgeoning biodiversity would have been possible. So let’s talk about soils. They are perhaps something we take for granted. For most people, soils are just somewhere to plant our potatoes and dahlias, a comprehensively dull and uninteresting dirty brown layer of stones, mud and sand. But if soils didn’t exist, almost nothing would grow, and the surface of our planet would be little more than a lifeless rocky place. By the time the last glacier melted around 11,700 years ago, the landscape was covered by a generous, chaotic and non-uniform layer of glacial debris. Scottish soils largely developed from this parent material bequeathed from the Ice Age. The exception is where the cover of dumped glacial debris was thin or indeed non-existent. In those circumstances, the nature of the bedrock is the main determinant of the type of soil that subsequently developed. The process of soil development started as soon as the ice melted away, and it continues to this day, influenced by many factors such as altitude and climate, lie of the land, movement of water and the speed with which the area was initially covered by pioneer species of trees and small shrubs. Our soils have only been developing since the end of the Ice Age, so are considered to be ‘young’. In areas of the world where soil development has continued uninterrupted by ice ages or other upheavals, soil formation has been going on for millions of years, resulting in some instances in soil profiles that are many metres deep. Soils are home to a myriad of micro- and macro-organisms. Soils have been compared with the rainforests of the Tropics in terms of the number of species of plants and animals they support. Just one teaspoonful of soil may contain up to several million micro-organisms – bacteria, fungi and protozoa, in addition to a selection of algae, microscopic worms called nematodes, beetles, mites, caterpillars, ants and the occasional mole. Many of these species play a vital role in the way that soils function, including decomposition of organic matter and the recycling of nutrients. Soils carry out many functions that help us to live a comfortable life. They act as a growing medium for our crops; they absorb rainfall and provide a buffering role, so that the land doesn’t flood every time it rains. Peat, in particular, fulfils a vital role in acting as a carbon store – especially important in these times when minimising our carbon footprint is a key issue. Soils also make the countryside what it is, supporting the wide variety of habitats for which Scotland is well-renowned.
Soils are hugely important in determining which habitats can be supported, so heavily influence the look of the countryside. For example, deciduous forests grow on brown earth soils; coniferous forests and heathlands on podzols; and rushy pasturelands are mostly associated with gley soils. In determining ‘what grows where’ and therefore the distribution of habitats throughout Scotland, there are many factors at work. Climate is key. There are significant differences in rainfall, wind speed, humidity and temperature across Scotland, and these are important factors in determining the type of vegetation cover that thrives and survives in each part of the country. Altitude also plays an important part. There is a marked vegetation change from the foot to the summit of mountain ranges. In this ‘environmental gradient’, perhaps the most important factor is the location of the tree-line. Above the tree-line, the countryside is dominated by hardy shrubs, grasses and other low-growing plants. This is the sub-alpine zone. The drainage characteristics of the soils are another important determinant of vegetation cover. Where drainage is poor, bogmoss or Sphagnum is the predominant vegetation that develops. It is one of the most widespread habitats in Scotland with around one million hectares covered by peatland. Let’s consider two of these habitats, fashioned by geological processes, in more detail: raised bog and machair.
Flanders Moss National Nature Reserve in the Forth Valley is an excellent example of a raised bog. Think of a national nature reserve and you are unlikely to picture a giant compost heap. But that is exactly how Flanders Moss could be described. It is a relic from a bygone age, dating back 11,700 years and more to the end of the Ice Age. As the ice melted, a lunar landscape of dumped glacial till and assorted debris emerged from the blanket cover of ice. In areas where the drainage was poor, a boggy wilderness developed and layer upon layer of vegetation accumulated to form a raised bog. It is called ‘raised bog’ because it has grown to a level higher than the surrounding land. At one time, these features were commonplace in the central part of Scotland, but agricultural improvements and drainage works over the past few centuries have destroyed many of these raised bogs. Those that remain have become all the more precious. Flanders Moss also holds evidence of huge environmental changes in the layers of peat that created the structure. Pollen grains from plants that grew close by or indeed formed part of the bog became trapped in the peat as the layers built up over millennia. As a rough guide, a metre of peat takes around a thousand years to accumulate. Sampling the surviving pollen grains from various levels within the peat bog gives us a picture of changing patterns of vegetation and other events that affected the area.
• Alan McKirdy has written many popular books and book chapters on geology and related topics and has helped to promote the study of environmental geology in schools. Before his recent retirement he was Head of Information Management at Scottish Natural Heritage. He is one of the authors of Land of Mountain and Flood. Set in Stone is published by Berlinn, priced £9.99