THE GEOLOGY OF ARDNAMURCHAN
By Peter J Mcfarlane
The Ardnamurchan Peninsula is an area of outstanding natural beauty with a rich and diverse collection of flora and fauna. Many of these organisms are specially adapted to the present ocean influenced climate and the underlying geology. However in the distant past conditions were very different and animals and plants existed in Ardnamurchan which have entirely disappeared today. Fortunately we can study the remains of some of these vanished creatures to build up a picture of how and where they lived. In particular the limestone present in Ardnamurchan contains fossils, which are the remains of creatures living at the time of the dinosaurs.
Other rocks tell of episodes in Ardnamurchan distant past when the conditions were too extreme for life to survive. Rocks and fossils are like 'postcards from the past' and reveal to geologists how the land changes over time. Today, the distribution of the many and varied habitats in Ardnamurchan plus the pattern of predominantly agricultural land use over the last 8000 years, still reflects the striking differences in the underlying geology. The area has been of great interest to scientists since it was first surveyed in 1932. Geologists come from all over the world to study this remarkable record.
Photograph of folded moine rocks near Camus nan Geal
The earth's surface is in a constant state of change because the plates, which make up the outer layer or crust, are constantly moving and colliding. Often in the past plates have collided, resulting in the formation of mountain chains. A great ancient mountain range similar to the present Himalayan range once included all of the Scottish Highlands Today after millions of years of erosion only the roots of these great mountains remain.
All of the rocks in the eastern part of the Ardnamurchan peninsula bear witness to this great event. Being far enough away from the main zone of deformation, the rocks all over eastern Ardnamurchan preserve structures within them that give an amazingly detailed insight into the environment in which they originally formed a billion years ago. It is rather remarkable that such a record has survived burial and repeated upheavals
Within this record the ghost remains of ripples and dunes formed on the ancient sandy seabed are well preserved
Sedimentary rocks are still being formed today and we can infer from present day processes what the conditions were like almost a billion years ago when the moine rocks were laid down as sediments. Often individual beds in the moine have layers containing gravel, sand and mud. Today gravel is deposited by water which flows too fast to allow sand to be deposited e.g. gravel filled channels in fast flowing tidal inlets. However the mud layers (altered to mica-rich schist) occurring between the sandy, quartzite layers show that calmer conditions also occurred, allowing deposition of mud from suspension perhaps at high tide. The pink colour present in the rock near the base of the photograph is due to the abundance of the mineral feldspar. Feldspar is very common in sediments deposited rapidly by fast flowing water.
The moine rocks are well exposed along both the north and south coast where coastal erosion has cut a cross section through important parts of the sequence. These areas are of great interest to geologists and have been designated as sites of special scientific importance.
Folds in the moine vary in scale from centimetres to kilometres. In the core of some of the largest folds called nappes even older rocks can be found. At Ardgour the Lewesian gneiss with its distinctive pink and banded texture can be seen in roadside cuttings. It is a staggering 2700 million years old.
The much younger moine formed a billion years ago when life was only at a very early stage and was comprised of cloned single celled marine organisms. The ocean was rather like a microscopic worldwide web of unicellular life. However, all that was set to change as the earth evolved geologically new complex life developed at first in the sea and later on land.
In contrast to the moine, the younger cover of sediments belonging to the Mesozoic are almost undeformed and packed with evidence of life.Internal mould of a Jurassic Nautiloid
One group of marine organisms with a long and distinguished history of achievement spanning over 500 million years are the marine molluscs.
In Ardnamurchan ancient marine molluscs, are represented by two groups, the ammonites and the belemnites. The former group are related to the Nautilus that still survives in the deep ocean today, the latter are part of the same family as squid and octopus also common today.
Interestingly both ammonites and belemnites are now extinct making their fossil record all the more important as this is the only way we can learn how the creatures lived and why they all died out.
Jurassic limestone beds outcrop on the north coast at Swordle and between Kilchoan and Ormsaigbeg. Evidence from these sites suggests that at this time (180 million years ago) the climate was tropical! Most of Ardnamurchan was periodically flooded by a warm shelf sea but as earthquake activity increased parts of the seabed subsided. The deeper stagnant bottom water allowed the formation of ironstone, the principal ore of iron.
In Ardnamurchan the pattern of settlement and agricultural land-use has benefited from these limestone deposits because the soils derived from the bedrock were fertile enough to support crops of corn. Until recently the old mill stood beside the Millburn in Kilchoan, adjacent to fields where corn would have been grown.
Internal skeleton of a Jurassic belemnite
180 million years old
Limestone was also of great benefit to early farmers as it was used to smelt iron from the ironstone. This provided tools and weapons that were a vast improvement on previous stone and flint implements.
Around 65million years ago a plume of molten material that had slowly risen from deep within the earth reached the surface. When the crust was breached, material from the plume began a serious of volcanic eruptions lasting for three million years.
Although much of the energy from the plume (also known as a hotspot) has dissipated, it has remained active creating the entirely volcanic landscape of Iceland, after Britain and North American drifted apart.Explosive eruption of Mt Pinotoba in the Philippians
At the time volcanic activity began, Greenland would still have been visible from Ardnamurchan. The climate was warmer than today and a thin coal seam at Ardslignish suggests that the sea had retreated allowing vegetation to grow.
The first eruptions were of quite fluid basalt lavas that flooded from fissures. One example is the Loch Mudle fault, which is surrounded by flat lying basalt lavas.
A change in activity led to the growth of large centralised volcanic edifices above distinct magma chambers. The ring of hills marks out the youngest magma chamber, clearly visible on the satellite image as the central depression around the township of Achnaha. Circular depressions like this are known as Calderas. Calderas form from cataclysmic explosive style volcanic eruptions where, the contents of the magma chamber are rapidly expelled due to the collapse of the roof of the magma chamber. Some of the volcanic rocks in Ardnamurchan are air-fall deposits and at Maclean's Nose there is evidence that air-fall or pyroclastic rocks were formed by surges and flows down the slope of a volcano. Additional evidence of explosive style eruptions can be seen from the occurrence of very large blocks and bombs, which have weathered out from cliff deposits of ash and other finer explosively formed deposits.
Around 45 million years ago the earth's climate began to cool as the continent of Antarctica drifted to become stationary over the South Pole. Large ice sheets began to build up first in the south and then over the continents and oceans in the northern hemisphere. Ardnamurchan became gripped in an ice age that has continued to the present day.
The advance and retreat of the northern hemisphere ice cap has had a profound effect on the landscape of Ardnamurchan and it is likely that much of the ancient volcanic landscape was ground away by ice. Today the rounded hills and raised beaches are only the latest stage in a continuing process of geological change that has produced a landscape of great beauty and interest to science.Please email Peter for more information.