More than 400 million years in the making
and it’s not over yet…
More than 400 million years in the making
and it’s not over yet…
People have used the landscape of Edinburgh and its geology in different ways over the last 10,000 years: finding suitable sites for settlement, defence and agriculture, and using the fine local sandstone for impressive buildings, and coal for fuel. The character of today’s city, with the Old and New Towns designated as a World Heritage Site, derives greatly from the bedrock, and the way in which this has been eroded.
There are two contrasting rock types in the area: sedimentary rock, derived from slow accumulation of sediment in river systems and shallow seas, mostly during the Carboniferous period, from 360 to 300 million years ago; and igneous rock, cooled from magma in volcanoes or trapped underground to form intrusions.
The big contrast between these rock types is in their hardness: igneous rocks tend to be tough and slow to erode, so they form the hills, including the Pentlands and the Seven Hills of Edinburgh. Sedimentary rocks are softer, and get worn down to create the gently undulating topography surrounding the hills, where most of the city is built.
Travel back 350 million years, and this area lay south of the equator, part of a large continent with a warm, wet climate. The mountains to the north were eroding, supplying sediment to large river systems flowing south. Over time, the environment changed in cycles, driven largely by changing sea level; you can see some of this variety at various places around the city (Sites: Water of Leith, Stockbridge; Barn’s Ness, East Lothian). At times river deltas and swamps dominated, covered in thick vegetation. Sand accumulated in river channels, forming layers of sandstone that have been exploited for Edinburgh’s building stone (Sites: Edinburgh city centre). In other places, swampy vegetation accumulated and eventually formed coal (Sites: Scottish Mining Museum, Newtongrange). Sometimes, sea level rose, flooding the central lowlands of Scotland with warm, shallow seas that thrived with life. Common lifeforms in the Carboniferous sea includes corals, crinoids and brachiopods and these helped to create limestone layers (Sites: Barns Ness, East Lothian).
There are older sedimentary rocks in this area too. In parts of the Pentlands, and further south in the Southern Uplands, you can find older layers formed in deep sea water, in the Iapetus Ocean: the ocean that once lay to the south of Scotland. This ocean disappeared about 420 million years ago as plate tectonic activity brought different continents together, and the intervening oceanic crust was subducted beneath the continents. Some of the ocean-floor sediment remains as contorted layers of sandstone and mudstone (Sites: Pentland Hills, Siccar Point).
The igneous rocks of this area were created by contrasting episodes of volcanic activity. To the south of the city, the Pentland Hills are made of tough igneous rock more than 400 million years old. This is a complex mix of lava flows and ash layers, the remains of an extensive volcanic field. The magmas that reached the surface had varying chemistry, forming a range of rock types including basalt, andesite, trachyte and rhyolite (Sites: Pentland Hills, Blackford Hill).
After a gap of about 70 million years, Edinburgh again saw volcanic activity: but this time dominated by small, short-lived volcanoes that built up low, discrete cones. These volcanoes were mostly basaltic in composition, with red, fluid lava flows. However, they erupted in a wet landscape, and sometimes mixing of magma and water created explosive eruptions that showered the surrounding area with flying blocks of lava and volcanic ash. Edinburgh city centre is dominated by hills made from different parts of volcanoes of this age, including the volcanic plug of Castle rock, and surface material at Calton Hill, Arthur’s Seat and Craiglockhart Hill. There are more extensive remains of volcanic fields of approximately the same age in East Lothian and the Bathgate Hills. (Sites: Arthur’s Seat, North Berwick, Bathgate Hills).
Towards the end of this episode of Carboniferous volcanic activity, it was more common for the rising magma to get trapped beneath the surface and spread out as flat, liquid layers between the existing layers of sedimentary rock. This forms sills, flat-lying intrusions composed of tough dolerite – a more crystalline rock than basalt, the result of slow cooling underground. Often these sills have been tilted by later earth movements. Dolerite makes a good road stone, it was quarried at Salisbury Crags and it is the main rock type of the old paved streets across the city. It is still quarried today for aggregate (Sites: Salisbury Crags, Corstorphine Hill).
The last rock formed here about 300 million years ago. Since then, this area has been land for most of the time, slowly eroding as rock weathered and sediment moved from the land into the North Sea. Britain has remained a solid block of continental crust, gradually moving north to its present location, and only splitting from Greenland and North America about 60 million years ago.
The most dramatic episodes of erosion have been in the last two millions years; the climate has cooled many times, with ice developing in the Scottish Highlands, slowly accumulating and moving downhill, until eventually the whole of Scotland was buried under an ice cap. The last time this happened, the ice was at least 1 km thick over Edinburgh. During these glacial episodes ice has travelled eastwards across the varied bed rock of Edinburgh, grinding away the sedimentary rock and exposing and polishing the harder igneous rocks. As the ice moved over and around the igneous rock outcrops, it scooped deeply around the sides, leaving glacial hollows. On the eastern side of each outcrop, softer sedimentary rocks might be protected and left intact: the Crag and Tail shape of the Castle Rock and the Royal Mile is an excellent example (Sites: City Centre).
After the peak of the last glacial period 20,000 years ago, the ice melted rapidly, and silt-laden meltwater rushed towards the sea. Sometimes this cut deep gorges in the bedrock (Sites: Water of Leith, Blackford Hill). The melting ice left a wet, raw landscape with many lochs and swamps. The first settlers preferred the high igneous hills for settlement and agriculture, but gradually the wet lowlands were cleared and drained.
The geology of Edinburgh and the surrounding area inspired one of the world’s greatest geological minds, James Hutton, born in the city in 1726. Hutton had an eclectic mix of interests, from the Chinese language to canal building, meteorology to agriculture. However, he is best remembered as the founder of modern geology. His key ideas about the action of natural processes in creating rocks, and on the huge expanse of geological time, were profoundly important on the development of the science of geology and influenced Charles Darwin’s development of the theory of evolution. You can visit the site of Hutton’s house in St John’s Hill, where he died in 1797, and some of the places where he made key geological observations (Sites: Hutton Memorial Garden, Salisbury Crags, Siccar Point).
The Edinburgh Geological Society is one of the UK’s foremost geological societies, whose aim is to promote public interest in geology and the advancement of geological knowledge. We are a friendly and informal organisation with a wide range of members of varied backgrounds and interests.
Charity registered in Scotland
No. SC 008011