Edinburgh’s rocks are more than 300 million years old. What evidence do they contain about conditions in the past? Sedimentary rocks often have fossils and other evidence that tell us about the environment and life of this area in the past. Igneous rocks contain evidence of volcanic eruptions and underground magma intrusion in this area.
The reconstruction shows what Central Scotland might have looked like 350 million years ago. Where is Edinburgh in this diagram? What types of rock are being formed?
Find out more …
Where was Scotland when Edinburgh’s rocks were being formed? The diagram on the right shows the approximate position of the continental plate containing Scotland at different times in Earth history. Scotland’s journey began in the southern hemisphere, and Scotland was located on the Equator 350 million years ago. This time period is called the Carboniferous, because coal is commonly found in rocks of this age.
What did the map of the world look like then? YouTube – Plate Tectonics animation, from 540 million years ago to today – Animation by Christopher Scotese.
What animals and plants lived on Earth 350 million years ago?
Sedimentary rocks found in the Lothian area include sandstone, mudstone and coal; they were mostly formed during the Carboniferous Period (359-299 million years ago) and reflect the changing environmental conditions when Scotland was close to the Equator and drifting slowly northwards. The climate was hot and wet, and central Scotland was slowly subsiding, and often covered by water.
Near Edinburgh there are also older rocks that formed in the Devonian Period (419-359 million years ago) in a hot, desert climate. When it rained, fast-moving flash floods deposited gravel, sand and mud washed from the mountains to the north, gradually forming rocks known as the Old Red Sandstone.
The oldest rocks in this area are found south of Edinburgh, in the Pentland Hills and the Southern Uplands. These sedimentary rocks were deposited in the Ordovician and Silurian Periods (488-419 million years ago), on the floor of a former ocean, the Iapetus Ocean. This ocean gradually shrank as the rocks of England and Scotland drifted towards each other, and the ocean finally disappeared about 420 million years ago. Find out more about the geological time scale and conditions in Scotland in the past – visit scottishgeology.com.
The igneous rocks of Arthur’s Seat were formed by volcanic eruptions around 340 million years ago. Arthur’s Seat had a violent beginning with volcanic explosions that showered the area with blocks of lava, producing clouds of black ash that blew away in the wind. Later, red-hot lava spilled from the crater and spread out over the flat ground. Eruption after eruption gradually built up cones of lava and ash from the central vent area, with other vents becoming active as the volcano grew. Eventually, the volcano became extinct and was slowly buried by layers of sedimentary rock over the following millions of years.
About 325 million years ago, more magma, trapped underground between layers of existing sedimentary rock, cooled gradually to form the tough rock of Salisbury Crags and Corstorphine Hill. This is a rock type called dolerite, which forms from slow cooling of magma underground. It is similar to basalt, but more crystalline and harder. This rock makes good road stone, because it resists erosion. It was quarried at Salisbury Crags and other locations around Edinburgh, and you can still see it used on Edinburgh’s roads.
The Edinburgh Geological Society is one of the UK’s foremost geological societies and aims to promote public interest in geology and advance geological knowledge. We organise varied programmes of excursions and lectures that bring together everyone from complete beginners to professional geologists interested in exploring the geology of Scotland and beyond. We publish the Edinburgh Geologist and excursion guides. Our geoconservation groups promote sites of local interest, publish leaflets and make sure that local geodiversity is understood and protected.
This web page is published by the Edinburgh Geological Society 2019 under a Creative Commons ‘Attribution Non-commercial’ (CC BY-NC) licence, which permits non-commercial reuse provided the original work is properly cited.