Edinburgh's Geology

Arthur's Seat - an extinct volcano in the heart of EdinburghWithin most towns and cities, there is always geology that can be explored along coastlines, river banks, in old quarries and amongst the building stones - and Edinburgh is no exception.  But beyond these typical rock exposures, Edinburgh has much more to offer.  The city proudly nestles amongst its 'Seven Hills', each offering a window into the past, not least of these being Arthur's Seat, the city's own extinct volcano. 

Read on for a description of the region's geological history, or click here to read about some of the sites worth visiting.

Lothian and Borders GeoConservation publish leaflets about the local area - many are available as pdf downloads.


Geological map of Edinburgh and the Lothian Region

Geological timescale showing Silurian to Carboniferous periodsMost of the rock beneath Edinburgh and the surrounding Lothian region formed during the Carboniferous period, although we also see rocks from the earlier Devonian and Silurian and Ordovician periods (covering a total time span from over 400 million years ago to about 280 million years ago).  The area has also been effected by Quaternary (2 million years to present) glacial activity.
 

Silurian and Ordovician

During the Silurian and Ordovician, the region lay near to the edge of the Laurentian continent, with the Iapetus Ocean lying immediately to the south.  Sediment from the uplands filled the ocean, whilst much further south, the continent of Baltica (of which England formed a part) crept ever closer as the ocean closed.  The final closure of the ocean led to the Caledonian Orogeny, and the formation of the Caledonian Mountains to the north. 

The Ordovician rocks that we see today consist of deep marine mudstones.  Graptolites are commonly found.  The Silurian rocks that we see show a transition from grey, marine sandstones, siltstones and mudstones, through to red, terrestrial sandstones that formed when the land emerged from the sea.  Fossils include trilobites, brachiopods, molluscs, crinoids, starfish, ostracods and early jawless fish.  Due to the mountain building Caledonian Orogeny, the once-horizontal layers of sediment are now tilted vertically.

Devonian

During the Devonian, the region lay in a large valley - the Midland Valley - that was a down-faulted block lying between two mountainous areas to the north and south.  The Caledonian Mountains were eroded heavily in the arid environment of the Lower Devonian, ultimately being worn down to level plains.  The eroded material was deposited in large alluvial fans on either side of the valley.  Later in the Devonian, in a less arid environment, rivers and lakes were typical features on the newly formed plains. 

The Devonian rocks that we see here today consist mainly of conglomerates from the alluvial fans, red continental sandstones and lacustrine siltstones and mudstones.  The local name for this period is the Old Red Sandstone, due to the colour of the vast majority of the rocks.  Rare fish fossils occur.  We also see Devonian basalts, andesites and acid lavas and tuffs.

Carboniferous

By the Carboniferous, 'Scotland' lay close to the equator, and bathed in a tropical climate.  With the Caledonian Mountains eroded, the area formed low-lying coastal plains.  Throughout this time, there were repeated cycles of subsidence (and therefore incursions by the sea), reclamation of land by the creation of deltas, and colonisation of the new land by forests.  During times when the region was covered by sea, seabeds were muddy, but clear, warm waters allowed the growth of limestones and coral reefs in the shallows.  Then, river deltas reclaimed the land through the deposition of sand, silt and mud, creating freshwater lagoons and alluvial plains.  Finally, this new land was colonised by dense lycopod forests (similar to the mangrove swamps of today).  Then through subsidence of the land, the process began again.

The Carboniferous rocks that we see here today consist mainly of marine and freshwater sandstones, siltstones, mudstones and limestones.  However, we also find coal and peat formed from the lycopod forests, and oil-shales formed from micro-organisms that thrived in the freshwater lagoons.  Historically, these fossil fuels formed an important role in the growth of the region's industrial sector.  The Carboniferous rocks are by far the most abundantly fossiliferous rocks within the region.  Fossils include: brachiopods, molluscs, corals, echinoderms, bryozoans, fish, plant material and land-living vertebrates.

The region was also volcanically active in the lower Carboniferous.  Many volcanoes dotted the landscape, erupting basaltic lavas and tuffs.  Arthur's Seat is the best preserved volcano, whilst other remnants of this activity can be seen today as plugs, sills, dykes and laccoliths.  Excellent examples include Salisbury Crags (sill), Edinburgh Castle Rock (plug), North Berwick Law (plug) and Traprain Law (laccolith).

Quaternary

The region has been effected by glacial activity over the last million years or so.  Glaciers travelling from the southern Highlands and Southern Uplands, headed east over the region.  This resulted in a number of glacial features being formed, including eskers, drumlins, terraces, tills and boulder clays.  But perhaps the most prominent glacial features are the crag-and-tails.  The best known crag-and-tail is Edinburgh Castle Rock and the Royal Mile.  It lies at the heart of the city and forms the land upon which the first settlement in Edinburgh was built.

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