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 Within
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.
 Most
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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