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.
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.
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.
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.
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.
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).
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.