| Tayside Biodiversity - Action
Plan - Tayside A Land Moulded |
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| THE
LINKS BETWEEN BIODIVERSITY AND GEODIVERSITY |
Tayside’s complex biodiversity only exists because of its underlying ‘geodiversity’.
Scotland’s geological history is recorded in its wide diversity of
rock types – in fact for its comparatively small size, Scotland has
the most varied
geological and natural landscapes of any country on Earth. From such beginnings
it is possible to see how Tayside’s own rich natural diversity came
about, which in turn has moulded our use of the land over many years.
An understanding of the geology beneath us is fundamental to the wider
goal of sustainable development because earth heritage features and processes
have created many of our important finite resources. It will also put into
perspective
our efforts to plan for effective biodiversity conservation. For instance,
protecting a vulnerable plant or insect community from erosion from the
sea will be unsuccessful if it was the erosion processes that created the
habitat niche
in the first place. Many species have evolved in dynamic environments and
some are able to withstand sudden habitat change. The Freshwater Pearl
Mussel, for example, buries itself deeper into the riverbed when water
levels start to rise.
Tayside’s rocks, soils and landforms are resources that provide the
essentials for life. These include water, raw materials for manufacturing
and construction, soil for agriculture and land for recreation. They also
support the all-important variety of habitats and species for which the
area is renowned.
The conservation
of geodiversity is therefore intrinsically linked to
the soil and what is living and growing on it. It is
therefore of direct relevance to the conservation of
biodiversity. Areas such as the coastal cliffs of Angus
are important tools in teaching the earth sciences.
Many Geological Conservation Review sites are now protected
as Sites of Special Scientific Interest. Other sites
receive limited protection through the non-statutory
designation of a Regionally Important Geological and
Geomorphological Site (RIGS). |
| TAYSIDE’S
GEOLOGICAL HISTORY |
| Cutting through the Tayside region is a prominent topographical feature,
the Highland Line, which traces a major geological fault (the Highland
Boundary Fault) from Stonehaven in the north-east of Scotland to the inner
Firth of Clyde in the south-west. This ancient fault line separates the
rugged and elevated mountains of the Scottish Highlands to the north-west
from the relatively subdued topography of the Midland Valley to the south-east.
Erosion has emphasised the more resistant nature of the mainly metamorphic
rocks of the Highlands in contrast to the generally softer sedimentary
rocks of the Midland Valley. Topography, weather, vegetation and land use
in the Tayside region are therefore all very clearly framed in the underlying
geology. |
Tayside’s
Rocks in the Making
Rocks have diverse and varied origins that reflect
the original nature of the material deposited or
formed and the processes that have happened
since their formation. They can be classified as one of three types:
Igneous rock is formed as molten magma
rises and cools. This forms a variety of rock types depending on the chemical
make-up of the rising magma
and the depth of cooling. Deep cooling tends to produce coarser-grained
rock types such as granites, whilst shallow cooling tends to give finer-grained
rocks such as basalt.
Sedimentary rock is, as its name implies,
formed from sediment and as such is often laid down in horizontal layers.
It may be derived from erosion
of earlier rock types such as sandstone or shale, or by chemical precipitation
(for example, limestone).
Metamorphic rock is formed when any rock
type is subjected to pressure, heat or chemical attack by hot fluids. This
can occur in the deep core
of a mountain range as it is being formed. The resulting metamorphic rock
can be a slate, schist or gneiss, depending on its original composition
and the pressure and temperature reached. If rock is heated above c.700°C
it may melt and form granite.
Each of these rock types - sedimentary, metamorphic and igneous - is found
within Tayside. |
| Tayside
Through Time |
The rocks of the Perthshire and Angus Highlands consist mainly of metamorphosed,
sedimentary and igneous rocks originally formed during the Precambrian
and Cambrian era, collectively known as the ‘Dalradian Supergroup’.
Major earth movements created by continental collisions led to the Caledonian
Mountain range forming 490 – 425 million years ago (Ma). These mountains
extended from what is now Northern Scandinavia through Northern Britain
to the Appalachians in North America. Around the same time the Dalradian
rocks were intruded by large masses of granite.
Further south lie sedimentary and volcanic rocks of mainly Devonian and
Carboniferous age (360 – 290 Ma); these were intruded by thick basaltic
rocks in the Late Carboniferous (c.295 Ma) period. |
| Evidence
of Early Life – Tayside during the Precambrian
to Cambrian |
Underlying Highland Tayside are Dalradian rocks which were originally many
kilometres thick and laid down as shallow marine sands, muds, silts and
limestones some 700 Ma - 510 Ma ago. Evidence for one of the earliest life-forms
ever found in the world comes from elsewhere in Scotland - small dung pellets
have been discovered which were probably produced by a worm-like species
inhabiting these ancient sediments.
These ancient rocks were strongly folded later during the Caledonian Mountain
building and much of the rock sequence is now inverted. The oldest Dalradian
rocks in Tayside, ‘the Grampian Group’ mostly consist of metamorphosed
sandstones. They underlie the mainly peat- and glacial till-covered plateaux,
glens and peaks in the north-west. Rocky crags and abundant scree slopes
are common in the deeper valleys and on the higher peaks. Soils are thin
and acid except in a few wider glens where small pockets of sand and gravel
occur. The overlying ‘Appin Group’ rocks are
metamorphosed quartzites, siltstones, black shaley mudstones and limestones.
Many of these rocks have been changed to schists. They are again strongly
folded and their outcrop displaced by the Loch Tay Fault. These give rise
to variable topography and soils with the quartzites forming the sharp,
scree-curtained peaks of Carn Liath and Beinn a’Ghlo, and the schistose
rocks weathering to form the valleys and lower ground.
The limestones are locally thick and extensive enough to give rise to rare
limestone pavements around Schiehallion and Blair Atholl. The limestone
areas weather to deep brown lime-rich soils which support species-rich
calcareous grassland. The succeeding ‘Argyll Group’ rocks are
also a mixture of metamorphic rock types with quartzites, schistose graphitic
mudstones and limy siltstones and mudstones dominant. Although the harder
more resistant rock forms prominent peaks such as Schiehallion and Carn
Mairg, the highest peak, Ben Lawers is formed of softer limy schistose
mud rocks which are prone to landslips. The resultant richer limy soils
and scree provide a haven for rare alpine and relict arctic flora. |
| World
Famous Fish, Insects and Plants - Devonian and Carboniferous |
Between 410 – 290 Ma Scotland was located 10 – 20° south
of the Equator. Rainforest covered much of the Midland Valley to the south
and coral reefs flourished in the surrounding tropical seas. The sedimentary
and igneous rocks of this era dominate the geology of Tayside south of
the Highland Border. The source of these sediments is, at least in part,
the erosion of the adjacent terrain and the rising mountains in what are
now Scandinavia and Greenland.
The sedimentary sandstone and mudstones represent the deposits of local
alluvial fans and major regional river systems. Such deposits occurred
when the climate was hot and semi-arid with seasonal rainfall. Plants,
including early ‘trees’, were restricted to boggy hollows and
freshwater lake margins. Some of these lakes were formed when volcanoes
erupted and the lava flows dammed the river system. The flagstones from
the Carmyllie quarries near Forfar were laid down in such lakes. World-famous
fossil fish and plants have been found in these rocks.
The Early Devonian lavas formed the Ochil and Sidlaw Hills, occurring also
along the Highland Border. These lavas were part of an extensive volcanic
province that stretched intermittently from Montrose in Angus to Ayrshire
in the southwest of Scotland.
Small volcanoes, laying down more lava flows and volcanic ash, were briefly
active about 340 Ma; these can be seen on the northern slopes of the Cleish
Hills. Subsequently, molten rock intruded between layers of sedimentary
rocks to form sills. These now make prominent landforms such as the upper
part of the steep northerly-facing scarp of Benarty Hill above Loch Leven
and the Cleish Hills. These rocks have long been exploited for hardrock
aggregate.
The solid rock record in the region terminates with the intrusion of the
Midland Valley Quartz-dolerite Sill and related dolerite dykes of the late
Carboniferous such as that seen at Corsiehill Quarry, near Perth. Soon
afterwards Scotland drifted northwards out of the Tropics and was well
on its journey to its present latitude where ice ages were to become inevitable
in the last two million years. Very few rock types of this younger age
are present in Tayside, though there is some evidence for the former presence
of some chalk cover which was eroded away leaving only flint fragments
in
some recent gravel deposits in North East Grampian. |
| The
Ice Ages – The Making of our Modern Landscape |
On a number of occasions during the last two million years Tayside, like
the rest of Scotland, has been covered by kilometre-thick ice-sheets. The
build up of the last main ice sheet dates to about 28,000 years ago. The
ice, which originated in the Highlands (notably Rannoch Moor), radiated
outwards, flowing south and eastwards through the major Highland glens
of Tayside.
In Lowland Perthshire the ice flowed mainly eastwards along the main straths,
including the Almond, the Earn and Strathmore. Armoured with rock debris
like sandpaper it eroded the bedrock into streamlined landforms such as
roche mountonée (glacially scoured and striated rock) and the crag
and tail features seen in the Lomond Hills of nearby Fife. Individual boulders
from the ice now litter the landscape as far-travelled erratics; these
include Samson’s Stone and the Cradle Stones at Crieff.
The characteristic mountain landscape with glacially eroded U-shaped straths
was fashioned by a number of glaciations in this late period. Frost action
developed corries and arêtes. The last main ice sheet left extensive
areas of glacial debris called till or boulder clay blanketing most of
the low-lying ground. Its sandy clay matrix is derived from the softer
mudstone; the angular and rounded boulders and pebbles tend to be from
the harder surrounding metamorphic rocks.
As the last main ice sheet retreated from its maximum extent 19,000 years
ago, meltwaters were released and deposited sand and gravel spreads in
the Earn valley below Crieff and in the Tay around Meikleour and Battleby.
Some of these spreads are in the form of mounds and ridges (eskers) and
others as staircases of river terraces. A feature of the moundy landscape
is the presence of closed hollows (kettle holes), often with lochans where
buried ice melted in situ about 14,000 years ago. Loch Monzievaird is one
such feature; Loch Leven the largest. Loch of the Lowes is one of
several between Dunkeld and Blairgowrie.
In coastal areas such as Montrose and in the Tay and Earn valleys, local
sea level was particularly high as the main ice cleared from the firth
and estuary. Consequently there are raised beaches and estuarine deposits
well inland of the present coastline. Some of the ‘raised’ clays
have been exploited - at Montrose, Dundee, Strageath and Pitfour for brick
and tile manufacture and the sands have been quarried at Friarton Bridge
(just east of Perth), Barry and Forgandenny.
About 13,500 years ago the sea extended inland up the Earn to Crieff and
occupied the Methven depression between this town and Perth. The sea also
reached north of Scone Palace in the Tay. Montrose Basin would have been
an even more impressive sea inlet than it is today. Pollen records show
that vegetation was present and although affected by fluctuations in climate
and the acidity of the surface material, species such as Birch and Willow
were beginning to recolonise the land, followed by Ash, Hazel and then
later by Pine, Oak and Elm.
There is evidence elsewhere in Scotland that around 8,500 years ago brown
bears, lynx and reindeer were present in a forest of pine, hazel and birch.
The sea still reached Scone in the Tay even 6,000 years ago when the extensive
former tidal flats of the carselands were deposited up to about 10m above
present levels.
At about this same time, post-glacial lochs also came into existence, some
of the smaller ones having been either drained or filled naturally with
clays, silts and sands, their former locations perhaps now marked only
by alluvial flats. The larger lochs, including Loch Tay and Loch Earn,
owe their origins and great depth, in some measure, to the erosional power
of the ice sheets. In fact Loch Tay is, at its deepest point, 55m below
sea level. Loch Faskally, on the other hand, is artificial and occupies
a dammed river valley.
As sea level reached modern levels about 4,000 years ago, modern flood
plains formed along rivers and burns with resulting deposits of alluvial
gravel, sand, silt and clay. Peat also accumulated in waterlogged hollows
and as raised bogs or mosses (for example, Methven Moss). On higher ground
blanket peat formed, as did extensive heathland.
With the colonisation of plants and trees, the land became more stable
and less debris was carried away by the rivers to the sea. The advent of
a wetter climate was, however, compounded with man’s activities in
tree felling. Man has also probably transformed parts of the River Tay
from being a braided river system to a meandering one by installing flood
embankments. |
| ROCKS
OF THE FUTURE |
| Geodiversity is a dynamic subject. Animals and plants growing today, plants
decaying to form peat bogs and soil washed off the fields during storms – all
are part of the process of creating rocks of the future. Such processes
support soil development, then plant and animal growth which in turn decay
and become part of the soil and rock formation cycle. All are parts of
the biodiversity cycle of life. Tayside’s incredible array of habitats
and species – now and in the future– cannot exist without this
cycle. |
“Biological
diversity is the key to the maintenance of the
world as we know it. This is the assembly of
life that took a billion years to evolve. It
has eaten the storms – folded them into its genes – and created the
world that creates us. It holds the world steady.”
Professor Edward O. Wilson, The Diversity of Life (1992)
(originator of the term ‘biodiversity’) |
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