Scotlands
Geodiversity - Rocks of Northern Scotland
For a small country, Scotland has a
remarkable diversity of rocks. This is the result of a rich and
varied geological history, spanning some 3 billion years of the
Earth's existance.
This Earthcache will take you to a place
where the diversity is displayed in a circle of stones which can be
found in norhtern scotland.
Formation of the diversity
Scotland consists of five geological blocks
that were brought together in a series of continental collisions
(called the Caledonian orogeny) around 480 to 425 million years
ago. Prior to this period, the foundation blocks lay far apart: in
the ancient continent "Laurentia", a chain of volcanic islands in
the lapetus ocean, Baltica (Scandinavia) and eastern Avalonia
(containing England). When this happened, Scotland lay south of the
equator. Since then, continental drift moved Scotland northwards to
its present position.
The rocks
The formed rocks of any time depended on
the environment Scotland was experiencing at that time. A vast lake
in a relatively arid climate produced muddy sediments with layers
rich in dead fish. These are now the flagstones of Caithness, full
of amazing fossils. A humid, tropical climate which existed when
Scotland lay on the equator, resulted in great thicknesses of dead
vegetation accumulated in swamps. This now forms the coal of the
Central Belt. Volcanic rocks also formed, initially as a result of
stretching centred on the Central Belt, and later as a result of
upwelling hot material and stretching associated with the opening
of the North Atlantic Ocean. These rocks are usually harder than
sedimentary rocks so often form hills or mountain
ranges.
Precambrian period
The oldest rocks of scotland and in europe
(and they are amongst the oldest in the world) are the Lewisian
Gneisses. They were formed in the precambrian period up to 3.000
million years ago. They can be found in several parts of the
northern highlands, west of the Moine Thrust and in the outer
Hebrides. They consist mainly of granitic gneisses. You'll find
examples of the Lewisian Gneisses and other precambrian rocks at
the cache location.
Cambrian/Ordovician period
The rocks of that period were mainly formed
by sedimentation. They are composed of a wide variety of materials
including mica schist, biotite gneiss, greywacke and quartzite. The
area that would become Scotland was at that time close to the south
pole. Fossils from the north-west Highlands indicate the presence
of trilobites and other primitive forms of life. A lot of the
quartzite lies direct on the Lewisian and are almost pure quartz.
The top of the quartzite is called Pipe Rock and contains burrows.
These pipes (you will see an example at the cache location) are
thought to be formed by worms before it was cemented together to
make the quartzite. The next rock sequence on the quartzite are
characteristically brown colored rocks called Fucoid Beds. They
contain a lot of different types of fossils and consists of
dolomitic siltstone.
As the landmass moved northwards during the Ordovician period
sandstone, mudstone and limestone was deposited. The upper part of
Durness limestone is from that period.
Devonian period
The scottish landmass now formed part of
the Old Redstone Continent. The Old Redstone is formed during the
Devonian and dominated by alluvial sediments progressing to a
combination of dune, lakes and river sediments. The color arises
from the presence of iron oxide. Many fossils are found within the
rocks including early fishes, arthropods and plants (take a look at
the big ORS at the cache location). ORS was frequently used as
building stone in Caithness. The Devonian was also a volcanically
active time in Scotland and you can find large evidences of lava
flows near Oban. In northern Scotland the remains of magma chambers
are found which formed granite.
Jurassic period
At the Jurassic the supercontinent Pangaea
began to brake up making the beginning of the seperation of
Scotland and North America. Sea levels rose and there are only
isolated sedimentary rocks of sandstone remaining on land - for
example on the Sutherland coast near Golspie. The area has
considerable significance as the burial of algae and bacteria below
the mud of the sea resulted in the formation of North Sea oil. At
the location you'll find to examples of sandstone and a fossil
tree.
To claim this earthcache:
Please send me an email using my profile
page with answers to the following questions:
1.) What is the given timeline for
the Cambrian/Ordovician period at the cache location?
2.) Describe in your own words the
differences you see between the stones from the Precambrian and the
Devonian period. And where might that differences come
from?
3.) There's a compass rose at the
location: Which rock (not the period!) lies to the west and which
lies to the south?
4.) (optional) Please post a photo
of yourself or your GPS at the location.
Send this answers to me - do not
post in your log. Your log will be deleted if no answers are
send.
Have fun!
Resources - Find out more about Scotlands
Geodiversity:
Wikipedia
Scottish Natural
Heritage
Assynt's
Geology