What is the Rhynie Chert?
What is it?
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Chert is a sedimentary rock comprised primarily of very finely
crystalline silica minerals, mainly a number of varieties of quartz. Some cherts are formed from the
accumulation of siliceous animal remains such as radiolaria, diatoms or sponge
spicules. Most, however, are formed by chemical precipitation from silica rich
fluids either within or replacing pre-existing sediments after they have been buried, or more rarely
as a primary surface deposit. The 'Rhynie chert' falls into this last category
and was originally deposited as sinter from ancient hot springs. It is a rather unusual variety of bedded
chert of predominantly dark blue-grey colours that is of Early Devonian
(Pragian) age, between 400 and 412 million years old (see inset right). The most remarkable feature of this particular rock
is that it contains exceptionally well preserved fossils of some of the earliest
plants and animals to colonise the land.
Right: A polished slab of the Rhynie chert. This shows a vertical section through a
chert bed displaying horizontal wavy laminae with occasional beds containing
fossilised upright plant stems (centre).
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Where is it?
The 'Rhynie chert' is located near the village of Rhynie
in north east Scotland, a village approximately 50km north west of Aberdeen (see
insets below). The rock itself does not naturally
outcrop at the surface. Most fossiliferous chert has been found as loose blocks
within the soil. The chert has been located in situ by trenching at
various times, and more recently by drilling
and coring during 1988 and 1997.
Above: Rhynie village looking southeast across
the Rhynie chert SSSI (field in foreground).
Right: Locality map for Rhynie, N.E.
Scotland. The Rhynie chert occurs towards the top of a sequence of
Devonian sediments in a narrow basin which sits on older igneous and
metamorphic rocks (for a close up map of the geology of the Rhynie area
see the section on Geology and setting).
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Why is it important to science?
There are a number of factors that give this rock its celebrated
status!
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The most important is probably the preservation, in
remarkable detail, of an Early Devonian terrestrial and freshwater community
of plants and animals, together with bacteria and fungi.
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The plants are sometimes preserved in such exquisite detail that their
internal anatomy can be described. They are the best preserved land
plants known from 400 million or more years ago and as such form a cornerstone
of palaeobotanical studies.
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The animals represent one of the earliest and certainly the
best preserved terrestrial fauna known from the Devonian. Recent finds make this the most
diverse associated fossil arthropod fauna of terrestrial and freshwater
origin from rocks of comparable age anywhere in the world.
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The Rhynie cherts are part of the surface expression of an
Early Devonian precious metal-bearing (contains minor gold) hot-spring
system. This is the oldest hot spring system known where
surface features such as geyser vents are preserved anywhere in the world.
How was it formed?
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For a short period in the Early Devonian there were hot springs
and geysers at Rhynie. The waters contained dissolved silica, and when the
erupted water cooled amorphous silica was deposited in the form of sinter.
Some of the silica coated and trapped plants and animals on the land surface or
within shallow ponds, and the organic structures were mineralised (see inset
right). Burial of the sinters over millions of years resulted in deposition of
more silica and the eventual conversion of sinter to crystalline chert.
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Above: A polished slab of recent
sinter from New Zealand showing upright moulds of plant stems coated by amorphous opaline silica.
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| At the present day hot springs depositing
siliceous sinters occur in many areas, notably Yellowstone National Park
in the USA, and near Rotorua in New Zealand (see insets right and below
right). |
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Right: Geysers in eruption at
Yellowstone National Park. Precipitation of amorphous silica from
successive eruptions and outflows of hot water create 'cones' and sheets
of sinter around the vent.
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Right: Ponded
run-off from Daisy geyser (middle distance) in Yellowstone National Park, creating a localised wetland
habitat on a degraded sinter surface colonised by plants.
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