From Earth Science On-Site
Meldon Valley, Devon
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At any one site it is helpful to think of interpreting the evidence preserved in the rocks as a recurring pattern of events, often referred to as the rock cycle. These events are:
The evidence for the events in The Rock Cycle can be “read” from the rocks in any exposure. However, some parts of the story are always missing, because geological evidence has many “gaps” in it caused by a combination of sediments never having been deposited and preserved in the first place, loss by erosion, and the fact that much is still buried and unknown. This means it is important to remember that the “story” at any one site is but fragments of a single Earth Science story that has an “invisible “prologue” and “epilogue” each millions of years long, but for which we cannot see the evidence at any one site, because it is not available to us.
In detail the area in question has a very complex geological interpretation. This briefing concentrates only on the story from the Meldon sites in the Earth Science On-Site itinerary, and describes the evidence for two Rock Cycles, an ancient one and a modern one, with the evidence for any intervening cycles being eroded away.
The First Rock Cycle
a. Weathering, Transport and Deposition
The (now metamorphosed) sedimentary rocks visited in this itinerary are Lower Carboniferous in age, as identified by the very few marine fossils they contain. They were deposited about 360 to 320 million years ago in one of a series of basins which stretched east to west from what is now southern Ireland, through Devon and Cornwall and into what is now Belgium and beyond. The evidence from areas to the north suggests that a shallow shelf sea area extended from around the Bristol area, north westwards to the Swansea area, and that further north still, was a land area, with physical and chemical weathering and erosion, but no deposition, occurring on it.
Weathered material would have been moved to the shallow sea by rivers and deposited there. However, on the edge of the shallow sea underwater “avalanches”, called turbidite flows, moved the sediment into deeper water, depositing them as black, bedded mudstones, and silica rich beds, called cherts. Cross bedding studies on these beds have shown that the turbidite currents flowing into the Meldon area followed the axis of the elongated basin and were dominantly from either east or west.
Some of the cherts seem to be organic in origin, showing traces of radiolarians, which are planktonic organisms. However, there are also volcanic tuffs, made up of solid material ejected from volcanoes, inter-bedded with the muds and cherts, indicating that some of the silica may be volcanic in origin. The blackness of the fine mud suggests the sea bed was deficient in oxygen, preventing bottom living animals from colonising the muddy sediment, and the oxidation of organic carbon which settled in the area. The fine grain size indicates that deposition took place below the effect of wave action. In addition there are some calcium carbonate deposits indicating that limestone sediment from the shallow sea was also moved into the area by turbidite flows.
b. Deformation: Folding Metamorphism and Igneous Intrusion
Towards the end of the Carboniferous period, about 300 million years ago, plate tectonic forces began to close up the depositional basins, which by now included many more hundreds of metres of sands and muds on top of the beds in our area. Plate tectonic theory suggests that there must have been a destructive plate margin to the south of the basin running east-west, bringing the piece of crust we now know as southern Europe into closer proximity with the piece we now know as Devon, and squeezing the sediments between into folds and thrusts. Full closure of this margin resulted in the uplift of a fold mountain range (called the Variscan mountains) extending eastwards into Europe from southern Ireland (see Figures 1 & 2). These beds were also faulted at this time, but the faults, like the folds, are not easy to see in the complex geology of the area today.
The granite of Dartmoor, Bodmin, St Austell, Carnmenellis, Land’s End and Scillies was also formed at this time, deep underground. The reason we can see this rock at the surface today is because the crust has been uplifted, and about 3 kilometres of overlying rock have been weathered and eroded away. Gravity surveys across the area indicate that the exposed moors are only the tops of one single large curving granite batholith (mass of underground igneous rock) which joins all of the exposures up below ground. In detail the granite shows evidence of a complex origin, but for our purposes it can be regarded as a magma, rich in silica, which crystallised to a pale coarse grained igneous rock, indicating a slow cooling history. It contains three minerals: feldspar (white, opaque); mica (black) and quartz (glassy). Parts of the granite show many larger white (feldspar) crystals, which are evidence for a slower period of cooling, preceding a faster (but still pretty slow) period of cooling which may have taken place over a period exceeding a million years. The result is a coarse grained rock with larger crystals embedded in it. Potassium – Argon age determinations of the black mica in the granite give an age of 296 (plus or minus 8) million years.
The granites are thought to have been intruded at a temperature of around 700 degrees Celsius, baking the “country rock” into which it was intruded for a distance of up to two kilometres. This resulted in the contact metamorphism of rocks which were also regionally metamorphosed by the fold mountain episode. These hard, fine grained metamorphic rocks are called “hornfels”. Mineralisation of the rocks along joints and faults occurred, forming sulphide minerals of arsenic and copper, and the tin oxide cassiterite (SnO 2).
A dyke of aplite occurs in the Meldon area. At its contact with the country rock it is a pale, fine grained igneous rock rich in silica and feldspar, with no black mica. The dark mineral in this rock is tourmaline, a mineral indicating a magma rich in boron. The dyke cuts across the bedding of the metamorphosed sedimentary rocks (hornfels) and so, according to the principle of cross-cutting relationships is later, and therefore younger than these sediments. Potassium – Argon dating of the lithium rich mica in the aplite gives an age of 254 (plus or minus 6) million years, i.e. slightly younger than the granite. This dyke is interpreted as a rock formed from the last parts of the granite magma, rich in the volatiles which had not been incorporated into the crystal lattices of the granite minerals. Before quarrying it was originally 20 metres wide at maximum, and dips 50 degrees to the south east (See Figure 2).
The effect of this fold mountain building episode was to turn some of the mudstones into slaty rocks showing a rough metamorphic cleavage, and to create folds which run north-east to south-west across the area (see Figure 2).
c. Uplift, Weathering and Erosion
Many millions of years before the present day, rivers cut through the Variscan fold mountains, and eventually reduced them to near sea level. They washed pebbles and sand northward depositing them in sandstone and conglomerate beds outside of our area. The evidence for this is the rounded, water transported pebbles from the metamorphic aureole of the granite which appear in (Permian) conglomerates near Crediton, ten miles to the east-north-east. Fortunately these beds are associated with lavas that have been dated around 281 Ma. It seems that the granite aureole was exposed by weathering within a few millions of years of its formation, and the evidence is preserved in the next rock cycle - but not in our area. In order to erode the lithosphere as deeply as this the crust must have also been uplifted to bring these deeply formed rocks above sea level.
[NOTE: The absolute timescale for dating rocks relies on radiometric methods. Geologists are normally only able to establish a relative time scale (i.e. one rock is older or younger than another). Only in very few areas can one timescale be linked to the other to give fixed time points in the relative timescale.
Here the formation of the granite at 296 (plus or minus 8) Ma (million years ago) and the age of the younger lavas at 281 (plus or minus 11) Ma associated with the conglomerates containing pebbles from its metamorphic aureole, are close enough to “bracket” a date in the relative time-sequence of events. This significant combination of evidence has allowed the date of the end of the Carboniferous, and the start of the Permian period to be established, with some degree of confidence, between these two fixed points, at 290 Ma.]
The Second Rock Cycle
d. Weathering, Transport and Deposition
In the Meldon area there is no evidence of deposition of beds after the Carboniferous: any that were deposited were eroded away millions of years ago. The next evidence in our story is the much more recent weathering and erosion of the rocks during the glaciations and inter-glaciations of the last 2 million years.
The effects of freeze – thaw activity at the end of the last glacial period produced blocks of rock physically weathered from the bedrock. These slipped some way down slope over the permanently frozen rocks below, and now lay in boulder fields called “clitters”, most clearly observed around the granite tors, (see Figure 3), but also present in our area east of Red-a-ven Brook. Granite tors are residual joint blocks of rock left behind as the surrounding granite has been weathered away.
Present day transport of material is to the north by the River West Okement and its tributaries, flowing off the granite to the south and through our area. The sediment trail northwards is to the River Torridge and the estuary and sea at Barnstable, on the north Devon coast. This is on the edge of an ocean with a constructive (not a destructive) plate margin, so it will probably be a great length of time before they are affected by plate tectonic forces, folding, faulting and uplifting them to begin the next cycle, far into the future.
Human activity may be regarded as a form of “weathering and erosion”, and in this area human activity has been significant. Metal mining has probably occurred since the Bronze Age, but peaked in the 1880s; kaolin, or china clay (chemically weathered feldspar) has been extensively mined at St. Austell, many miles to the south west.
The London and South West Railway line between London and Exeter was completed in 1871, but extended to Lydford, between 1872 and 1874 to allow passenger and freight traffic to reach Exeter and beyond. The viaduct across the valley was built at this time, 163 metres long and 46 metres high and stayed open till the 1960s. It is now a scheduled monument, being a rare example of this type of construction. It was during the opening of the railway cutting that the hard metamorphosed rock called hornfels was discovered, leading to quarrying for railway ballast in 1894. The quarrying activity continues today, conducted by Aggregates Industry, which from 1994 has produced 150,000 tonnes per annum, of ballast, roadstone, concrete aggregate, and building stone from the area.
The Meldon Aplite Quarry was worked till the 1970s producing raw material for enamelling, abrasives and road metalling. In the 1920s the aplite was used in a glassworks to produce bottles of a light green colour. The works closed after a few years due to technical problems related to achieving consistent quality of the glass.
Earth Science Principles
In this area it is possible to demonstrate the following Earth Science principles.