Tag Archives: Geological Time

Geological Time Part 2 – The Periods in Shropshire

As part of my geological time series I thought I’d have a look at where rocks/features of the periods can be found in Shropshire.  Shropshire is a very geologically diverse county and if you look hard enough you can find examples from all of the time periods except for the Cretaceous (though the Tertiary is a little dodgy) .  I will say that the examples I am giving are not the only ones to be found in the county.  There are many other formations, rock types and sites that can be used to represent the time periods, I’ve just pick a selected few.  So prepare for a few explanations and quite a lot of pictures.  Just as a remind ma stands for millions of years ago.

Shropshire’s Geological Timeline

Shropshire Geology Time 2

Shropshire Base Map 1

 

I’ve included a brief timeline and a small map giving the rough location of the sites I visited (blue circles) with the major towns in the county for reference.

Quaternary Period (2ma to present)

As we are still living in the Quaternary then many landforms can be seen to represent what has and is still going on in the county.  For a more pre-historical landform a good example would be the glacial relics, of which the kettle holes and peat bogs around Ellesmere would be a good place to start.

DSCF2941DSCF3579These features were formed after the ice of the Devensian Glaciation retreated around 11,000 years, leaving layers of sand and gravel (commonly called glacial till).  The kettle hole forms when a block of ice falls of the retreating glacier and melts, leaving a water-filled depression.

Neogene, Paleogene and Cretaceous Periods

Sadly these formations are missing from Shropshire.  There is rumour of a Tertiary (possibly Neogene) outcrop near Whitchurch, but I haven’t been able to confirm it and I can’t find it on a map, so for now it’ll have to remain a mystery.

Jurassic (200 to 145ma)

You’ll be lucky to find this one.  There is only one real outcrop of Jurassic age rock in the county and that is around the village of Prees, about 5 miles south of Whitchurch.  There is some Jurassic bedrock under the layers of glacial till, but Prees is the only place it comes to the surface.  You can see one exposure, just east of the A49.  To get to it you can follow a public footpath.  The rock is mudstone from 183-190ma and the environment was once a shallow, tropical sea.

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Triassic (252 to 200ma)

The Triassic and Permian are both present in Shropshire, but the exact boundary has caused a little confusion in the past.  This is largely down to the similarities in the formations and the fact that they rest one on top of the other in the same locations.  Personally I think the best place in the county to see both Triassic and Permian is around Bridgnorth, though there are some other impressive red-rock formations that form several ridges in the north of the county.  The Triassic rocks are to the east of Bridgnorth, further up the hill and form part of the Kidderminster Formation.  It is mostly made up of red sandstone and conglomerate.  The image below is that of an exposure on the A442 near Allscott, just north of Bridgnorth and shows a layer of conglomerate sandwiched between two layers of sandstone.  It is a good example of cross-bedding between the strata and is representative of a river running through a desert environment.

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Permian (299 to 252ma)

As I said in the Triassic section, Bridgnorth is the place to see Permian rocks, in fact most of the town is built on it and some of the older buildings are made from it.  The Permian sandstone is also red, shows cross-bedding as the result of wind-blown desert sand, but seems to be a little tougher.  My Triassic samples half crumbled as I was extracting and transporting them.  Again the ancient environment was a desert.

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Carboniferous (359 to 299ma)

For the Carboniferous Period I’ve chosen something a little special; the Tar Tunnel.  Unlike my other choices this is something of a minor tourist attraction and as such will require the parting of a couple of coins.  It’s found in the east end of the Ironbridge Gorge.  The rock in the surrounding area is a mixture of limestones, coals, sandstones, mudstones, and conglomerates from the late Carboniferous Period.  When the Industrial Revolution was in full swing in Ironbridge a tunnel was built to act as short cut between the mines and the River Severn.  Upon digging the tunnel the walls started to ooze natural bitumen.  This was then extracted for a number of years before the industry moved on.  Is there likely to be an oil rush in Shropshire?  Not likely but it is an interesting phenomenon and one that I don’t think can be seen anywhere else in the UK.  If like me you are a ‘geo-geek’ it’s worth seeing.

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If it was red you’d think you were in a cheap horror movie.  The image below shows an pool of tar in a side chamber.

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For a more mundane sample from further up the gorge (Jigger’s Bank) you can see an exposure of the Lydebrook Sandstone.  This layer is made up of a pebbly sandstone and includes layers of conglomerate, and was what I used in the geological timeline above.

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Devonian (416 to 359ma)

Devonian rock actually forms the bulk of Shropshire’s tallest hill; Brown Clee Hill (not to be confused with the neighbouring Clee Hill or Titterstone Clee Hill).

Below is an image of Brown Clee Hill taken from the Wenlock Edge.

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Silurian (443 to 416ma)

The Silurian is very well represented in Shropshire.  One of the best formations in the county (and possibly the country) to see rocks of Silurian age is the beautiful Wenlock Edge.

The Edge is an escarpment that runs for almost 20 miles in a north-east to south-west direction and is made up of of a knoll-reef limestone with lime-rich mudstones & shales surrounding it.

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There are numerous walks along the Edge and due to the old (and current) quarrying activities there are plenty of places to see the local geology.  The above picture showing some wonderfully defined bedding planes.  It is also a good place to go fossil hunting.  There are several places both on and under the Edge were you can collect some nice samples like the one shown below.

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Ordovician (488ma to 443ma)

Like the Silurian the Ordovician Period is well represented in Shropshire, but I’m going to go for a well known formation; the Stiperstones Hills.  The Stiperstones are made up of quartzite and like the Wrekin Quartzite is a misnomer as it is a hard, white sandstone and not a metamorphic rock (like true quartzite is).  The Stiperstones have the added advantage that besides the Ordovician rock, you also have the tors formed from millennia of ice/frost shattering.

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Cambrian (542ma to 488ma)

The Cambrian witnessed a massive diversification of animal lifeforms (often called the Cambrian Explosion) and there some good locations and rocks to be seen in Shropshire.  The Ercall quarry has some wonderful quartzite to sandstone formations, showing beach ripples, conglomerates and inclined strata (see my Ercall post for more)

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The Wrekin Quartzite gives way to the Comley Sandstone.  The type site is the sadly neglected Comley Quarry (located on the north-east slope of Caer Caradoc) where Shropshire’s first Cambrian trilobites were found.  Unfortunately the rock faces are now overgrown and difficult to see.  It can be seen in better condition a the Ercall.

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Precambrian (4600ma to 542ma)

Being such a long eon the Precambrian goes from the formation of the earth to around 542ma.  The Precambrian rocks in Shropshire are mostly from towards the younger end; around 570ma with the gneiss and schist of Primrose Hill possibly being older.  My choice for the Precambrian is the Uriconian Volcanic formation which makes up a number of hills, including the Wrekin, Caer Caradoc and the Lawley.

This photo was taken from the Long Mynd (itself a sedimentary formation from the Precambrian) showing Caer Caradoc (centre right), the Lawley (centre left, and a bit in the distance) and on the left horizon is the Wrekin.  These hills are made up mostly of rhyolite, andesite and basalt.

Below is a sample of basalt from the Wrekin.

IMG_1862Well there you have, a brief geological tour of Shropshire.  The county has seen volcanoes and beaches, deserts and tropical seas and now the efforts of an enthusiastic geo-geek.  Hope you enjoy.

 

Geological Time Part 1- Concepts of Geological Time

In some of my posts I have made mention of certain geological time periods, such as Precambrian, so I thought I’d write a few posts about geological time.  I’m only going to do a brief overview as there are many other sources in which you can get more information.  Most geological text books will cover the subject and even the wikipedia article is worth a look for a little more info.  Oh and in case you were wondering the featured imaged is of the Isle of Hoy, part of the Orkney Islands.  I took the photo several years ago and if you look carefully you can see the geological strata in there.  The cliff itself is about 300m high.

4004 BC, Catastrophism & Uniformitarianism

For most of the past 2000 years the western world has been dominated by Christian theology and this extended to concepts in science.  Genesis was taken literally by many people for the early history of the world, including those who wanted to understand it.  One of the most famous calculations about the age of the earth came from James Ussher, Archbishop of Armagh (lived 1581-1656).  He used the genealogies in the Old Testament to calculate that the world began the night before Sunday 23rd October 4,004 BC.  This calculation became popular (and in some circles this figure is still considered to be the ages of the earth).  Not long after Bishop Ussher scientists – most notably Isaac Newton and Comte de Buffon – started to make their own calculations based on experimentations.  One such experiment was to heat up iron balls and measure how long it takes for them to cool down.  By using such experiments De Buffon eventually calculated that the world was about 74,832 years old.  The venerable physicist Lord Kelvin even weighed in on the issue and using similar ideas thermal diffusion estimated the age of the earth at 20 millions years.  All of these estimates are considerable older than the biblical chronology would have it, but considerably younger than the 4.6 billion years of the current estimate for the earth’s age.

As part of the bible-influenced ideas, one that prevailed was that the geology & geomorphology of the world was shaped by short-lived, dramatic events, in particular Noah’s Flood.  These events were used to explain such things as why their were fossil shells in mountainous areas and why the deep, wide valleys of northern Europe contained rivers that seemed too small to carve them out.  This idea is known as catastrophism.

It didn’t take to long for early geologists working in the field to realise that ideas of catastrophism weren’t seen in the rocks they studied.  What was seen was evidence of processes going on today; rivers depositing sediment, glaciers carving out valleys, sand dues creating angular layers etc.  One of the first geologists to recognise this was James Hutton and he developed the concept of uniformitarianism.  This idea is basically that the laws of nature and the processes that go on now are the same as those that went on in the past.  Catastrophes were not needed to explain the earth’s geology, and in most cases the slow but continuous processes like uplift, erosion and deposition explain what is seen much better.

In modern geology uniformitarianism is seen as being basically true in the ‘day-to-day’ processes, but there is the recognition that dramatic events do happen.  Meteorites impact the earth, large volcanic eruptions and flood basalts do take place and such events can have a dramatic impact on the local (and very occasionally global) geology.

Geological Time Scale

As the study of geology developed several distinct time periods started to be noticed.  Along with uniformitarianism a very simple principle was recognised; the law of superposition.  This is a simple concept; sedimentary layers are deposited in a time sequence with the older layers being at the bottom and the younger layers at the top.  Uplift, folding & faulting can disrupt this sequence, but such processes are usually evident.  Fossils were used to help define such layers and a sequence began to emerge with simpler organisms at the bottom and more complex ones higher up the sequence (yes this is a bit of a generalisation I know).  The presence of the same fossils in different rocks around the world is usually an indication that the rocks are the same age.

Below is a simple diagram of the geological time scale as we currently understand it.

Geological Time 1

The earth is currently estimated to be around 4.6 billion years old.  The above diagram shows the Eons that earth’s history is divided into.  It is roughly to scale.  The Proterozoic, Archaean and Hadean Eons are collectively known as the Precambrian and represents the early earth.  Rocks from the Precambrian form the base of the continents and are often metamorphic in nature.  To give you an idea of scale the most famous extinct organisms, the dinosaurs, lived in the highlighted section of the Phanerozoic (green box).  Being more recent the rocks of the Phanerozoic Eon are more prolific, easier to define and better preserved.  Starting from about 542ma the Phanerozoic is divided as below.  The dates to the side are approximations as there is a margin of error of a couple of ma either side, with some sources quoting slightly different dates.

Geological Time 2

Again this table is roughly to scale.  Click on the image and you should get a better resolution.  The Palaeozoic (formally know as Primary) has early life and sees the evolution of arthropods & fish in the earlier periods, amphibians, reptiles & proto-mammals in the later periods.  The Mesozoic (formally Secondary) is the age of dinosaurs and tends to be the era most people think of when you mention prehistoric life.  You also had further development of mammals and later birds, along with flowering plants.  The Cainozoic is often called the age of mammals with the Neogene and Palaeogene often being called the Tertiary.  The current time period is the Quaternary and is usually defined as being the last 2 million years up to the present, and contains the so called ice ages.

Telling Time Geologically

Until the discovery of radioactivity there was no absolute way of determining how old a rock was, hence the variations in trying to determine the age of the earth.  Early geologists could only date rocks relative to each other; so called relative dating.

Relative Dating: this uses the law of superposition and uniformitarianism to date the rocks relative to each other.  Unless the layers have been disturbed through processes such as faulting, folding or intrusions, the older layers of rock will be at the bottom and the younger ones at the top.  You can then compare the fossils in each layer to see for differences.  Extinction and evolution will lead to some creatures disappearing and being replaced by other organisms in a sequence up through the rock layers.  This can then be compared to rocks from elsewhere and if you find the same fossils then the rocks will be of a similar age.  This then allows you to compare the fossils & layers in the new site for differences there and so.  This allows you to compare & contrast layers to each other so that you know how old a rock layer is in relation to others around it.  You can then compare current rates of erosion and deposition to then make estimates of how long it took to for the geographic process (say a river depositing a layer of silt) to form a layer of rock that think.  Thus giving you an idea of how old the rock may be.  Even in areas where the rock has been deformed you can study how the layers have changed and get a good idea of how old they are in relation to each other.

Absolute Dating: after the discovery of radioactivity and radioactive decay it was realised that you could get a more accurate date for rocks.  The most common method used is radiometric dating.  I’m only going to do a brief intro to this as to give it a full description would take pages.  The most well known type of radiometric dating is radiocarbon dating.  This uses the radioactive carbon-14 to determine a date.  It can only be used for objects containing carbon (e.g. living material) and only for objects up to about 70,000 years old.  Other radioactive isotopes can be used to for older materials (such as Uranium-Lead, Potassium-Argon and Rubidium-Strontium).  Regardless of the isotopes used the method is the same.  As the atoms decay they lose electrons, protons & neutrons and the original atom (parent atom) becomes a different atom (daughter atom).  This radioactive decay is a constant (there are a few special circumstances such a nuclear fusion that resets this clock).  By determining how long it takes for half of the parent atoms to decay to the daughter atoms you can then measure the ratio between the parent & daughter atoms in a sample of rock.  This will then give you a good estimation for the age of the rock.  This can only be done to igneous rocks as when the molten rock solidifies the crystals that contain the radioactive material are fixed in the rock until you take the sample.  With sedimentary rocks the original material has been eroded away, the crystals are no longer complete and so you cannot get a good ratio reading.  Even if you did get an accurate reading you’d be getting a reading for the crystal not the sedimentary rock it sits in.  For metamorphic rock the crystals have been deformed or broken up by heat & pressure again preventing you from getting an accurate reading.s7001758.jpg

(Metamorphic Lewisian Gneiss, with granite mixed in from north-west Scotland)

There are other absolute dating methods that can be used; dendrochronology, fission-tracking, sediment layers (e.g. in mud), annual snow fall/melt layers in ice to name but a few.  By combining absolute and relative dating methods geologist can determine the age of the earth and the various rock layers that make up its surface.

If you wish to know more about geological dating methods I’d suggest you find a decent geological text book, that’ll describe these methods in greater detail.  For the purposes of my blog writing this brief introduction should suffice.  For part 2 I have been working on something a little special and it has taken a bit of time to gather the necessary materials.

References: Fundamentals of the Physical Environment (3rd Edition) by P. Smithson, K. Addison & K. Atkinson (2002).  Geology (2nd Edition)by S. Chernicoff (1999).  Geology of Shropshire (2nd Edition) by P. Toghill (2006).  The Geology of Britain – An Introduction by P. Toghill (2006).  Fossil Revolution – the finds that changed our view of the past by D. Palmer (2003).