Introduction
Today the hill this EarthCache is on rises to a height of 265 metres above sea level, but the rock the hill is formed from was laid down millions of years ago on a sea bed.
The hill is made up of three types of Carboniferous limestone which range in age between approximately 343 and 331 million years.
These limestone layers rest directly on even older rocks from the Silurian Period, which are around 425 million years old.
What's most intresting though is that there are no layers of rock here from the period of time between the end of the Silurian and the beginning of the Carboniferous. In other words, there's no rock here from the Devonian Period - a span of around 60 million years! How can that be? 😲
Let's look at some of the environmental conditions and geologic processes which led to the formation of this hill - and the apparent disappearance of around 60 million years-worth of rock from the Devonian Period.
Logging Tasks
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Based on your observations at the EarthCache location and the information on this page you should be able to tell me:
- Between waypoints ST and FK you will pass lots of accumulated scree (broken rock fragments) on the slope of the hill. Waypoint SS is roughly the midway point along this scree slope. What's the average fragment size of this scree from the following options?: medium gravel (8mm - 16mm), coarse gravel (16mm - 32mm), very coarse gravel (32mm - 64mm), cobble (64 - 256mm), or boulder (> 256mm)
- The scree is a mixture of Carboniferous limestones which are mostly white and grey in colour, plus some fine-grained Carboniferous and Silurian sandstones which are pale to dark brown in colour. Approximately what fraction or percentage of the scree is NOT limestone?
- At waypoint DL you will see an exposure of Farleton Knott's oldest limestone - the Dalton Limestone. Here the limestone is divided into obvious layers. With horizontal being zero degrees and vertical being 90 degrees, roughly what angle are these beds tilted at?
- At the published coordinates you will find a large boulder of Borrowdale Volcanic rock - an erratic dropped here by melting glacial ice around 10,000 years ago. This rock is around 450 million years old and was formed before the collision of Avalonia, Baltica and Laurentia. Describe how it differs from the rocks in the scree.
- Please add a non-spoiler photograph of yourself or a personally identifiable item taken at the given parking coordinates - with Farleton Knott in the background.
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
This prominent hill that I’ve passed many times on journeys up the M6 motorway has held my interest for some time now.
The rubbly appearance of its northwest flank used to make me wonder, before I developed an interest in geology, if it were man-made - a huge pile of spoil from a mining or quarrying process somewhere nearby perhaps.
I've since learned it’s actually a completely natural feature, made up of layers of different types of limestone surrounded by a landscape heavily shaped by glacial ice - millions of years of geological history just waiting to be explored.
On top of that I also learned how Britain, or at least the pieces of the Earth's surface layer which have joined together to form the British isles as we know them today, has travelled thousands of miles to its current location and that this journey is key to which types of rocks appear here today.
Climatic Belts or Zones
Weather is the condition of the atmosphere at a particular place over a short period of time.
Climate on the other hand refers to weather patterns and their averages over long periods of time - which suits our purposes here because geologic time spans millions and millions of years.
Now this is an EarthCache so our subject is geology rather than climatology, but climate is a key ingredient in many geologic processes.
Anyone who is aware of the differences between the polar ice caps and the world's hot deserts - even if that's only by seeing them on television or in a book - will be aware that the climate around the globe varies significantly. These variations in climate affect the Earth's geology. One way in which climate affects geology is in the types of sedimentary rocks laid down in different parts of the world 😉
The Sun heats the Earth, and its atmosphere - but it heats some parts more than others and this is what makes the climate differ across different parts of the planet.
It's a common misconception that because the Earth is round and therefore part of it is closer to the Sun, that part will be hotter. The truth is that it's the angle at which the Sun's rays hit the planet that affects the amount of energy absorbed and thus the amount of heating taking place.
The parts of Earth which are heated most are those where the Sun's rays hit head-on. The Earth absorbs less energy from rays which hit at oblique angles. The more oblique the angle, the less of the Sun's energy is absorbed and the less heating occurs.
Climatology, the scientific study of climate, divides the Earth into several climatic zones, (sometimes referred to as climatic belts) - the tropical zone, the temperate zones and the polar zones.
The tropical zone tends to be hotter and more humid - because the sun's energy hits this zone head-on.
The polar zones tend to be cold and dry - because the sun's energy hits them at oblique angles which leads to much of the moisture in these zones being locked up in snow and ice.
The temperate zones - one of which is the site of Britain today - are somewhere in between these two extremes.
The types of sedimentary rock which accumulate most in each zone are affected by the associated climate.
Sedimentary Rocks
Farleton Knott's three types of limestone and the Silurian rocks the hill sits on are all sedimentary rocks.
Sedimentary rocks are formed from fragments of pre-existing rocks or parts of once-living organisms. These ingredients are called sediments.
Sediments settle under the force of gravity, commonly onto the floors of bodies of water such as seas and lakes but also in other environments such as deserts. Over time these sediments accumulate and with enough time get so deep that the lower layers get compressed under the weight of higher layers and then cemented together into solid rock.
Some sea creatures extract calcium carbonate from the water in order to build shells and skeletons and when they die those shells and skeletons become the sediments from which most limestones are formed.
These sea creatures typically live in warm, shallow seas in the tropical zone, or slightly outside it in the warmer parts of the temperate zones (also known as the subtropics), where the direct sunlight can penetrate the clear, shallow water most effectively.
This is why limestones are most common in these parts of the Earth's surface - because conditions there support limestone formation.
Britain's Epic Voyage
The present geographical shape of Britain has only existed for about 10,000 years (and it’s slowly changing).
When we talk of Britain in the geologic past we really mean the bits of the Earth’s crust which eventually became Britain as we recognise it today. At times, millions of years ago these bits of crust were separated by thousands of miles of ocean.
If you check the coordinates of this EarthCache you'll see that they are 54° north of the equator, yet in the Ordovician this part of Britain was around 60° south of the equator suggesting, as each degree of latitude is equivalent to roughly 69 miles, that this piece of crust has travelled almost 8,000 miles northward between then and now 😲.
How is all this possible? Plate tectonics 😉
Plate Tectonics - Earth's Conveyor Belts
The outer layer of the Earth - the crust - is made up of fifteen or so enormous tectonic plates, which shuffle around on the surface - very, very slowly.
The speed at which these tectonic plates move has been compared to the speed at which human fingernails grow.
The processes and mechanisms behind tectonic plate movement are something of a scientific hot potato right now with older explanations being discounted by new scientific theories. Happily, perhaps, this EarthCache looks only at the movements of tectonic plates over the Earth's surface and not at the underlying processes which make that happen.
For the purposes of completing this EarthCache we can just imagine plate tectonics as a series of very powerful natural conveyor belts, capable of slowly moving the enormous tectonic plates around the Earth's surface over distances of thousands of miles over millions of years.
Britain in the Ordovician - approx 490 to 444 mya
Around 490 mya (million years ago), in the early Ordovician Period, Britain was split across two continents.**
The northern part of Britain, including Scotland, was on the continent of Laurentia on the west side of the Iapetus Ocean.
The southern parts of Britain, including Ireland, were thousands of miles away on the east side of the Iapetus ocean on a land mass called Avalonia which had broken away from a larger land mass (a supercontinent) called Gondwana.
The diagram to the right shows this arrangement, with the parts of Britain shaded red.
In the late Ordovician Avalonia, Laurentia and a third continent, Baltica, began to move toward each other, a process which continued for millions of years through the Silurian Period and into the Devonian Period.
Britain in the Silurian - approx 444 to 419 mya
By the early part of the Silurian the component parts of Britain were still in the temperate zone south of the equator and still separated by the now much smaller Iapetus ocean, but having moved much closer to each other than they had been.
Avalonia had moved, through the processes of plate tectonics, to a latitude of around 30° south and had merged with Baltica.
Around 425 mya the slow collision* of the combined land mass of Avalonia and Baltica closed the Iapetus ocean completely, uniting Britain's component parts in a minor supercontinent called Laurussia (OR Euramerica depending on which book you read - take your pick! 😄).
As the massive continental rock plates collided they buckled under the enormous pressure, in an event called the Caledonian orogeny, forming a mountain range which covered much of north and west Britain.
Despite massive amounts of erosion since the orogeny, evidence of these mountain ranges can still be seen today in the Scottish Highlands, the Lake District and North Wales.
The Silurian rocks directly underneath the limestones of Farleton Knott belong to the Kirkby Moor Formation, turbidite sandstones laid down toward the end of the Silurian Period in a high energy shallow marine environment.
*Bear in mind that when we talk about continents colliding we're not talking about an event that happens quickly like, say, two cars colliding. Remember that tectonic plates are incredibly huge and incredibly heavy and move incredibly slowly - about the same speed our fingernails grow - so a continental collision can last millions of years 😉.
**In September 2018 scientists at Plymouth University, having subjected rock samples from 22 sites in Devon and Cornwall to detailed analysis, concluded that the British mainland was formed from the collision of not two, but three ancient continental land masses.
They discovered that the chemical properties of rocks in parts of Devon and Cornwall more closely match those in rocks from France and other parts of mainland Europe.
These scientists take this as strong evidence that these parts of Devon and Cornwall are from the microcontinent of Armorica and that this explains why tin and tungsten are so abundant in Devon and Cornwall but nowhere else in Britain.
Britain in the Devonian - approx 419 to 360 mya
By the Devonian Period Britain had moved closer to the equator, to a latitude of between 10° and 15° south, and had become effectively landlocked inside the continent of Laurussia.
Here, inside the tropical zone, Britain was subjected to a semi-arid desert climate suited to the formation of red sandstones.
These particular red sandstones have become known as Old Red Sandstone, as opposed to the New Red Sandstone laid down later in the Permian and Triassic age.
It was during the Devonian though that thousands of metres of rocks were weathered and eroded away, leaving only very limited exposures of Devonian sandstone remaining in Britain today - none of which will be found at the EarthCache location.
Britain in the Carboniferous - approx 360 to 299 mya
A global drop in sea level at the end of the Devonian Period reversed early in the Carboniferous Period and Britain, by now at the equator, became covered again by a warm, shallow sea.
These early Carboniferous conditions suited the laying down of significant amounts of limestone - including the Dalton, Park and Urswick limestone which forms Farleton Knott.
Farleton Knott's limestones were laid down directly on top of the earlier Silurian rocks although, sadly, you don't get to see where they meet at this location 🙁.
The only examples you'll see here of Silurian rocks are erratics which were picked up from further north by glacial ice and then dropped here when the ice melted around 10,000 years ago.
If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there 
.
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