Discover
Torteval’s Geology - a guided
walk by George Le Couteur
A suggested walk through Torteval to discover for yourself the secrets of how Torteval came to be as it is today. This itinerary is produced in conjunction with the "Spirit of Torteval" Exhibition being held during 2006 at the Torteval Church Hall in June and at the Priaulx Library in October
Our suggested walk will take you on a round trip through some of the most picturesque parts of our parish, concentrating on the coastal areas. We shall be stopping off at various points of interest, giving you the opportunity of taking in panoramic views of the landscape, and learning how it came to be shaped the way it is today. During this walk we shall be seeing examples of each of four different types of rock and learning something about them. These rocks are L’Erée Granite, Pleinmont Metasediment, Perelle Gneiss, and Icart Gneiss. In spite of the fact that the names of some of these rocks suggest locations outside of Torteval, these are the main rock types of which our parish is composed.
The walk starts and finishes at the car park at Portelet. The total distance is about 4.5 miles, and should be within the capability of most people to complete in 3 hours at a leisurely pace. People with small children should be advised that the walk will take them up to the cliff paths and down into the beaches to examine rock formations, and that a certain amount of scrambling over rocks is involved. They should use their parental control and good judgement as to where they should allow their children to roam and where not. It is advisable to do this walk roughly at low tide, since some of the features cannot be seen at high tide. A pair of binoculars will be very useful on the cliff path section of the walk. Please refer to the map included with this guide. Each location of interest is shown numbered in sequence.
Location 1 - Portelet Harbour
Proceed to the top of the slipway leading to Portelet harbour. Notice
how this sheltered bay has been carved out by the action of the sea.
Notice the steep earth banks surrounding Portelet which have largely
become covered in vegetation. These lead up to a platform of higher
ground overlooking the bay, between the pine trees and the beach. This
is much better seen on the photograph reproduced here, which was taken
before most of the pine trees appeared. Notice how there is evidence
of continual slippage of earth from this platform into the sea. Minor
landslides occur here nearly every winter. A major land slip occurred
several decades ago making it necessary to replace part of the sea
wall with a breakwater of large boulders. So inch by inch, Portelet
is growing bigger at the expense of the platform of land overlooking
it. We shall be discovering more about how this platform of land came
to be there later in the walk.
Now go down into the bay and examine the low outcrop of rock in the middle of the bay just sticking up above the sand (tide permitting of course). This rock is a type of Granite, and is known here in Guernsey as L’Erée Granite. You will find that you can see the detail of the rock texture far better if you wet the surface. This rock is found throughout much of Rocquaine bay up to Pezeries Point. The most obvious and attractive feature of it is the large pink crystals of the mineral Feldspar embedded within it which are sometimes over an inch long. Other minerals comprising this rock are quartz, Mica, and Hornblende. This rock was originally a molten lava very deep underground which slowly cooled, and as it did so, the crystals within it grew into solid rock. It was formed some 646 million years ago during a mountain building event known as the Cadomian orogeny. We shall be seeing this rock again at Fort Pezeries and along the cliffs all the way to L’Angle.
Return back up the slipway and turn right onto the road which goes to Pezeries Point. Notice that you are now on the platform of land overlooking Portelet. Walk past the former Trinity Cottages and note that on your left the ground rises again through the pine forests to the higher Pleinmont plateau. You are now walking on what would have been a beach 125,000 years ago, and the pine forests to your left would have been the cliffs. When the sea level subsequently retreated, the old beach was stranded above sea level, and it became covered in deposits eroded from the higher ground and blown by the wind. Proceed to the section of granite wall overlooking Portelet, which is location 2.
Location 2 -the granite wall overlooking Portelet
Look at the panoramic view in front of you.
In the foreground is Portelet, and in the background is the imposing sea wall starting under the Imperial Hotel, and stretching past La Lague, the entire length of Rocquaine bay to L’Erée, interrupted only by the causeway to the rocky Island on which Fort Grey is built. You will notice that the coastal land bounded by the sea wall is relatively flat, and is now extensively built upon, first by fishermen’s cottages, and more recently with a host of other homes. Now imagine if you can, the sea level rising up by about 18 metres or 60 ft from where it is today. Where you are now standing would be submerged by several feet of sea. All of the fisherman’s cottages and other houses between The Imperial hotel and Fort Grey would be submerged. Only the upper part of the Imperial hotel would be sticking up above the water, and the new shoreline would be defined by the foot of the hills rising up behind the Imperial hotel to the higher Torteval plateau. This is how things would have looked about 125,000 years ago. This is not a great deal of time in Geological terms. This particular sea level rise was caused by the melting of the Continental ice caps in a warm period between two ice ages. When we look back into the geological past we see repeated rises and then retreats in sea level, which make our current worries about the moderate expected sea level rises due to the effects of global warming pale into some sort of Geological context.
This ancient beach, now an area of relatively flat coastal land (or Marais) can be found all along the West Coast of the Island.
Now look behind you, and take the opportunity of examining La Varde Rock. The rock stands some 100 ft above ordinance datum, and it forms a distinctive feature, with quite a flat top. La Varde is likely to have been shaped about 180,000 years ago by wave action when the sea level was even higher during a very warm interglacial period. This feature is what is known as a wave cut notch. Now continue the walk, uphill passing La Varde rock on your left, and just after you reach the highest point of the road, at the junction with the zigzag lane, you will see on your right hand side a narrow track leading down to a slipway. Follow that track to the bottom of the slipway. You are now in location 3
Location 3
Look back towards the slipway, see photograph. You will notice a vertical
slice of dark, fine grained rock (between the dotted lines on the photo)
surrounded by the pinker L’Erée granite which you saw
at Portelet. This rock has some deposits and lichen covering it, but
you should be able to see
it’s structure underneath. This type of structure is called a
dyke. There are many such dykes in Guernsey and we shall be seeing
some spectacular ones on the cliffs later in the walk. If you imagine
the L’Erée granite forming by the cooling of a molten
rock miles underground 646 million years ago, as the rock cooled, it
contracted and in so doing, it cracked. Several millions of years later,
in a different geological event, a new molten rock flowed into the
cracks in the L’Erée granite. This is how this dyke was
formed. The dark rock of which it is formed is called a dolerite.
We suggest that you can now return up the slipway to the road, and turn right, and make your way towards Fort Pezeries, which is our next location.
Location 4
When you arrive at Fort Pezeries, walk down the slipway to the right
of the Fort. The first thing to note is the continuation of the L’Erée
granite on the rocks which border the left hand side of the slipway.
This type of rock should by now be easily recognisable to you because
of it’s large pinkish crystals of Feldspar.
Now turn right and go across the pebbles to the low earth bank which borders the beach. Take a close look at the bank, and you should see an accumulation of rounded pebbles within the bank itself.
This is part of a raised beach. The pebbles were at one time part of a beach when the sea was at a higher level than it is today. We have already seen that the coastal platform of land overlooking Rocquaine bay is an ancient beach formed by a large rise in sea level 125,000 years ago. After that event there was another ice age, during which the sea retreated to a very low level compared to what it is now. There then followed yet another period of relative warmth approximately 100,000 years ago, which resulted in yet another sea level rise, and that formed the more recent raised beach which we are now looking at. The reason why the pebbles are covered in earth, is because the sea subsequently retreated again a very long way indeed, and this left the old beach pebbles to become covered with bits of rock and earth sliding down from the higher ground above. After this particular raised beach had been formed, the sea retreated so far that Guernsey became joined to France again, and the English Channel became just a river valley between England and Europe. This particular raised beach is known as the 8 metre raised beach and is seen at several points on the island. It is not particularly well seen here in Torteval, but If you are interested in studying it further, you will find a much better example of it in the earth banks at the top of the slipway leading to Lihou causeway.
Looking at the earth bank again, you will also notice that at higher levels there are other fragments of rock, but these are not rounded, having jagged edges. These are not beach pebbles, and are not part of a raised beach. These are pieces of rock which, through repeated freezing and thawing have weathered away from the slopes above, have worked their way down the slopes and have become buried over the millennia. This type of deposit is called "head" or "combe rock". It appears extensively in the earth banks bordering our beaches.
Make your way back to the slipway, and note that just before the slipway you will be walking over some strikingly pink, fine grained rock. This rock is called Felsite, and it was emplaced in a similar way to the dolerite dyke which we examined at location 3. However, it differs from dolerite significantly in composition, because it contains a very high proportion of pink Feldspar, giving it it’s attractive colour. Now proceed to Location 5, which is the slipway on the right hand side facing the easily recognised "gap in the rocks" with La Table Des Pions up the slope on your left. Walk down the slipway and examine the earth bank overlooking the small pebble beach.
Location 5
You are looking at a cross section through the earth bank on which
La Table Des Pions is situated. This is the remnant of the same 18
metre beach level which we discussed in location 2. Note that the 8
metre raised beach pebbles, which we saw at the last location, are
not visible here. They are probably present, but at a lower level,
now behind the sea wall. A considerable part of this bank consists
of rock fragments (head) which have slumped down from the higher ground
as explained for the previous location. The upper part of the bank
is finer grained and of a lighter colour. This is called "loess",
which is a fined grained wind-blown dusty-sand deposit resulting from
the weathering and eventual disintegration of the surface rocks during
the ice ages. Much of our soil in Torteval is loess based and it provides
a very friable, workable topsoil into which our farmers could easily
plough in manure and vraic to enhance the fertility, giving our parish
it’s famed green fields and valleys.
Now proceed eastwards to location 6, which is the gap in the rocks which you can see a bit further on from this beach.
Location 6
When you reach the gap in the rock, you will see from the rough-hewn steps, and from the appearance of the rock face, that this gap is man made. It is likely to have been done in Napoleonic times to provide access to the next headland where there is a gun battery.
The rock which you are now looking at is one of the few rocks in Guernsey which has a sedimentary origin. The age of this one, known as the Pleinmont Metasediment, is somewhat uncertain, but likely to be between 1,000 and 700 million years old. Within Guernsey, this rock only occurs in Torteval. It was originally formed in deep water, and was originally a mud or silt. This then became buried, and it eventually changed under pressure and heat into the greyish, rather unattractive, uniform rock which it is today. The rough-hewn steps into this rock have been polished by countless pairs of feet and give an opportunity of examining polished surfaces of this rock. If you examine the face of the cutting at eye height, you will see how the rock fractures into multiple micro-fractures when quarried.
It will come as no surprise to learn that it has never been quarried for use as a building stone, the tendency to fracture in this way would be a stone-mason’s nightmare.
Before leaving this site, take a look at the rock near the bottom of the cutting. Although it is very similar looking to the surrounding rock, this is in fact a small dolerite dyke of the same composition, and probably the same age, as that seen at location 3. The dyke is therefore younger than the surrounding Pleinmont Metasediment.
Now go through the gap in the rocks and proceed to the next location which is nearly to the Napoleonic gun battery at the next headland.
Location 7
The beach which you are now overlooking consists of pebbles which are
mainly Pleinmont Metasediment. Take a look at a prominent pebble filled
gully. This is the site of another dyke. But this dyke is unlike the
ones we have already seen. Notice that the action of the sea has worn
away the rock within the dyke so that it now forms a gully, and has
filled it with pebbles. So the rock which formed the dyke was relatively
soft. Depending on wave action and storms, the level of pebbles within
the gully varies enormously. At times the pebbles are partly removed
and the original rock which formed the dyke can be seen.
Take a close look. If you are lucky enough find it relatively free of pebbles, you may see that at the bottom of the gully is a rock that looks very like reddish brick. This rock is called a "lamprophyre" and lamprophyre dykes can be seen in several locations around Guernsey. Lamprophyres, are normally of dark grey colour with many crystals of Mica which sparkle in the sun. However, when they are exposed to the weather and sea, they often weather to a brick red colour. This is the youngest rock type to be found in Guernsey, having the relatively "young" age of 296 Million years. The molten magma which formed the lamprophyres was formed by the intense energy produced during the process of continental collision which formed the Alps when Africa collided with Europe. This event is known as the "Variscan Orogeny".
Guernsey’s lamprophyres also have the distinction of being our most radio-active rocks, although they are by no means dangerous, and it is perfectly safe to handle them.
When you have finished examining this location, proceed up the grassy slope behind you where you will find a track leading up to the cliff path which leads along the cliffs eastwards. When you reach the cliff path turn right up a relatively steep portion with steps, and right again at the stone sign which directs you to Creux Mahie . Follow the path to the car park near to the partly demolished square German fortification, and onwards to Location 8 which overlooks where you have just been.
Location 8
The stunning view from this location takes in many of the places you
have just visited. Take a look out towards La Grosse Rocque. You may
notice even from this distance that it is distinctly pinker than the
rocks further towards the shore. You may even be able to see some pink
streaks within the rock. La Grosse Rocque is composed of L’Erée
granite, and the L’Erée granite at this location is extensively
covered by pink Felsite veins. The contact between the L’Eree
granite and the Pleinmont Metasediment is shown by the dotted line
on the photograph and is marked by a long rock pool between La Grosse
Rocque and the beach. This well illustrates the fact that the contact
between these rocks is a line of weakness which in the past has seen
the rocks near the contact being crushed and sheered. The sea has exploited
this weakness and eroded the crushed rocks more than the surrounding
rocks, and this has resulted in this long rock pool. Looking out to
sea, towards the Hanois Lighthouse, there are many reefs to be seen.
Their detailed geology has not yet been properly recorded.
The walk from now on will be on the "official" cliff path to Les Tielles. Guernsey’s steep cliffs provide many locations of Geological interest, but they are also very dangerous and inaccessible without the right equipment. We shall therefore confine ourselves to discovering Torteval’s Geology by what we can see from the immediate vicinity of the cliff path. For this, a pair of binoculars will be invaluable.
Location 9
The first
thing we should notice is that we are now on the highest point of Torteval.
During the walk that follows you will see that the
highest places in Torteval are relatively flat. Where you are standing
once represented the approximate level of the sea around 10 million
years ago during the later Tertiary period. The erosive action of the
sea caused a Plateau to be formed at about 200 ft. So 10 million years
ago, our cliff tops would have formed a largely submerged reef, probably
of coral.
Proceed on the cliff path to location 9, which is Les Grandes Cotils, from which you will have a clear view of the rock known as La Pointe de Pleinmont.
This section of the cliffs is entirely composed of L’Erée granite. Notice how La Pointe de Pleinmont is joined to the mainland by a very narrow coupée of grass covered rock which is currently undergoing severe erosion. There are examples of this type of structure all along our cliffs, and these coupées represent the last stages before an outlying rock finally becomes detached and surrounded by the sea.
Our next stopping location will be La Congrelle, near to the German tower at L’Angle, so proceed along the cliff path until you pass the ruins of the Victor Hugo watch house, and then onwards past the newly re-instated gun to the car park at Battery Dollmann. At this point, the cliffs still consist of L’Erée granite, as does the prominent rock which you can see from here, known as the Tas de Pois D’Aval (Upper Peastacks). The cliff path then continues downwards with the motocross track on your left. Soon you will notice that you are passing over outcropping rocks on the path which are grey crystalline rocks, unlike the pinkish L’Erée granite. You have just passed over the transition from L’Erée to Perelle gneiss.
Perelle gneiss started it’s life as a member of the granite family (called a diorite). It is like a granite, but has rather more darker minerals such as mica and hornblende. However, some 1,000 million years ago it was subjected to very great temperature and pressure, and under these conditions it re-crystallised into it’s current form This is the well known local "Grey or Blue Granite".
Proceed until you get to the German tower at L’Angle. At this point you will leave the cliff path temporarily, and although it is quite safe, this part is not advised if you have children, because there are dangerous sheer drops nearby. Proceed carefully between the tower and the protective wire netting fence to your left, down the scree slope below the tower where you will eventually find a small quarried area. This is location 10.
Location 10
If you look eastwards you will see La Congrelle, a deep fissure into
which the sea comes at high tide.
The rock face of this fissure is mainly composed of grey Perelle Gneiss, but in the upper part of it a strip of reddish rock can be seen. This is a dyke of rock called Felsite, and consisting mainly of the Pink mineral Potassium Feldspar. The age of this felsite dyke is not accurately known, but it is probably somewhere between 300 and 400 million years old.
Now turn around and look westwards in the direction in which you have just come. Look at the cliff face below battery Dollmann. You will see a prominent line running down the cliff in a curve to a cave at the bottom. This line is shown dotted in the photo. This is a fault line, a discontinuity in the rock sequence which represents the join between L’Erée granite and Perelle Gneiss. These fault lines often reveal themselves because they are often softer, and more vulnerable to the erosive action of the weather and the sea. Notice that in this case, the sea has exploited the weakness in the rock and hewn out a cave. Return to the cliff path and continue walking towards the Mont Herault watch house which should by now be visible. You will pass the clay pigeon shooting range on your right. Just after the clay pigeon range, there is a field. Walk along side the field in the direction of the sea until you can see the view below. This is location 11
Location 11
The scene looks west over Baie de la Forge along the route we have
come. If there is a swell and from about 3 hours after low tide, there
is a good
possibility that you will see one of Guernsey’s best Souffleurs
or blow holes, forcing the sea spray into a fountain of foam and spray.
A Souffleur is the result of sea erosion, where a fissure in the rock
becomes a small cave, and eventually the sea erodes a hole in the roof
of the cave, forming a chimney, which funnels the spray upwards. Souffleurs
are geologically speaking very short-lived features, because the very
force of the sea which makes them work provides the erosive force which
eventually destroys them. Return to the cliff path. Walk past Mont
Herault watch house. After this point we have crossed over the parish
boundary into St. Pierre du Bois. Walk on past Belle Elizabeth, and
follow the cliff path up some steps until you have a view of the next
bay in which can be seen Le Long Cavaleux and Les Tielles in the distance.
This is location 12
Location 12
A striking feature of the cliffs below both Le Long Cavaleux and Les
Tielles is that both headlands are crossed by prominent bands of very
dark rocks. These are impressive examples of dolerite dykes which were
originally molten magma which found it’s way into cracks in the
much older rock whereupon the magma solidified. These are perhaps the
best examples of this type of feature on our cliffs, but a careful
examination of other locations will reveal that our cliffs are full
of very dark dykes such as we see here, and sometimes the lighter coloured
or reddish dykes such as we saw at La Congrelle. These serve to illustrate
the extremely complex and tangled geological history of our cliffs.
Another feature which stands out from these views is the colour of the rocks into which the dark dykes intrude. They are very distinctly reddish, quite unlike the Perelle Gneiss which we have recently left behind. This is the last rock type which we shall be examining on this walk. It is known as Icart Gneiss. Some 2,600 million years ago this rock was a red granite, but it then re-crystallised into it’s current form under great temperature and pressure. It is not only one of Guernsey’s oldest rocks, but one of the oldest in the British Isles or France.
Although we will be walking past Le Long Cavaleur and having a better view of it, it is worth noting here that this is another example of a narrow coupée being eroded away, which in this case has resulted in a hole through the headland along a fault line. This process will continue until eventually, the bridge over the hole will collapse.
Location 13
Continue the walk almost to Les Tielles car park. Keep a sharp eye
to your right and you will see a small quarried area between the path
and the cliff edge. Caution is urged here, since the drop at the edge
of the quarry is sheer. This is a good opportunity to examine the pinkish
Icart Gneiss. The pink colour is due to large red crystals of potassium
feldspar. You may be able to see that the rock has a "grain" to
it, and you can imagine that it would tend to split an a preferred
direction. This is typical of gneisses.
Now proceed down the lane which leads back to the main Torteval Road, turn left and walk to the road leading to Torteval Church.
Location 14
Although it may seem curious to have our Parish Church as a location
of geological interest, in fact the examination of materials used in
buildings provides a lot of information about the geology of the area.
In the case of Torteval church, you will find of two types of Guernsey
stone. Most of the building consists of grey Perelle Gneiss (found
in Torteval), but at the base of the structure there is a pleasing
addition of some pink Cobo Granite.
Location 15
Our final location is reached by proceeding behind Torteval Church, and down Rue Des Clercs to the coast road, stopping at Fort grey to take in a panoramic view of Torteval.
Summary of how Torteval came to be the way it is now.
The walk which you have just undertaken has taken you on a circular route around Torteval, and the features of the Geology which you have looked at have not been in any chronological order. We have looked at various types of rock of extremely different ages, and considered the evidence in our landscape for certain events, and you should by now be asking yourselves how everything came together to look the way it is today. Before attempting to answer that question, I should say that we certainly do not know the whole story, and in any case it is certain that the whole story of the formation of even such a small area as Torteval is exceedingly complex. But we have already seen some pointers which enable us to bring together all of our observations into a picture which makes some sort of historical sense. The story presented here is of necessity very simplified, and there are many other publications and articles for the more experienced geologist to study, see the references at the end.
Our story begins some 2,500 million years ago. An almost unimaginable time, when the earth was less than half the age it is today. What we do know, is that at a very great depth below where we are standing, there were huge tectonic events, caused by continental plates colliding, resulting in the generation of intense heat deep below the earth’s surface. This caused the partial melting of the earth’s crust beneath us, and molten lava was generated which eventually cooled to become a granite. This was the origin of the "Icart Gneiss", our oldest rock, and one of the oldest rocks in the whole of France and the British Isles. Even after the rock had solidified, there continued to be further tectonic events generating heat and extremely high pressures, and this caused the granite to re-crystallise, to become a metamorphic rock, called a Gneiss, causing the rock to become the Icart Gneiss which we recognise now.
Towards the end of this period, which has become known as the Penthevrian, another similar cycle of events resulted in a molten lava of somewhat different composition to crystallise, first into a rock of the granite family, and known as a granodiorite. This rock had darker minerals within it, and as with the Icart gneiss, it also suffered great pressure and heating deep underground, causing it to re-crystallise to form our familiar "blue Granite" or "Perelle Gneiss". These events are though to have happened around 1,500 to 1,000 million years ago.
The next significant event for us was the formation of the Pleinmont Metasediment. This took place somewhere between 1,000 and 700 million years ago. We can surmise that we were near to the shore of an ancient sea. Mud and silt accumulated on the sea bottom, and eventually, over millions of years, this buried mud solidified to be come the rock which became the Pleinmont Metasediment of today.
Our area was then subjected to the most significant of all our geological events, a process which not only formed most of our remaining rocks, but also thrust the rocks so far formed upwards from deep below the ground to roughly where they were today. This event was a massive collision of two tectonic plates in what was known as the Cadomian Orogeny. Orogeny means a mountain building event. We were roughly at the point of this collision, a process which produced intense heat and new magmas. These in turn solidified to form first, the L’Erée granite 646 million years ago, and several million years later, the other rocks of Guernsey, such as the Cobo granite, the Bordeaux Diorites, and the St. Peter Port Gabbro. The important part of this process for us is that it lifted the new magmas and older rocks up though the earth’s crust, such that when later erosion took place, the rocks that were to become Jersey, Guernsey, Alderney and the other islands were on higher ground than adjacent areas. So around 500 million years ago, the main Channel Islands would have been present as significant peaks with mountains to the south of Jersey where Brittany now is.
Mention was made in connection with the Lamprophyre dyke we saw at location 7 of the Variscan Orogeny. This was yet another great mountain building event associated with the collision between the European and the African continents, a process which formed the Alps. The Variscan Orogeny had a profound effect on the French and British landscapes, but in our area, the only significant event was the formation of the Lamprophyre dykes 296 million years ago.
For the next part of our story we need to fast forward in time to the Tertiary period which began 64 million years ago, when the Atlantic Ocean was being formed. During the Tertiary it is known that there were many periods of very high sea level, and it is likely that around 10 million years ago, the sea must have spent a considerable period of time at levels of 200 to 300 ft above current level. At that time, the main Channel Isles would have had significant peaks sticking up above the water, but would otherwise have had recognisably the same shapes they have today. However, bit by bit, these peaks were eroded away by the action of the sea until our islands were no more than reefs of rocks just above sea level in places, but otherwise largely flat. This is how the flat top to Guernsey was formed.
At that time the sea was relatively warm, and judging from the limestone which is extensively found around the Islands on the sea bed, it is likely that the Channel Islands at that time would have been Coral reefs.
At the end of the Tertiary era we move into the Pleistocene era. This was the era that finally shaped the island into what it is now. It was a time of repeated Ice ages and consequently of climatic extremes. In general, during a period of glaciation, the sea level dropped, because so much water trapped was in the form of Ice over the Northern Continents. In the warmer interglacial stages, the Ice tended to melt to a greater or lesser degree, with consequent rises in sea level.
We have seen evidence of two such events in our walk. The earlier one, happened during a warmer period about 125,000 years ago, and resulted in the sea leval rise to 60 ft (18 m) which as we have seen, formed the beach platform which later became the low lying Marais along the coast road. There then followed another period of Glaciation known as the Wolstonian glaciation, followed, about 100,000 years ago by another period of warmer weather, during which our 25 ft (8 m) raised pebble beaches were formed.
The last period of glaciation was the Devensian, about 45,000 to 15,000 years ago. During the early part of this period, the Head deposits which we saw at location 5 were formed, and the towards the end of the Devensian, the wind blown Loess deposits which form the basis of our arable land were formed. During the Devensian, the English channel became just a river bed between Britain and France, and we were joined to France.
Finally, from about 10,000 years to the present, the sea level gradually attained it’s current level as the Ice sheets of the Devensian melted.
If you have enjoyed this walk, I would appreciate your feedback or suggestions. You are welcome to contact me on georgelec@cwgsy.net
A suggested walk through Torteval to discover for yourself the secrets of how Torteval came to be as it is today. This itinerary is produced in conjunction with the "Spirit of Torteval" Exhibition being held during 2006 at the Torteval Church Hall in June and at the Priaulx Library in October
Our suggested walk will take you on a round trip through some of the most picturesque parts of our parish, concentrating on the coastal areas. We shall be stopping off at various points of interest, giving you the opportunity of taking in panoramic views of the landscape, and learning how it came to be shaped the way it is today. During this walk we shall be seeing examples of each of four different types of rock and learning something about them. These rocks are L’Erée Granite, Pleinmont Metasediment, Perelle Gneiss, and Icart Gneiss. In spite of the fact that the names of some of these rocks suggest locations outside of Torteval, these are the main rock types of which our parish is composed.
The walk starts and finishes at the car park at Portelet. The total distance is about 4.5 miles, and should be within the capability of most people to complete in 3 hours at a leisurely pace. People with small children should be advised that the walk will take them up to the cliff paths and down into the beaches to examine rock formations, and that a certain amount of scrambling over rocks is involved. They should use their parental control and good judgement as to where they should allow their children to roam and where not. It is advisable to do this walk roughly at low tide, since some of the features cannot be seen at high tide. A pair of binoculars will be very useful on the cliff path section of the walk. Please refer to the map included with this guide. Each location of interest is shown numbered in sequence.
Location 1 - Portelet Harbour
Proceed to the top of the slipway leading to Portelet harbour. Notice
how this sheltered bay has been carved out by the action of the sea.
Notice the steep earth banks surrounding Portelet which have largely
become covered in vegetation. These lead up to a platform of higher
ground overlooking the bay, between the pine trees and the beach. This
is much better seen on the photograph reproduced here, which was taken
before most of the pine trees appeared. Notice how there is evidence
of continual slippage of earth from this platform into the sea. Minor
landslides occur here nearly every winter. A major land slip occurred
several decades ago making it necessary to replace part of the sea
wall with a breakwater of large boulders. So inch by inch, Portelet
is growing bigger at the expense of the platform of land overlooking
it. We shall be discovering more about how this platform of land came
to be there later in the walk.Now go down into the bay and examine the low outcrop of rock in the middle of the bay just sticking up above the sand (tide permitting of course). This rock is a type of Granite, and is known here in Guernsey as L’Erée Granite. You will find that you can see the detail of the rock texture far better if you wet the surface. This rock is found throughout much of Rocquaine bay up to Pezeries Point. The most obvious and attractive feature of it is the large pink crystals of the mineral Feldspar embedded within it which are sometimes over an inch long. Other minerals comprising this rock are quartz, Mica, and Hornblende. This rock was originally a molten lava very deep underground which slowly cooled, and as it did so, the crystals within it grew into solid rock. It was formed some 646 million years ago during a mountain building event known as the Cadomian orogeny. We shall be seeing this rock again at Fort Pezeries and along the cliffs all the way to L’Angle.
Return back up the slipway and turn right onto the road which goes to Pezeries Point. Notice that you are now on the platform of land overlooking Portelet. Walk past the former Trinity Cottages and note that on your left the ground rises again through the pine forests to the higher Pleinmont plateau. You are now walking on what would have been a beach 125,000 years ago, and the pine forests to your left would have been the cliffs. When the sea level subsequently retreated, the old beach was stranded above sea level, and it became covered in deposits eroded from the higher ground and blown by the wind. Proceed to the section of granite wall overlooking Portelet, which is location 2.
Location 2 -the granite wall overlooking Portelet
Look at the panoramic view in front of you.In the foreground is Portelet, and in the background is the imposing sea wall starting under the Imperial Hotel, and stretching past La Lague, the entire length of Rocquaine bay to L’Erée, interrupted only by the causeway to the rocky Island on which Fort Grey is built. You will notice that the coastal land bounded by the sea wall is relatively flat, and is now extensively built upon, first by fishermen’s cottages, and more recently with a host of other homes. Now imagine if you can, the sea level rising up by about 18 metres or 60 ft from where it is today. Where you are now standing would be submerged by several feet of sea. All of the fisherman’s cottages and other houses between The Imperial hotel and Fort Grey would be submerged. Only the upper part of the Imperial hotel would be sticking up above the water, and the new shoreline would be defined by the foot of the hills rising up behind the Imperial hotel to the higher Torteval plateau. This is how things would have looked about 125,000 years ago. This is not a great deal of time in Geological terms. This particular sea level rise was caused by the melting of the Continental ice caps in a warm period between two ice ages. When we look back into the geological past we see repeated rises and then retreats in sea level, which make our current worries about the moderate expected sea level rises due to the effects of global warming pale into some sort of Geological context.
This ancient beach, now an area of relatively flat coastal land (or Marais) can be found all along the West Coast of the Island.
Now look behind you, and take the opportunity of examining La Varde Rock. The rock stands some 100 ft above ordinance datum, and it forms a distinctive feature, with quite a flat top. La Varde is likely to have been shaped about 180,000 years ago by wave action when the sea level was even higher during a very warm interglacial period. This feature is what is known as a wave cut notch. Now continue the walk, uphill passing La Varde rock on your left, and just after you reach the highest point of the road, at the junction with the zigzag lane, you will see on your right hand side a narrow track leading down to a slipway. Follow that track to the bottom of the slipway. You are now in location 3
Location 3
Look back towards the slipway, see photograph. You will notice a vertical
slice of dark, fine grained rock (between the dotted lines on the photo)
surrounded by the pinker L’Erée granite which you saw
at Portelet. This rock has some deposits and lichen covering it, but
you should be able to see
it’s structure underneath. This type of structure is called a
dyke. There are many such dykes in Guernsey and we shall be seeing
some spectacular ones on the cliffs later in the walk. If you imagine
the L’Erée granite forming by the cooling of a molten
rock miles underground 646 million years ago, as the rock cooled, it
contracted and in so doing, it cracked. Several millions of years later,
in a different geological event, a new molten rock flowed into the
cracks in the L’Erée granite. This is how this dyke was
formed. The dark rock of which it is formed is called a dolerite.We suggest that you can now return up the slipway to the road, and turn right, and make your way towards Fort Pezeries, which is our next location.
Location 4
When you arrive at Fort Pezeries, walk down the slipway to the right
of the Fort. The first thing to note is the continuation of the L’Erée
granite on the rocks which border the left hand side of the slipway.
This type of rock should by now be easily recognisable to you because
of it’s large pinkish crystals of Feldspar.Now turn right and go across the pebbles to the low earth bank which borders the beach. Take a close look at the bank, and you should see an accumulation of rounded pebbles within the bank itself.
This is part of a raised beach. The pebbles were at one time part of a beach when the sea was at a higher level than it is today. We have already seen that the coastal platform of land overlooking Rocquaine bay is an ancient beach formed by a large rise in sea level 125,000 years ago. After that event there was another ice age, during which the sea retreated to a very low level compared to what it is now. There then followed yet another period of relative warmth approximately 100,000 years ago, which resulted in yet another sea level rise, and that formed the more recent raised beach which we are now looking at. The reason why the pebbles are covered in earth, is because the sea subsequently retreated again a very long way indeed, and this left the old beach pebbles to become covered with bits of rock and earth sliding down from the higher ground above. After this particular raised beach had been formed, the sea retreated so far that Guernsey became joined to France again, and the English Channel became just a river valley between England and Europe. This particular raised beach is known as the 8 metre raised beach and is seen at several points on the island. It is not particularly well seen here in Torteval, but If you are interested in studying it further, you will find a much better example of it in the earth banks at the top of the slipway leading to Lihou causeway.
Looking at the earth bank again, you will also notice that at higher levels there are other fragments of rock, but these are not rounded, having jagged edges. These are not beach pebbles, and are not part of a raised beach. These are pieces of rock which, through repeated freezing and thawing have weathered away from the slopes above, have worked their way down the slopes and have become buried over the millennia. This type of deposit is called "head" or "combe rock". It appears extensively in the earth banks bordering our beaches.
Make your way back to the slipway, and note that just before the slipway you will be walking over some strikingly pink, fine grained rock. This rock is called Felsite, and it was emplaced in a similar way to the dolerite dyke which we examined at location 3. However, it differs from dolerite significantly in composition, because it contains a very high proportion of pink Feldspar, giving it it’s attractive colour. Now proceed to Location 5, which is the slipway on the right hand side facing the easily recognised "gap in the rocks" with La Table Des Pions up the slope on your left. Walk down the slipway and examine the earth bank overlooking the small pebble beach.
Location 5
You are looking at a cross section through the earth bank on which
La Table Des Pions is situated. This is the remnant of the same 18
metre beach level which we discussed in location 2. Note that the 8
metre raised beach pebbles, which we saw at the last location, are
not visible here. They are probably present, but at a lower level,
now behind the sea wall. A considerable part of this bank consists
of rock fragments (head) which have slumped down from the higher ground
as explained for the previous location. The upper part of the bank
is finer grained and of a lighter colour. This is called "loess",
which is a fined grained wind-blown dusty-sand deposit resulting from
the weathering and eventual disintegration of the surface rocks during
the ice ages. Much of our soil in Torteval is loess based and it provides
a very friable, workable topsoil into which our farmers could easily
plough in manure and vraic to enhance the fertility, giving our parish
it’s famed green fields and valleys.Now proceed eastwards to location 6, which is the gap in the rocks which you can see a bit further on from this beach.
Location 6
When you reach the gap in the rock, you will see from the rough-hewn steps, and from the appearance of the rock face, that this gap is man made. It is likely to have been done in Napoleonic times to provide access to the next headland where there is a gun battery.
The rock which you are now looking at is one of the few rocks in Guernsey which has a sedimentary origin. The age of this one, known as the Pleinmont Metasediment, is somewhat uncertain, but likely to be between 1,000 and 700 million years old. Within Guernsey, this rock only occurs in Torteval. It was originally formed in deep water, and was originally a mud or silt. This then became buried, and it eventually changed under pressure and heat into the greyish, rather unattractive, uniform rock which it is today. The rough-hewn steps into this rock have been polished by countless pairs of feet and give an opportunity of examining polished surfaces of this rock. If you examine the face of the cutting at eye height, you will see how the rock fractures into multiple micro-fractures when quarried.
It will come as no surprise to learn that it has never been quarried for use as a building stone, the tendency to fracture in this way would be a stone-mason’s nightmare.
Before leaving this site, take a look at the rock near the bottom of the cutting. Although it is very similar looking to the surrounding rock, this is in fact a small dolerite dyke of the same composition, and probably the same age, as that seen at location 3. The dyke is therefore younger than the surrounding Pleinmont Metasediment.
Now go through the gap in the rocks and proceed to the next location which is nearly to the Napoleonic gun battery at the next headland.
Location 7
The beach which you are now overlooking consists of pebbles which are
mainly Pleinmont Metasediment. Take a look at a prominent pebble filled
gully. This is the site of another dyke. But this dyke is unlike the
ones we have already seen. Notice that the action of the sea has worn
away the rock within the dyke so that it now forms a gully, and has
filled it with pebbles. So the rock which formed the dyke was relatively
soft. Depending on wave action and storms, the level of pebbles within
the gully varies enormously. At times the pebbles are partly removed
and the original rock which formed the dyke can be seen.Take a close look. If you are lucky enough find it relatively free of pebbles, you may see that at the bottom of the gully is a rock that looks very like reddish brick. This rock is called a "lamprophyre" and lamprophyre dykes can be seen in several locations around Guernsey. Lamprophyres, are normally of dark grey colour with many crystals of Mica which sparkle in the sun. However, when they are exposed to the weather and sea, they often weather to a brick red colour. This is the youngest rock type to be found in Guernsey, having the relatively "young" age of 296 Million years. The molten magma which formed the lamprophyres was formed by the intense energy produced during the process of continental collision which formed the Alps when Africa collided with Europe. This event is known as the "Variscan Orogeny".
Guernsey’s lamprophyres also have the distinction of being our most radio-active rocks, although they are by no means dangerous, and it is perfectly safe to handle them.
When you have finished examining this location, proceed up the grassy slope behind you where you will find a track leading up to the cliff path which leads along the cliffs eastwards. When you reach the cliff path turn right up a relatively steep portion with steps, and right again at the stone sign which directs you to Creux Mahie . Follow the path to the car park near to the partly demolished square German fortification, and onwards to Location 8 which overlooks where you have just been.
Location 8
The stunning view from this location takes in many of the places you
have just visited. Take a look out towards La Grosse Rocque. You may
notice even from this distance that it is distinctly pinker than the
rocks further towards the shore. You may even be able to see some pink
streaks within the rock. La Grosse Rocque is composed of L’Erée
granite, and the L’Erée granite at this location is extensively
covered by pink Felsite veins. The contact between the L’Eree
granite and the Pleinmont Metasediment is shown by the dotted line
on the photograph and is marked by a long rock pool between La Grosse
Rocque and the beach. This well illustrates the fact that the contact
between these rocks is a line of weakness which in the past has seen
the rocks near the contact being crushed and sheered. The sea has exploited
this weakness and eroded the crushed rocks more than the surrounding
rocks, and this has resulted in this long rock pool. Looking out to
sea, towards the Hanois Lighthouse, there are many reefs to be seen.
Their detailed geology has not yet been properly recorded.The walk from now on will be on the "official" cliff path to Les Tielles. Guernsey’s steep cliffs provide many locations of Geological interest, but they are also very dangerous and inaccessible without the right equipment. We shall therefore confine ourselves to discovering Torteval’s Geology by what we can see from the immediate vicinity of the cliff path. For this, a pair of binoculars will be invaluable.
Location 9
The first
thing we should notice is that we are now on the highest point of Torteval.
During the walk that follows you will see that the
highest places in Torteval are relatively flat. Where you are standing
once represented the approximate level of the sea around 10 million
years ago during the later Tertiary period. The erosive action of the
sea caused a Plateau to be formed at about 200 ft. So 10 million years
ago, our cliff tops would have formed a largely submerged reef, probably
of coral.Proceed on the cliff path to location 9, which is Les Grandes Cotils, from which you will have a clear view of the rock known as La Pointe de Pleinmont.
This section of the cliffs is entirely composed of L’Erée granite. Notice how La Pointe de Pleinmont is joined to the mainland by a very narrow coupée of grass covered rock which is currently undergoing severe erosion. There are examples of this type of structure all along our cliffs, and these coupées represent the last stages before an outlying rock finally becomes detached and surrounded by the sea.
Our next stopping location will be La Congrelle, near to the German tower at L’Angle, so proceed along the cliff path until you pass the ruins of the Victor Hugo watch house, and then onwards past the newly re-instated gun to the car park at Battery Dollmann. At this point, the cliffs still consist of L’Erée granite, as does the prominent rock which you can see from here, known as the Tas de Pois D’Aval (Upper Peastacks). The cliff path then continues downwards with the motocross track on your left. Soon you will notice that you are passing over outcropping rocks on the path which are grey crystalline rocks, unlike the pinkish L’Erée granite. You have just passed over the transition from L’Erée to Perelle gneiss.
Perelle gneiss started it’s life as a member of the granite family (called a diorite). It is like a granite, but has rather more darker minerals such as mica and hornblende. However, some 1,000 million years ago it was subjected to very great temperature and pressure, and under these conditions it re-crystallised into it’s current form This is the well known local "Grey or Blue Granite".
Proceed until you get to the German tower at L’Angle. At this point you will leave the cliff path temporarily, and although it is quite safe, this part is not advised if you have children, because there are dangerous sheer drops nearby. Proceed carefully between the tower and the protective wire netting fence to your left, down the scree slope below the tower where you will eventually find a small quarried area. This is location 10.
Location 10
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The rock face of this fissure is mainly composed of grey Perelle Gneiss, but in the upper part of it a strip of reddish rock can be seen. This is a dyke of rock called Felsite, and consisting mainly of the Pink mineral Potassium Feldspar. The age of this felsite dyke is not accurately known, but it is probably somewhere between 300 and 400 million years old.
Now turn around and look westwards in the direction in which you have just come. Look at the cliff face below battery Dollmann. You will see a prominent line running down the cliff in a curve to a cave at the bottom. This line is shown dotted in the photo. This is a fault line, a discontinuity in the rock sequence which represents the join between L’Erée granite and Perelle Gneiss. These fault lines often reveal themselves because they are often softer, and more vulnerable to the erosive action of the weather and the sea. Notice that in this case, the sea has exploited the weakness in the rock and hewn out a cave. Return to the cliff path and continue walking towards the Mont Herault watch house which should by now be visible. You will pass the clay pigeon shooting range on your right. Just after the clay pigeon range, there is a field. Walk along side the field in the direction of the sea until you can see the view below. This is location 11
Location 11
The scene looks west over Baie de la Forge along the route we have
come. If there is a swell and from about 3 hours after low tide, there
is a good
possibility that you will see one of Guernsey’s best Souffleurs
or blow holes, forcing the sea spray into a fountain of foam and spray.
A Souffleur is the result of sea erosion, where a fissure in the rock
becomes a small cave, and eventually the sea erodes a hole in the roof
of the cave, forming a chimney, which funnels the spray upwards. Souffleurs
are geologically speaking very short-lived features, because the very
force of the sea which makes them work provides the erosive force which
eventually destroys them. Return to the cliff path. Walk past Mont
Herault watch house. After this point we have crossed over the parish
boundary into St. Pierre du Bois. Walk on past Belle Elizabeth, and
follow the cliff path up some steps until you have a view of the next
bay in which can be seen Le Long Cavaleux and Les Tielles in the distance.
This is location 12Location 12
A striking feature of the cliffs below both Le Long Cavaleux and Les
Tielles is that both headlands are crossed by prominent bands of very
dark rocks. These are impressive examples of dolerite dykes which were
originally molten magma which found it’s way into cracks in the
much older rock whereupon the magma solidified. These are perhaps the
best examples of this type of feature on our cliffs, but a careful
examination of other locations will reveal that our cliffs are full
of very dark dykes such as we see here, and sometimes the lighter coloured
or reddish dykes such as we saw at La Congrelle. These serve to illustrate
the extremely complex and tangled geological history of our cliffs.Another feature which stands out from these views is the colour of the rocks into which the dark dykes intrude. They are very distinctly reddish, quite unlike the Perelle Gneiss which we have recently left behind. This is the last rock type which we shall be examining on this walk. It is known as Icart Gneiss. Some 2,600 million years ago this rock was a red granite, but it then re-crystallised into it’s current form under great temperature and pressure. It is not only one of Guernsey’s oldest rocks, but one of the oldest in the British Isles or France.
Although we will be walking past Le Long Cavaleur and having a better view of it, it is worth noting here that this is another example of a narrow coupée being eroded away, which in this case has resulted in a hole through the headland along a fault line. This process will continue until eventually, the bridge over the hole will collapse.
Location 13
Continue the walk almost to Les Tielles car park. Keep a sharp eye
to your right and you will see a small quarried area between the path
and the cliff edge. Caution is urged here, since the drop at the edge
of the quarry is sheer. This is a good opportunity to examine the pinkish
Icart Gneiss. The pink colour is due to large red crystals of potassium
feldspar. You may be able to see that the rock has a "grain" to
it, and you can imagine that it would tend to split an a preferred
direction. This is typical of gneisses.Now proceed down the lane which leads back to the main Torteval Road, turn left and walk to the road leading to Torteval Church.
Location 14
Although it may seem curious to have our Parish Church as a location
of geological interest, in fact the examination of materials used in
buildings provides a lot of information about the geology of the area.
In the case of Torteval church, you will find of two types of Guernsey
stone. Most of the building consists of grey Perelle Gneiss (found
in Torteval), but at the base of the structure there is a pleasing
addition of some pink Cobo Granite.Location 15
Our final location is reached by proceeding behind Torteval Church, and down Rue Des Clercs to the coast road, stopping at Fort grey to take in a panoramic view of Torteval.
Summary of how Torteval came to be the way it is now.
The walk which you have just undertaken has taken you on a circular route around Torteval, and the features of the Geology which you have looked at have not been in any chronological order. We have looked at various types of rock of extremely different ages, and considered the evidence in our landscape for certain events, and you should by now be asking yourselves how everything came together to look the way it is today. Before attempting to answer that question, I should say that we certainly do not know the whole story, and in any case it is certain that the whole story of the formation of even such a small area as Torteval is exceedingly complex. But we have already seen some pointers which enable us to bring together all of our observations into a picture which makes some sort of historical sense. The story presented here is of necessity very simplified, and there are many other publications and articles for the more experienced geologist to study, see the references at the end.
Our story begins some 2,500 million years ago. An almost unimaginable time, when the earth was less than half the age it is today. What we do know, is that at a very great depth below where we are standing, there were huge tectonic events, caused by continental plates colliding, resulting in the generation of intense heat deep below the earth’s surface. This caused the partial melting of the earth’s crust beneath us, and molten lava was generated which eventually cooled to become a granite. This was the origin of the "Icart Gneiss", our oldest rock, and one of the oldest rocks in the whole of France and the British Isles. Even after the rock had solidified, there continued to be further tectonic events generating heat and extremely high pressures, and this caused the granite to re-crystallise, to become a metamorphic rock, called a Gneiss, causing the rock to become the Icart Gneiss which we recognise now.
Towards the end of this period, which has become known as the Penthevrian, another similar cycle of events resulted in a molten lava of somewhat different composition to crystallise, first into a rock of the granite family, and known as a granodiorite. This rock had darker minerals within it, and as with the Icart gneiss, it also suffered great pressure and heating deep underground, causing it to re-crystallise to form our familiar "blue Granite" or "Perelle Gneiss". These events are though to have happened around 1,500 to 1,000 million years ago.
The next significant event for us was the formation of the Pleinmont Metasediment. This took place somewhere between 1,000 and 700 million years ago. We can surmise that we were near to the shore of an ancient sea. Mud and silt accumulated on the sea bottom, and eventually, over millions of years, this buried mud solidified to be come the rock which became the Pleinmont Metasediment of today.
Our area was then subjected to the most significant of all our geological events, a process which not only formed most of our remaining rocks, but also thrust the rocks so far formed upwards from deep below the ground to roughly where they were today. This event was a massive collision of two tectonic plates in what was known as the Cadomian Orogeny. Orogeny means a mountain building event. We were roughly at the point of this collision, a process which produced intense heat and new magmas. These in turn solidified to form first, the L’Erée granite 646 million years ago, and several million years later, the other rocks of Guernsey, such as the Cobo granite, the Bordeaux Diorites, and the St. Peter Port Gabbro. The important part of this process for us is that it lifted the new magmas and older rocks up though the earth’s crust, such that when later erosion took place, the rocks that were to become Jersey, Guernsey, Alderney and the other islands were on higher ground than adjacent areas. So around 500 million years ago, the main Channel Islands would have been present as significant peaks with mountains to the south of Jersey where Brittany now is.
Mention was made in connection with the Lamprophyre dyke we saw at location 7 of the Variscan Orogeny. This was yet another great mountain building event associated with the collision between the European and the African continents, a process which formed the Alps. The Variscan Orogeny had a profound effect on the French and British landscapes, but in our area, the only significant event was the formation of the Lamprophyre dykes 296 million years ago.
For the next part of our story we need to fast forward in time to the Tertiary period which began 64 million years ago, when the Atlantic Ocean was being formed. During the Tertiary it is known that there were many periods of very high sea level, and it is likely that around 10 million years ago, the sea must have spent a considerable period of time at levels of 200 to 300 ft above current level. At that time, the main Channel Isles would have had significant peaks sticking up above the water, but would otherwise have had recognisably the same shapes they have today. However, bit by bit, these peaks were eroded away by the action of the sea until our islands were no more than reefs of rocks just above sea level in places, but otherwise largely flat. This is how the flat top to Guernsey was formed.
At that time the sea was relatively warm, and judging from the limestone which is extensively found around the Islands on the sea bed, it is likely that the Channel Islands at that time would have been Coral reefs.
At the end of the Tertiary era we move into the Pleistocene era. This was the era that finally shaped the island into what it is now. It was a time of repeated Ice ages and consequently of climatic extremes. In general, during a period of glaciation, the sea level dropped, because so much water trapped was in the form of Ice over the Northern Continents. In the warmer interglacial stages, the Ice tended to melt to a greater or lesser degree, with consequent rises in sea level.
We have seen evidence of two such events in our walk. The earlier one, happened during a warmer period about 125,000 years ago, and resulted in the sea leval rise to 60 ft (18 m) which as we have seen, formed the beach platform which later became the low lying Marais along the coast road. There then followed another period of Glaciation known as the Wolstonian glaciation, followed, about 100,000 years ago by another period of warmer weather, during which our 25 ft (8 m) raised pebble beaches were formed.
The last period of glaciation was the Devensian, about 45,000 to 15,000 years ago. During the early part of this period, the Head deposits which we saw at location 5 were formed, and the towards the end of the Devensian, the wind blown Loess deposits which form the basis of our arable land were formed. During the Devensian, the English channel became just a river bed between Britain and France, and we were joined to France.
Finally, from about 10,000 years to the present, the sea level gradually attained it’s current level as the Ice sheets of the Devensian melted.
If you have enjoyed this walk, I would appreciate your feedback or suggestions. You are welcome to contact me on georgelec@cwgsy.net
Candie Gardens, St Peter Port, Guernsey GY1 1UG, Channel Islands.
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