Field trip to Ploumanac'h / Côte Granit Rose

June 21st to 25th 2013

On Friday evening the fieldtrip leader Dr Emilie Thomassot (Nancy) gave a brief introduction to the geological setting of Ploumanac’h, a Hercynian / Variscan (H/V) granite pluton. Continental academics tend to use the name Hercynian, derived from the Harz Massif, whereas British geologists tend to use Variscan because it is the Variscan orogeny that affects British rocks.

Plate movements in the Palaeozoic caused a collision between Laurentia and Baltica, resulting in the Caledonian orogeny; later this combined continent collided with Gondwana in the Variscan orogeny. As the Gondwana plate consisted of a large number of microplates (including Armorica and Avalonia), the Variscan orogenic belt is complicated and Armorica occupies a central position, and so is important for its interpretation.

Ploumanac’h is on the northern coast of Brittany, between Trébeurden and Perros Guirec in the Trégor region. The pluton has a surface area of about 12 x 8 km with excellent exposure at the coast and the numerous islands and islets and some parts of the pluton also have extensive quarries. It is also in the northern part of Avalonia, where Variscan deformation is minimal or absent.

In NW France there are a number of H/V granites in a lineation shown in Figure 1, from Aber Ildut (Finistère) to Barfleur (Cotentin) intruding into some of the oldest rocks in France (Icartian gneiss around 2 Ga) and also into the Cadomian (570-560 Ma) granite of Perros Guerec.

Now, advances in techniques including oxygen isotopes have enabled new hypotheses on the formation of these granites to be tested.

Trend line of Hercynian/Variscan granites in NW France
Fig. 1: Trend line of Hercynian/Variscan granites in NW France

In NW France there are a number of H/V granites in a lineation shown in Figure 1, from Aber Ildut (Finistère) to Barfleur (Cotentin) intruding into some of the oldest rocks in France (Icartian gneiss around 2 Ga) and also into the Cadomian (570-560 Ma) granite of Perros Guerec.

Now, advances in techniques including oxygen isotopes have enabled new hypotheses on the formation of these granites to be tested.

The plutons may have a common source and have been formed by similar geodynamic processes. There is also a Bougier gravity anomaly beneath the Brittany plutons.

Most previous work has focused on the Aber Ildut granite (including a thesis by former OUGS Chairman Joe Jennings) but there had been little study of Ploumanac’h since the 1970s (Michael Barrière’s thesis). Tragically Barrière died a couple of years later.

The granite complex of Ploumanac'h
Fig. 2: The granite complex of Ploumanac'h (Côte d'Armor). The solid black line is the 'coastline'; the pale colours in the key represent the geology between high & low tide level.

The Ploumanac’h pluton shows concentric outcrops around the Ile Grande granite, shown in Figure 2, as a result of successive intrusions of magma. There are three types of granite in the area:

1 Red ‘Gros Grain’ – (type la Clarté or Traouiéros) which has some mafic enclaves.

2 Light pink, medium grain intermediate granite, representing a different crystallization process but both 1 & 2 are from magmatic fractionation.

3 Grey granites – (type Ile Grande) which have the same composition but with some having more aluminium, resulting in the presence of muscovite. This represents crustal contamination of the magma as little Al is found in mantle melts.

The age of emplacement is late Variscan (303 ± 15 Ma, Vidal 1980) but the context of emplacement is still a matter of debate.

The trip would concentrate on establishing the field relationships between the various outcrops, to find out which granites intruded earliest and which ones were later.

Crossing to Île Milliau
Fig. 3: Crossing to Île Milliau, in coarse red granite

 

Day 1 Saturday June 22nd

Locality 1 Ile Milliau (SW section of Figure 2): Contact of granite & country rock

At low tide we traversed coarse-grained red granite to the island (Figure 3), seeing aplite dykes and (reportedly) amethyst in the sand.

 

Contact of granite with metasediment
Fig. 4: Contact of granite (above) with metasediment (banded) and later aplite vein cutting through both, at Île Milliau

Most of the island is composed of this granite but on the southern, seaward side, of the island we found a contact with very dark grey, vertically-oriented greywacke/arkose metasediment shown in Figure 4, with cordierite (metamorphic mineral). The precise age of these country rocks is not known but they are believed to be light & dark layers of explosive volcaniclastic sediment (silica-rich), all reworked.

The contact between the granite and metasediments was a roughly horizontal planar surface shown in Figure 4. It was clear that the granite was injected into the metasediments with some veins petering out. There were also enclaves of the country rock, with sharply defined edges indicating hot magma intruding cold metasediment, and cordierite and sillimanite, formed by contact metamorphism in conditions of high temperature and low pressure. Some of the enclaves were quite spectacular (Figures 5 and 6).

enclave of metasediment and mafice enclave with feldspars in granite
Fig. 5: Two types of enclave: metasediment with sharp straight sides; mafice enclave with feldspars showing reaction rims, at Île Milliau
enclave of metasediment in granite
Fig. 6: Enclave of metasediment in granite, at Île Milliau
Gneiss of Trébeurden
Fig. 7: Gneiss of Trébeurden, with augen, at Porz Pabu

Using a P/T diagram for the metamorphic minerals, the pressure & depth of emplacement of the granite can be calculated (approx. 2.5 kb, or 8 km depth). The zone of contact metamorphism follows the edge of the pluton.

We also saw black tourmaline (boron-rich mineral) in the aplite veins that cross-cut all the rock types and are therefore later, indicating the remobilization of elements such as boron.

Locality 2: Porz Pabu (coast west of Trébeurden) Country rock (contact with granite not seen)

A Pleistocene loess cliff is easily eroded, so the back of the bay Porz Pabu has large granite blocks to protect the cliff. We found the paragneiss known as the gneiss of Trébeurden (Icartian gneiss) that contained feldspar augen (Figure 7) and also garnets, but not a contact with the granite.

The gneiss has been dated using an ion probe or laser ablation technique on zoned zircons, where one ion is replaced by uranium that decays to lead and gives the true age of crystallisation. The outside rim gives a date of 600 Ma (the last event, deformation during the Cadomian 570-560 Ma) and the interior 2 billion years old.

Enclave of red coarse-grained granite within the fine-grained granite
Fig. 8: Enclave of red coarse-grained granite within the fine-grained granite, at Toënot

Locality 3: Toënno (middle LHS of Fig. 2 Toënot) Three types of granite

On the southern side of the peninsula, the coarsegrained Type 1 pink granite (the ‘outer’ ring on the map Figure 2) is in contact with the grey Type 3 (Ile Grande) granite. The fine-grained granite perhaps looks less interesting but it is a stronger construction material and so has been extensively quarried. There are enclaves of the pink granite within the finegrained granite (Figure 8) with sharp edges and no reaction aureole, showing that the fine-grained granite came last but was of a similar composition. Pink granite (with aplite veins) contained pink feldspars with preferred orientation, which here was E-W, i. e. Concentric with the pluton. There were also enclaves of mafic material (gabbro) that were elongated in the direction of flow of the feldspar phenocrysts.

rear side of the Menhir of St Uzec
Fig. 9: Menhir of St Uzec, rear side

Locality 4: Mehir of St Uzec (location on Figure 2)

The menhir stands 7.5m high & is 3m x 2m at base, so weighs in excess of 100 tonnes (Figure 9). It is of fine-grained granite with enclaves of pink granite and was sourced from a quarry near its present site.

It was reputedly erected in 5,000 BC (we all had difficulty imagining how it was erected, given its mass) and was ‘Christianised’ (recarved) in the 16th Century. There were discussions in the field as to the imagery shown on the menhir and also whether the grooves on the reverse were from natural erosion or human carving, the question being that, having seen outcrops that represented ca 500,000 years of erosion, could the deep grooves on the mehnir have resulted from only 5,000 years of natural erosion since it was quarried?

Chapel of St Uzec
Fig. 10: Chapel of St Uzec

Locality 5: Chapel of St Uzec

This was a small votive chapel (no cemetery etc.) that has been beautifully restored since 1957 (and the bells in 2012), shown in Figure 10.

We ended the day enjoying a coffee/tea/cider at a waterfront café in St Guerec, Ploumanac’h, overlooking the Oratoire and Château de Costaeres surrounded by large rounded outcrops of the beautiful pink granite. A shrine first built here in the 12th century called L'Oratoire de Saint-Guirec (the Oratory of Saint Guirec) stands in the bay with a chapel on the facing beach. Female pilgrims have come for centuries to call upon the prayerful intercession of the monk saint for their seafaring husbands' safety. Young women also come to ask for Guirec's prayers to soon find a husband.

Personal comment.

I learned a great deal during the trip – although I had seen these rocks many times before, it was really good to hear up-to-date ideas and techniques applied. However, it seems that there is still great deal to be learned about the implication of the dates and sequence of events of the Ploumanac’h complex and also the plate tectonic implications of the localities we visited.

Dee Edwards

OUGS SW branch

 

<h>Day 2, Sunday June 23rd

Wet weather gear was donned as we ventured out to grapple with a spray laden on-shore gale to walk along the coastal path near our lodgings.

Baltic brown granite
Fig. 1: Baltic brown granite

The first stop was at the neighbouring cemetery to examine a monument stone fashioned from rapakivi type granite known commonly as “Baltic Brown” (Figure 1).

Emily (our leader) explained that the large oval to circular crystals were of alkali feldspars, rimmed with plagioclase within a fine grained black matrix often containing pyroxene. The circular shape was due to preferential dissolution of the corners of the feldspar to minimise surface energy of the crystal in response to pressure variations. This type of granite is found within the Ploumanac’h granites but the size of the orbicular crystals varies from place to place.

granite types on Île Grande
Fig. 2: Île Grande granite

Retracing the steps taken on Friday evening we again examined the darker coloured, coarse grained biotite granite (shaded violet on Figure 2) and the lighter, finer grained muscovite granite (shaded black).

Muscovite (KAl2(AlSi3O10)(OH,F)) and cordierite ((Mg,Fe)2Al4Si5O18) were observed within one boulder suggesting the magma had been contaminated with aluminium obtained from the assimilation of sedimentary rock through which the magma had travelled or was emplaced.

Undulating and diffuse margins
Fig. 3: Undulating and diffuse margins

Our mission was to find the contact between these two granites, with the overarching objective to deduce the order in which the magmas had been injected into the pluton. Firstly, we explored the rocks on the south-western area of the promontory as highlighted in Figure 2. The low tide revealed an exposure where an undulating boundary between a lighter and darker coloured granite could be seen (Figure 3).

In places the lighter granite had bled into the darker granite but elsewhere the dark appeared within the light. Mixing therefore would have occurred before crystallisation when the magmas were both of putty type consistency. This area was not considered to be the contact we were looking for. A short distance to the southeast, triangular shaped lighter granites ranging in size from 5 cm to 75 cm were seen clearly within the body of the darker granite.

Biotite vortex
Fig. 4: Biotite vortex

Crossing over to the north-eastern side of the promontory a loose boulder was seen which did display the characteristics of the contact between the two granites. The low tide on the north-eastern side exposed several examples of a biotite vortex within the granite. These were thought to represent eddies created by circular motion within the magma chamber. Figure 4 appears to show feldspar and quartz crystals oriented as if flowing from left to right round the vortex.

This suggests the vortex was created before the granite completely solidified.

Bowen’s Reaction Series shows that biotite crystallises out of the mix at a higher temperature than that needed for orthoclase (KAlSi3O8), muscovite (KAl2(AlSi3O10)(OH,F)2) and quartz (SiO2) (Geology, 2007; Cook and Kirk, 1995).

The next stop was to examine the Allée couverte de l'Île-Grande, a Neolithic (2300 BC) burial site. Some of the group walked along the coastal path and admired a modern granite memorial dedicated to the stone masons who for generations had worked the granite quarries along the coastline and the offshore islands.

Archaeological work on this covered walkway had found no skeletons but carved ornamental artefacts had been recovered, although were subsequently lost. Originally the granite structure would have been covered in soil. It is thought that this type of burial structure was aligned (in this case to the east) towards the direction of the rising sun on the day that the buried dignitary died.

La Greve Blanche was our next destination and luckily the weather was improving. We were now standing upon the coarse grained pink granite of the outer ring of the pluton. We climbed a short way up a prominent granite stack and finding shelter from the wind we ate our more than ample packed lunch bathed in sunshine.

Figure 5 depicts the glorious view of the beach giving a panoramic vista of the geological features seen. To put into context, the three areas visited during the afternoon have been marked upon the geological map of the area (Figure 6).

La Grêve Blanche
Fig. 5: Panoramic view of La Grêve Blanche granitic rocks
Geological map of the northern section of the Ploumanc'h granites
Fig. 6: Geological map of the northern section of the Ploumanc'h granites
Gabbro intruded into red granite
Fig. 7: Gabbro intruded into red granite

A rather interesting cliff face (Figure 7) awaited us on the beach below the granite stack. Various observations can be made of this exposure. The gabbro intrusion, at ~1200 °C, would have been the main source of heat and can clearly be seen intruding through the coarsegrained pink granite and an aplite dyke.

An exposed band of dark rounded mafic enclaves appear to be oriented parallel to the right but not obvious to the left of the aplite dyke. Aplite is a fine-grained, quartz-feldspar rock representing the crystallisation of the last remnants of the magma chamber.

Erosive wave action has created a veil of gabbro upon a pink granite pavement. Here, cross sections of the rounded gabbro enclaves were exposed within which pink and white feldspar phenocrysts which had been entrained into the gabbro from the pink granite could be clearly identified. Time did not allow for discussion of these features to be further elaborated upon.

Walking westwards along the beach, the profile of a jagged outcrop could be seen obviously different from the smooth weathering of pink granite. These rocks were thought to be diorite. Gabbro enclaves with feldspar phenocrysts and small aplite dykes could be intermittently seen in rocks exposed by the low tide.

Hornfels are the general name for country rock which has been influenced by contact metamorphism. Andalusite (Al2SiO5) porphyroblasts within such rocks are characteristic of higher temperature and low pressure metamorphism of clays (Rocks & Minerals, 1991). Such minerals were seen in parts of the massive enclave of country rock which outcropped at the end of the promenade. Scrambling over a jumble of large boulders on the beach beneath the outcrop, banding was observed which could be foliation. However, after closer inspection, Dave Williams thought these to be bedding planes. Finding a particular boulder confirmed this conclusion as preserved sediment bedding could clearly be seen with strong evidence of possible bioturbation.

On reading Rennes University Ploumanac’h geological summary subsequently forwarded on to us by Elisabeth, a photo almost exactly the same as Figure 8c was depicted with a description which concurred with the above conclusion.

Schlieren within Tourney Beach rocks
Fig. 8: Schlieren within Tourney Beach rocks

Our next port of call was Tourney beach. A beautiful bay with a desirable des-res castle perched on a central island. Whilst exploring the western side of the beach ‘schlieren’ could be seen within the pink granite boulders (Figure 8).

The swirls of biotite (KAl2(AlSi3O10)(OH,F)2) and amphibole are thought to represent mixing within magma chamber. Much discussion took place as to the mechanics of this but as mentioned previously fractional crystallization within the magma chamber may be significant.

Madame Odile Guervin, our local geologist, explained that some of the enormous boulders strewn around the bay had once been partly buried in the soil. Pedogenic processes had acted upon that part of the rock which was buried causing the rock to appear bleached and undercut when uncovered.

At Ploumanac’h we joined the tourists at the eastern end of “Sentier des Douaniers” to glimpse the famous strange rock sculptures that the elements had carved. A short detour was made to visit the Pars Kamor Lighthouse, one of the many around Brittany.

gneiss and red granite
Fig. 9

Unintentionally, I had taken a photograph with a dark loose rock in the foreground which looks to be gneiss against a back drop of pink granite (Figure 9).

The tour culminated with a most welcome drink partaken in the sun at a bar by the sea in St. Guirec, serenaded by a cacophony of sound as the wind battled with the patio awnings.

Josephine Brown

 

REFERENCES

Geology (2007), Definitions and images to illustrate geological terms, links to images and website articles

[online], http://bio-geo-terms.blogspot.de/2007/11/bowens-reaction-series.html accessed 3rd July 2013.

Cook, D. and Kirk W. (1995) Pocket guide Rocks & Minerals, London, Larousse plc

Day 3, Monday June 24th

Tréguier is a rich historical city and capital of the province of Tregor. Nested in the central square, at the core of the medieval town, Cathedral Saint Tugdual was a splendid start to the day. Ile Grande granite, schists and various other stones were the building blocks for the cathedral itself and the monuments, bas relief and tombstones in the aisles and chapels. This elegant structure dedicated to St Tugdual and St Yves is romanesque to gothic in style, with more recent additions.

Tréguier cathedral
Fig. 1: Tréguier cathedral

It is modestly challenged by a statue of Ernest Renan placed nearby. 19th century philosopher, philologist and historian, Ernest Renan was a native of Tréguier, whose upbringing in a very catholic family logically led him to pursue his studies at the seminary.

A brilliant mind, as his skepticism grew, his writings expressed very controversial critical views of religious dogma. He was at the time variously accused of blasphemy or revered for his outstanding intellectual qualities. The latter view is now predominant.

The cathedral area is an important touristic landmark. All sorts of gifts and delicacies are available, among these the Kuinaman, a local cake and must sampling for anyone with a demanding day ahead. With its special recipe of about 300% butter, sugar and flour evenly distributed, there will be no calorie shortage for the day.

pillow lavas with jasper
Fig. 2: Paimpol pillow lavas with jasper (see hand lens in the center for scale)

We wound our way along Tréguier’s narrow streets bordered by houses whose façades display very finely ornate beam work; then headed east towards the Baie of St Brieux and town of Paimpol. From there we headed due east again, along the Baie de Poulafret and the chemin de Guilben to enter the peninsula and reach the pillow lava formation. The peninsula, named Pointe de Guilben, is remarkable for the formation known as “Spillite de Paimpol”, Cadomian pillow lavas, formed 600 Ma ago, well before the Hercynian orogeny, as a result of extensive marine volcanism which occurred at fairly shallow depth. The basalt signature points to island-arc type volcanism.

The pillow basalts are injected as pipes through the country rock, itself of lighter colour and heavily fractured. The basalt pipes are younging on the coast side.

The basalts are vesicular, have often turned a green colour by alteration to serpentine over several tens of thousands of years, and filled with zeolites at the core of the pipes. The pillows are cemented by numerous large opaque dark red, more or less nodular bodies of iron-rich polycrystalline siliceous material as jasper, interstitial to the pillows. A dyke of lighter colour, more silica-rich lava, can also be seen intruding through the pillows onto which it forced its flow direction.

These basalts make up a band 1500 m wide, over 20 km length, much of which continues buried under the more recent formations of the pointe de Guilben. A sedimentary formation of aerial origin is visible as conglomerates on both sides of the pillow basalt formation.

Sillon de Talbert
Fig. 3: Sillon de Talbert

From Paimpol we retraced our steps to drive west/north-west towards the Sillon de Talbert, commune of Pleubian. Le Sillon tips the Lezardrieux Peninsula, projecting north/north east between the outlets of two rivers: Le Trieux and Le Jaudy, flowing near parallel to each other and the Sillon direction. Locally named “ria”, and further west in Finistère “aber”, these river outlets are dug deep, fjord-like, and subjected to the amplitude of tides for quite a considerable distance inland. Such currents also contribute to form the Sillon.

The remarkable ongoing geological process of the Sillon is a 3 kilometre-long stretch of pebbles into the sea, and is the northernmost point of Brittany. Its maximum width is 35 m. As it takes the impact of waves and erosion, the Sillon protects the coastline to the east. Meanwhile, its pebbles accumulation tips the outreaching finger with a gently sloped concave shape to the West forming La Grande Grève, and a slightly sharper and narrower slope convex to the East, La Petite Grève. Better sheltered from wind from the sea, this is the side where we opted to sit for our lunch picnic (see image 3).

As in a dune process, materials (here pebbles) impacted from the west can migrate eastward towards the coastline, progressively moving the line of the Sillon. Monitoring of this move shows an average yearly displacement of one meter, although this is variable according to the occurrences of battering storms and varying tidal amplitudes. Adding to the general instability of this environment, pebbles will move along different scenarios:“floated pebbles” can travel over 30 km away as they are lifted along by the algae to which they are attached, old beaches are continually being “recycled” by the erosion and rolling of their individual pebbles, while new pebbles originate from broken Tors. Pebbles can migrate up to 4 or 5 meters in one powerful tempest.

In this natural reserve of an area of 18 ha, the ecology is both fragile and sturdy. Though unstable, this substrate hosts a rich biodiversity which includes a protected vegetation of “cabbages”. Their roots can reach 4 or 5 meters and woody seeds are carried by rough seas to disseminate further.

For long the locals had a tradition of gathering seaweed there into rafts named “dromes”, several meters wide, in order to burn them to obtain soda salts, locally used in the manufacturing of glass, while iodine was extracted for an antiseptic, and seaweed generally used as fertilizer. Extraction of soda went on well until the 18th century.

Informative signs along the path on the Sillon itself and nearby beach make sure the visitor is both careful and appreciative of this fragile environment. The beach has receded by an average of 1m/year over the last 40 years, moving as much as the Sillon itself. Migrating birds find a haven there and the nearby coastal cliff faces are home to sand martins and lone bees. Above the cliffs the soil is pebbly loess: Small pebbles were emplaced during glaciations, and the soil consists of loess, windblown and of land origin.

Evidence of ancient activities of salt production, glass and pottery making have been uncovered by

archaeologists in the loess layer: a salt factory, a pottery making workshop, the firing, at low temperature, achieved by burning reeds and seaweeds.

One indicator of a salt production site consists in pebbles split by fire, as pebbles used to be heated in fire and placed in the brine to speed up evaporation and salt crystallisation.

St Gonery church with a leaning tower
Fig. 4: St Gonery church

Leaving the Sillon on our drive to Plougrescant, we crossed the vast Trieux estuary and headed north after the bay, reaching the chapel of St Gonery. This chapel is originally a Romanesque chapel, further transformed in the 15th and 16th century, which added a gothic component. Its curious feature is a tower added in 1612, built of lead covered wood, which tilts very visibly. This chapel dedicated to 6th century St Gonery, is of much historical importance. Unfortunately, due to both our tide-time constraint and ongoing restoration at the site, we could see only some of its external features, missing out on remarkable early wood paintings inside.

Continuing north from St Gonery, we reached Pors Hir and the Pointe du Chateau. Walking along the grassy path, we passed the iconic home of Castel Meur, dwarfed by the high granite Tors between which it was built in 1861. Its back to the sea, it is protected by the tors from the frequently violent winds and marine storms.

Plougrescant: Castel Meur
Fig. 5: Plougrescant: Castel Meur
Le Gouffre near Plougrescant
Fig. 6: Le Gouffre

The path leads to Le Gouffre (see image 6), a narrow gully where waves push through over the dark dolerite between two fractured cliffs of pink granite , while the scenery out at sea is made up of numerous tors, boulders released from them, small islands and, among them, seaweeds and abundant fauna.

The gouffre was formed by a massive post Cadomian and pre Hercynian – between 600 and 350 Madolerite intrusion into cadomian granite. Apart from the gully of Le Gouffre, around the site, the dykes follow 3 main directions, expressing a large scale volcanic magma source. Veins can be followed over 20 or 30 meters, and crosscut one another (see image 7).

Dolerite dykes near Plougrescant
Fig. 7: Dolerite dykes

Some dykes have enclaves of the granite as angular fragments, ripped by the dolerite rise. Conversely, the Cadomian granite has enclaves of burst diorite showing that granite has intruded the diorite.

Where diorite enclaves are angular, diorite was already crystallised when intruded. It is also possible that intrusions are angular if the melting point of the enclave is higher than that of the molten material, while some enclaves show nibbled rims as they are more similar in composition to the host and in near chemical equilibrium. Another variation, in grain size, of the granite, could also result from its composition, and not necessarily from cooling speed.

Mafic enclaves near Plougrescant
Fig. 8: Mafic enclaves

These variations is interpreted as resulting from successive different pulses from a vast complex magma chamber producing different magmas as a result of fractional crystallisation (see images 8 and 9).

These variations is interpreted as resulting from successive different pulses from a vast complex magma chamber producing different magmas as a result of fractional crystallisation (see images 8 and 9).

In the Gouffre itself, as the granite better resisted the battering of waves, erosion of the broad dolerite dyke created this dark hole/gully continually further eroded, being the lowest and easiest channel for wave and tide attack.

Mafic enclaves near Plougrescant
Fig. 9: Mafic enclaves with nibbled rim

At Nearby tors, as at many tors in Plougrescant, evidences of human occupation of Neolithic or Gaul (iron age = 2500 BP) times have been found.

On the top of tors near Le Gouffre, Some small spindles and flints were found by archaeologists. It is understood that the tors were used as natural watch towers by human groups protecting their community from dangers.

Walking away from Le Gouffre, along the path of the nature reserve, we paused with due respect to let a lazy adder take its time to cross the path, before passing another site of archaeological interest for its early soda extraction from seaweed and an inland formation of a migrated pebble “dune.”

On the return drive to Ploumanach we passed the 1727 CRAC windmill, built with La Clarté granite, a well restored witness of early wind technology, while a tide activated mill dated late 14th century can still be seen close to the town. This tide powered mill was in use until 1932. Odile, our expert local guide, was very helpful in pointing out such historical elements during our study trip, and this was particularly interesting in these sites of ancient and continuous settlements, such as Neolithic stone structures revisited over the following millennia of human interaction with the environment.

Text: Brigitte Revol Macdonald

Images: Dee Edwards

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