A weekend in Vosges/FR

My birthday is in August and last August I was lucky enough to be given a fine geological hammer and a definitely up-market hand lens – which came with an invitation from my husband to spend a weekend in the Vosges inaugurating these splendid gifts and exploring some of our regional geological delights.

I therefore busied myself on the Internet trying to work out a suitable route which may yield us the most interesting geological finds and, although not all the locations were fruitful we did have a wonderful weekend and came home with the car definitely heavier towards the back and the hammer and handlens well and truly broken in!

First, here is a rough summary of the geological history of the Vosges:

In Precambrian times (4600 – 570 Ma) the gneisses found today in certain areas of the Vosges were formed from relics of ancient mountain ranges. The oldest, in the Climont region of Val de Villé, have been dated at around 1 billion years old and the most recent can be seen near St Marie Aux Mines. These formed the backbone of the Caledonian Range 430 Ma ago.

Towards the end of the Primary era the Hercynian range occupied a large part of Europe, stretching from Britain to Bohemia. It was raised mainly during the Carboniferous era between 340 and 280 million years ago, as a result of continental collision between two slabs, one producing the Northern (sandstone) Vosges and the other the middle and Southern (granite) Vosges. The suture between these two slabs is visible today at the bottom of the Val de Villé, between Lalaye and Lubine. During that time various geological phenomena took place with volcanic eruptions, deformation and folding, metamorphism of ancient rocks and production of granite.

During the Permian, about 250 Ma ago a major climate change took place and the area became much more arid. The lakes dried up and the forest regressed to be replaced by vast wind-swept tracts of land between the peaks. The Hercynian range was eroded and the finest sediment swept by the wind, formed sand dunes, the orientation of which shows that the dominant winds at that time were easterly. These Permian aeolian dunes gave rise to Champenay clay which is still used for construction. Volcanic activity during this same period produced the red porphyry found at Nideck in the Bruche valley today.

During the Triassic, the Hercynian range continued to erode and the entire eastern part of France became a transit zone for erosion products from the Paris Basin, swept across the country by a network of rivers into the Germanic Sea which then covered a large part of Eastern Europe. Prolonged droughts were punctuated by shorter wet periods during which large quantities of sand and pebbles were carried along and then deposited as the water courses dried up again, leaving an alluvial plain which eventually built up into the vast banks of sandstone found today.

220 Ma ago the Vosges and Alsace formed a low plain bordering the sea with a vast delta channelling water – and eventually giving rise to the clay used later to build Strasbourg cathedral. By the Upper Triassic the Germanic sea had regressed into a series of lagoons into which were decanted clay particles enriched with calcium, which gave rise to iridescent marns in greens, reds and violets. The hot Triassic climate dried up these lagoons and potash deposits developed in the Mulhouse region – the last remaining signs of the lost Germanic Sea.

During the Jurassic period the sea invaded the Vosges and Alsace once more and this was when the bituminous schists formed. Shallow warm seas covered Europe for nearly 20 million years and about 125 million years ago the area occupied by the Vosges was transformed into a huge carbonate platform, with the formation of oolithic sands. Reefs in Southern Alsace towards the end of the Jurassic are most visible today in the limestone cliffs of the Alsatian Jura.

About 150 Ma ago the Vosges and Alsace emerged from the water once more. For more than 100 million years, until the middle of the Tertiary era it existed as emergent land under a tropical climate. This was the time of flowering plants and dinosaurs but the main event of which we have evidence here was the formation of the Alps.

Around 50 million years ago during the Eocene period, a North-South oriented depression formed in the middle of the emergent Vosges and the Black Forest, and this gradually broadened to a width of about thirty kilometres, extending from Basle to Frankfurt – separating the two ranges. The plain of Alsace had arrived. Along the fractures between the plain and the Vosges lava was erupted in association with the formation of the Alps further south. In the meantime the Vosges were raised further and erosion increased. In the region of the Ballons in the southern Vosges the cover was eroded and old Hercynian basement revealed. In the north, where the Vosges were lower, erosion stopped at the Triassic sandstone.

During the Quaternary, 2 million years ago, the climate deteriorated and ice covered Northern Europe as far as the Alps. The Lorraine slopes of the Vosges developed a sea of ice over about 40 km. The Alsatian side is much steeper so that smaller glaciers carved their way through the valleys and their effects are seen today in the High Vosges. Successive periods of cold and warmth left their marks and the latest period of climatic warming, begun 10 000 years ago, saw the last glaciers disappear from the Vosges, leaving evidence of their existence only in the relief we see today.

Vosges valley

We live at the northern end of the Vosges in the Northern Vosges Regional Nature Park and right next door to an interesting geological feature known as the oil lagoons of Pechelbronn. 40 million years ago, the sea which then occupied the current location of the Alps, invaded the Rhine depression and a climatic change, which led to 10 Ma of hot dry summers turned the plain of Alsace into a dead sea bordered with swamps. This was when Potassium and Gypsum were deposited in the Mulhouse region of S. Alsace – where potash is still exploited today. Lagoons of briny water developed with plankton proliferating on the surface. As these died and fell to the bottom of the lagoons, stagnant water preserved their remains under anoxic conditions. Chemical maturation enriched this organic material with hydrocarbons and oil formed as a result. This oil was exploited from the 18th century to 1963 – one of the world's first oilfields. Local springs in the area now provide hot water which is currently being exploited industrially by a geothermic company.

However – to get on with our trip. Our first port of call was Bouxwiller, west of Haguenau where I hoped to find the remnants of a vast lake which formed there during the first half of the Tertiary era. A 50 Ma old community of fossils in clay and calciferous rock was what we were looking for – but alas it is long gone and the only quarry we found was the usual Vosges sandstone, of which more anon.

We turned south and headed for the Dabo rock, near Saverne. John is a keen cyclist so knows our area like the back of his hand. He had recently climbed a col which led him to the Dabo rock, a well-known local landmark peak, and he thought there were interesting markings in it.

Dabo rock

Each of these deposits shown in the photo may have been made in minutes, hours or days, but there may have been centuries between each one. Oblique layers, several millimetres thick were built up, according to current fluctuations with coarse sandstone full of pebbles demarcating each layer as can be seen in the photo. The Dabo rock is now the culmination of a considerable climb, yet the signs of ancient currents are still clearly visible today.

Our next port of call was Nideck in the north of the Bruche Valley – an area of Aeolian Dunes, although these are difficult to see today because the whole area is well forested!

However, during the Permian, volcanic eruptions took place at the same time as the dunes were developing. The Château de Nideck (ruins) was built on red porphyry derived from the ancient lava. A fine waterfall shows columnar bedding of this rock with a total thickness of 200 to 300 m .

Pink porphyry at Nideck

This porphyry is very hard rock (as my hammer could tell you!), a reddish violet in colour and sprinkled with particles of white quartz and feldspar with angular fragments of darker volcanic rock. In fact, the red porphyry developed from the cooling of very viscous lava produced by the above-mentioned volcanoes and classified as a rhyolite. The Nideck volcanoes became extinct 250 Ma ago and the location of their craters is unknown. This formation of red porphyry is the only remaining evidence of those volcanoes. We were able to find excellent samples without attacking any of the formations themselves. The castle ruins are a tourist spot and well sign-posted – you need good walking shoes as it's a relatively fun walk down to the falls, not to mention the climb back up!

Continuing south through the Bruche valley which hosts all the high spots of Vosges geology, we visited the villages of Russ and Hersbach which face each other across the valley. Hersbach has a huge quarry which is exploited today by Wenger et Petit. My notes told me that these quarries exploited ancient reefs from the Devonian (375 Ma ago) when Alsace was in the tropics. Grey limestone with a reddish tinge, known as Russ marble, is quarried here. To the western end of the quarry is a different area revealing thin slabs of grey or black schists with concentrations of pyrite. Black schists are said to hold fossil sponges, holothurians, gastropods and lamellibranches as well as molluscs and radiolarians, deposited along the littoral of a shallow sea.

This was a disappointment to us – we found the schist certainly and trespassed on private property for a closer look – picking up a few samples that were lying around, but not a fossil in sight that we could find. Most of the area was well fenced off by the quarry company and on a Saturday afternoon no-one could be found who might let us in for a closer investigation, so we headed for Climont in Val de Villé, NE of Séléstat, to hunt for Precambrian rock instead!

This was difficult as sandstone covers everything in this area and we toured the region of Climont which is known to have outcrops of gneiss and schist which have been dated at 1 billion years, hoping for a sight of such an outcrop. All we could see was sandstone and yet more sandstone and finally we decided we'd done enough riding and it was time to walk again. We spotted a nice trail leading up a pretty steep little hill that seemed just right for stretching our legs so off we went – hammer in rucksack and handlens at the ready anyway, because even sandstone may have some interesting inclusions now and then. The trail zig-zagged up the hill, through the forest – the soil remained infuriatingly red, until we came to end of a zig about halfway up and began to zag back across the hillside when rock suddenly emerged through the red soil as definitely grey and schistic – we had stumbled (literally) onto an outcrop of Precambrian gneissic schist!! We took a few samples and that was the only small area where this rock was visible because the next zig was back in the sandstone again – and indeed we found ourselves in a war zone. The top of the hill still bore all the signs of devastation caused by the December 26th 1999 hurricane which we remembered only too well. By August 2001 the broken and fallen trees were pretty much dead up there and not yet cleared away – we had to give up our attempt to reach the summit because the fallen trees were just too numerous to clamber over – we got some photos though!

Storm damage

After a pleasant stay in St Dié (the Ibis hotel usually serves us well) we set off south again in the morning to explore the Southern (Granite) Vosges. We drove through Ste Marie aux Mines – a town squatting in the bottom of a high-sided valley that must be very dark in winter! Here there is 600 Ma old gneiss with lens formations of marble formed from the recrystallization of calcium compounds (thought to be from ancient coral reefs) during metamorphism. Apparently ancient eroded mountain ranges were deposited as sand and mud in geosynclinal seas, subducted and subjected to 9000 – 10 000 times atmospheric pressure at 750°C to form this gneiss. Vertical movement brought it to the surface and erosion produced the current outcrops. These were impossible to visit – we spotted one or two outcrops towering over our heads at the top of the very steep valley, but nothing would have been reachable without a major expedition. We did finally obtain a sample on the way back home from a slightly different area nearby. 340 Ma – 280 Ma years ago the Hercynian range, which then stretched from Britain to Bohemia, was lifted and folded and melted magma rose and crystallized to produce the various Vosges granites. I confess to a particular love of granite and I was not to be disappointed that Sunday in August as we headed along the peaks route towards the Ballons des Vosges. We stopped at various pull-ins and spots with panoramic views where granite boulders were used with gay abandon to mark parking perimeters or even provide picnic tables and benches. There are some enormous quartz and feldspar crystals in some of the granite found along this route and all are fascinating. The granite is quite pink with feldspar in some cases, pale grey with quartz in others and all like rich fruit cake! I collected so much granite of various shapes and hues, the car was beginning to groan by the end of the day.

We took in the Petit Ballon and Grand Ballon – eroded remnants of volcanic granite hills but these are heavily tourist-infested so we decided to head for home. In a way we are lucky to live at one end of the Vosges because the whole area is geologically fascinating and you can drive the whole length in about three hours. We spent a very happy two days there and intend to repeat the experience. Our September holidays took in the Cévennes and the Pyrenées Orientales – but that's another story!

PS If you read French there's much information to be found on a website called La Passion des Minéraux et Fossiles: La Région Alsace. The link is: http://jasper.kioskdu.com/r_g2.html

Gill Ewing