Fig. 1. Location map and seven one-day itineraries in the CCvennes, Causses and Garrigues.

A field trip to southern France M o r e than 40 million people take their holidays in France each year between June and September, mostly flocking to the beaches fringing the Mediter- ranean. A short drive inland, almost anywhere, brings the visitor to unspoilt country, a landscape rich in fascinating geological sites, many of which also feature as three-star tourist sites in the green Michelin guides. This is an ideal place for the geologist on holiday with a family.

A University of London extra-mural class recently visited the Cevennes, Causses and Garrigues, west of Avignon (Fig. l ) , on a ten-day trip, following on from an evening class entitled ‘All abroad for a geological holiday’, which had been held at the Geological Museum. The party, in five private cars belonging to members of the group, covered the long journey across France by Motorail, with cars and passengers travelling overnight between Boulogne and Avignon. The group took advantage of a package deal offered by Hoverspeed, which included the Channel crossing, Motorail travel for the cars and passengers, and holiday insurance with medical and vehicle breakdown cover.

The region falls into easily distinguishable areas: the mountains of the Cevennes, the high table-lands of the Causses, the densely wooded Garrigues, and the fertile lowlands and valleys. These are due to vari- ations in geology, which reflect in land use and natural vegetation as well as in the available building materials and the architectural styles these inspire (Fig. 2).

The geological history of the region is bound closely to the story of the Tethys Ocean, which separated Eurasia and Africa through Mesozoic and early Tertiary times. The Cevennes rocks originated in an earlier (Palaeozoic) ocean and were folded, rneta- morphosed and intruded by granites in the Variscan earth movements which ushered in the Mesozoic era. Within the Tethys Ocean, Triassic sandstones and conglomerates were laid down, followed by great thicknesses of Jurassic limestones and dolomites. During the Cretaceous, the sea was less extensive, and limestones with reefs built by strange-shaped bi- valves, the rudists, were formed. Tertiary and more

recent sediments were deposited in areas of con- tinuing subsidence in the closing stages of the Tethys Ocean.

The Causses and Garrigues display wonderful examples of the solution and deposition landscape features so typical of limestone regions. This is in sharp contrast to the landscape of the impermeable basement of schist, gneiss and granite which makes up the Cevennes.

The group first visited sites near the head of the RhBne delta. Near St Remy de Provence are the excavations of the Roman city of Glanum, built of Miocene limestone. Since Roman times St Remy has been a quarrying town, and nearby are many spec- tacular quarries, some of which are open to the public. Here it is possible to visit quarries where bauxite has been worked, and to visit Les Baux, the town after which bauxite was named.

West of the RhBne, the group came to the Pont du Card (Fig. 3). The River Card itself is pleasant for swimming and canoeing, and the oak forest provides shade for a picnic on a hot day. Steps and a footpath lead up to the aqueduct at the top of the bridge, and visitors can walk through the water channel itself or along the top of the structure, on the limestone slabs which roof over the water channel. The Romans used Miocene limestone as a construction material. It is more yellow in colour than the St Rimy stone, shelly and false-bedded. The water channel is partly blocked by calcareous tufa deposited from the transported water. The tufa grew to such a thickness that it reduced the flow of water to less than one-third of the original capacity of the aqueduct, which in its heyday had carried over 100 000 cubic metres of water a day to the town of Nimes.

The extra-mural class stayed in hotels near Barjac, and. spent the following three days exploring the Cretaceous limestone scenery of the Garrigues, visit- ing the gorges of the Ardeche and Ceze rivers, panning for gold in the Ceze, and seeing dry valleys, caves, a river actively depositing calcarous tufa, and the solution-weathered ‘stone forest’ of the Bois de Paiolive (Fig. 4).

The reef limestone of the Garrigues is massive,

4 . 138lGEOLOGY TODAY J j u l y - A ~ g ~ ~ t 1989

mj 4 p-=l I w 2 i-7 3

I Granites, gneisses and schists of the Chvennes

2 Jurassic limestones and dolomites o f the Causses

3 Cretaceous limestones and shales o f the Garrigues

4 Tertiary sediments

white and fine-grained. In the warm sea of Tethys, bivalves developed into forms in which the two shells are quite unlike each other. The lower shell, which was attached to the sea bed, was cup-shaped, while the other acted as a lid. Collecting these rudists is easiest in the more massive beds in new road cuttings, where road widening has provided easy parking.

A highlight of the days in Barjac was the visit to the Aven d’Orgnac, one of the finest caves in the world, discovered by Robert de Joly in 1935 (Fig. 5). The cave is a vertical sequence of rooms magnificently garnished with huge stalagmites and stalactites. Near the entrance, the group spotted rudists in the walls. The cave guide was unconvinced that these were fossils, assuring us that these were nothing more than scratches on the wall, made by visitors waiting for the guided tours!

The days in Barjac included a drive into the moun- tains of the Cevennes, to look at the granite, gneiss and schist of the upland where the local rivers rise. Here is the source of hard quartz pebbles, the tools with which the rivers mechanically erode the lime-

A Fig. 2. Simplified geological map of the Cevennes, Causses and Carrigues.

Fig. 4. The ‘stone forest’ of b the Bois de Paiolive. Solution runnels (karren) have formed on the exposed surface of Jurassic limestone. (Photo: Jill Clarke.)

I Fig. 3. The Pont du Card. This 50-m-high aqueduct spans the gorge of the River Card as i t meanders through the Cretaceous limestone of the Garrigues. Initially constructed to.convey only water, the Pont has been modified by the addition of a road bridge.

stones downstream. The river waters draining the Cevennes are acid, so they are especially active a t dissolving limestone. The renowned limestone gorges, with incised meanders of the Rivers Card, Ceze and Ardeche to the east, and the Tarn, Jonte and Dourbie to the west, owe their origin to a combination of these two facrors, coupled with continuing uplift of the limestone terrain (Figs 6 and 7).

In the Cevennes in Pleistocene times there were only small glaciers, but periglacial conditions were pervasive on the high ground. Granite that had been deeply weathered during Tertiary times was stripped bare on the summits as loose material slid valleywards by mass-flow. Continuing rapid erosion is exhuming spectacular pinnacles of granite corestones, which stand like sentries on the precipitous slopes above the valleys. I t was interesting to compare these weather- ing and erosion features with their counterparts in south-west England.

Crossing the Cevennes by the Corniche des Cevennes route, the party moved on to stop two nights in Florac, situated on the River Tarn. It is worthwhile in Florac to spend some time in the Chateau, the headquarters of the National Park of the Cevennes. Here are displays on the natural history of the park, and a bookshop which sells popular guide- books in three languages (including English) as well as specialist and scholarly publications in French on all aspects of the region.

The group took a boat trip downstream from La Malene, passing through the narrowest part of the gorge of the Tarn, where faulting has brought massive dolomites of the Middle Jurassic down to the present- day river level and where, further east, they make up the surface of the Causse plateau, 600 metres higher.

Back in the cars, the convoy climbed by hairpin bends to the top of the Causse Mejean and saw the

GEOLOGY TODAY J ~ l y - A u g ~ ~ t 19891 139

4 Fig. 5. Aven d’Orgnac. The group stands in front of some of the larger stalagmites, said to date back to the Miocene. (Photo: Lab-Cine Photo, Vallon Pont d’Arc.)

arid, stony frost-shattered surface, punctuated with lusher areas that carry cereal crops. The arable patches are solution hollows that retain moisture as well as soil, so they are the only easily cultivated parts of the plateau. Some hollows are no longer blocked with soil, but are open shafts (avens) leading into cave systems in the solution-dissolved limestones and dolomites below. The dolomitic horizons of the Middle Jurassic have a distinctive form of erosion

Fig. 7. The origin of meander caves. A combination of chemical solution and mechanical erosion on the outside of the bends in the river course leads to enhancement of the meandering course.

b

4 Fig. 6. Meander caves in Cretaceous limestone, Les Concluses, Gard. Known locally as ‘baumes’, meander caves are a conspicuous feature of the cliffs above the river valleys. As the limestone mass rises, and the riverbed is carved deeper, meander caves are left suspended above today’s river level. (Photo: Jill Clarke.)

considered to resemble a ruined town (Fig. 8). Major joint planes, widened by solution, make alleyways leading between the prominent, often roughly rectangular, areas, seen as ‘houses’.

Within the Causes, the group visited the Abime de Bramabiau. This horizontal cave system is the under- ground passageway of the River Bonheur and was first explored by E. A. Martel towards the end of the last century. The visitor enters the cave from downstream where the river returns to the daylight, and the guided visit follows the subterranean course upstream for a couple of hundred metres. In competition with the deafening roar of the river, the French commentary is delivered through a megaphone. Keen visitors can also take the Grand Randonne footpath half a kilo- metre to the point where the River Bonheur descends into the cave system.

The group stayed the last night at a hotel high in the Cevennes. The proprietress had arranged for an after-dinner film show about the National Park of the Cevennes, and a representative from the Park answered our many questions, ranging from where specific orchids could be found, to attitudes of the locals to outsiders buying holiday cottages, and to planning for quarrying.

One of the unexpected charms of this region was the warm welcome and magnificent provision we received almost everywhere we went, coupled with the feeling of friendship that followed us as we left. It almost hurts to let you know where these places are,

140IGEOLOGY TODAY July-Augu~r 1989

in case it gets as crowded and spoilt as many other lovely parts of Europe have become. Geology Today readers are special though; they will know not to tread on the lark’s nest as they walk across the Causse, not to hammer the karren at Paiolive, and not to throw stones and Coke cans into the avens . . . won’t they?

A repeat of this excursion, arranged by the Univer- sity of London Centre for Extra-Mural Studies, took place in June this year. There may be another in 1990; so anyone interested in participating should contact the Centre on 01 636 3854.

Easily accessible further reading in English hardly exists. The green Michelin guide for Provence has an English version and contains some geology. The ‘Touristic Guide Book’ of the National Park of the Cevennes, by M. Sabatier (1987), in English trans- lation, can be bought from the Chateau de Florac for 54 French Francs. In French, three of the Masson Guides Geologiques Regionaux are needed to cover the area: Provence; Causses, CCvennes, Aubrac; and Languedoc Mediterranee. The BRGM 1 : SO 000 geological maps are published for most of the area, SUSANNA van ROSE and each contains an explanatory handbook, Geological Museum, London

describing at least the rock types for each sheet. Handbooks to the more modern sheets contain a great deal more information, particularly on the extractive industries.

Fig. 8. Ruiniform landscape at Nirnes-le-Vieux on the Causse Mejean. (Photo: Jill Clarke .)

Museum File 17: Oxford University Museum Address: Parks Road, Oxford OX1 3PW. Tele- phone: (0865) 272950.

Administration: University of Oxford, Committee for the Scientific Collections.

Admission: Free.

Times of opening: Monday-Saturday 12.00 noon to 5.00 p m . Closed Sundays, for a number of days over Christmas, and three days before Easter. Access for research visitors, schools and other parties by arrange- ment.

Historical notes: The collections of the Oxford Uni- versity Museum are based on the natural history specimens belonging to J. Tradescant (1587-1638) and his son J. Tradescant (1608-1662). The latter passed his private collection to Elias Ashmole (1617- 1692), who presented it, together with his own ethnographic and other collections, to the university in 1677. They were initially housed in the original Ashmolean Museum (now the Museum of History of Science) which opened in 1683 and was devoted to the teaching of natural sciences. The first Keeper was Robert Plot (1640-1696), who wrote a Natural His- tory of Oxfordshire in 1677, with illustrations of fos- sils and minerals. His successor Edward Lhwyd (1660-1709) published a catalogue of the mineral and fossil collections in 1699, a few of which survived the curatorial neglect of the eighteenth century. The pre- sent University Museum building was opened in 1860 and is a three-storey neo-gothic building that origi- nally housed all the university science departments as well as the Raddiffe Science Library. It now

houses the University Entomological, Geological, Mineralogical and Zoological collections, which together rank second only to the National Collections.

The geological collections: These comprise more than 500 000 specimens, of which over 700 are types, at least 4000 figured, and a further large number cited. The principal collections are those of William Buckland (178&1856), John Phillips (1800-1874, Yorkshire Carboniferous, Oxfordshire Jurassic), Sir Charles Lye11 (1791-1875, Tertiary molluscs of Oxford.

~ i ~ . 1. The exterior of the University Museum,

GEOLOGY T O D A Y J U L ~ J - A U ~ U S ~ 19891 141