"readers' words"

Leaving Auvergne, on the road back to Belgium passing between Issoire and Besse, the visit of a troglodyte fortress dug in the heart of the volcanic tuff allows a beautiful discovery: the caves of Jonas.

The Jonas fissural volcano, with its aligned cinder cones, and the nearby volcano, Saint-Pierre peak, have mixed pyroclasts with the lava to form a red tuff soft enough to be dug down to a depth of 5 meters.

The cliff is 500 meters long and 100 meters high; it is the result of volcanic activity that took place between -18 and -3Ma.

The cliff was first dug in the 4th century BC by the Celts, who established a sanctuary there. The Romans took over the place, then Christian monks made an oratory there.

The caves served as a refuge during the Barbarian invasions, between 850 and 915.

At the beginning of the 12th century, the knight Anet Dalmas de Jaunac, owner of the land, had a stronghold built there, capable of resisting invaders and controlling the valley.

From generation to generation, the site will prosper to pass in 1683 to the Montal-Nozières family.

The caves were classified as Historic Monuments in 1886.

The frescoes of Jonas date from the 9th to the 11th century. They represent episodes from the end of Christ's life.

Only a few pigments were used: red ocher, yellow ochre, black and white (lime).

Sources:
- Sacred places – Canalblog – Saint-Pierre-Colamine – Jonas caves
- Auvergne Center France – the troglodyte site of Jonas
- Wikipedia - Caves of Jonah

Puy Mary consists of the remains of a cumulo-dome dated 7.2 Ma (M.Krafft), largely eroded by glaciers.

It consists of benmoreite, a viscous lava rich in phenocrysts of plagioclase, sanidine, and brown hornbland, embedded in a paste of microcrystals of sanidine, biotite and tridymite. Veins of basalt or phonolite lard it.

From the summit, at 1,783 m., the entire summit part of the Cantal massif can be discovered.

On the way back, Salers, one of the most beautiful villages in France, can be discovered along the cobbled streets.

This city, fortified during the Hundred Years War, preserves ancestral houses in dark stones, with turrets and slates, architectural characteristics of the region, which are completed by a remnant of a rampart and its belfry.

The statue of Tyssandier d'Escous, renovator of the Salers cattle breed, with a dark mahogany robe and light lyre-shaped horns, enhances part of the main square.

Sources:
- Guide to the volcanoes of Europe and the Canaries / Cantal - M. Krafft and Larouzière, ed. Delachaux & Niestle
- Salers - an architectural and historical heritage.

In the heart of Cantal, the Cère valley has been deepened into umbilicus, separated by locks of hard breaches. These locks have been cut by the rivers in gorges, sometimes in waterfalls.

The Faillitoux waterfall, in the Thiézac region (Aurillac), crosses a flow of ankaramite (*) prism vertically. The prisms, 0.5 m in section at the base, rise to the full height of the cliff.

This flow was dated at - 9.5 +/- 0.5 Ma (K/Ar).

The waterfall is fed by the Lasmolineries stream which rises on the southern slope of the Élancèze and flows into the Cère.

(*) Ankaramite is a volcanic rock close to basalts and basanites. It is a dark rock (melanocratic), with a porphyritic microlithic structure. The phenocrysts include black automorphic olivines and clinopyroxenes (= augite) which can reach 2 cm in size. The matrix contains microlites of the same nature to which are added plagioclases associated with traces of titanomagnetite and biotite.

It belongs to a flow interbedded in trachyandesitic breccias and topped by trachyandesites and supracantal basalts.

Le Chaos de Casteltinet, on the left bank of the Cère:

The landslides of large volumes of breccias that occurred during the disappearance of a glacier 12,000 years ago, as a result of decompression phenomena on the edges of the glacial valley and water infiltration from the liberated plateaus first, forming listric faults (**) which destabilized the cliff.

The upper part of the cliff which overlooks this chaos is made up of prismatic columns corresponding to an old trachyandesitic flow which overcomes mud and debris flows (piling of lahars with carbonized wood) interstratified with lava flows, themselves overcoming sandstone formations.

The breach, thus detached, takes on the phantasmagorical shapes of needles, of pyramids. The "lion's door" is an arch made of these debris and which evokes this animal.

(**) : A listric fault is a normal fault of great spatial extension which is concave at depth, involving a deep shear level.

A quick look at the passage to Puy Griou.

It is a phonolite dome located in the center of the remains of the Cantal stratovolcano. It would have been in place 6 Ma ago, after the paroxysmal eruptive phases of Cantal (between 8.5 and 6.5 Ma).

The dome was remodeled under the action of freezing and thawing which cut it into slates piled up in a scree sleeve.

Murat is a very old city, cited in the texts of the evangelization of Auvergne from 270.

Human traces have been discovered on the rock of Chastel-sur-Murat, dating from the Middle Neolithic.

In the Celtic language, Murat could mean "steep rock ".

In the Middle Ages, the fortified town nestled under the rock of Bonnevie, the summit of which was occupied by a castle, demolished by Richelieu in 1633.

Sheltered by its walls, this city was very active: fairs and markets, crafts, liberal professions, inns, etc... During its rich historical past, Murat was fortified on many occasions: there are no less of three successive enclosures.

In 1944, the city was one of the highest places of the Cantal Resistance. Today, this small industrial center is a highlight of Cantal tourism.

To see the old houses of the XVIIth century, with arcades in handle of basket on column, nicely covered with lauzes and typical of the constructions of the city.

The city is framed by three basaltic peaks:
- the rock of Bonnevie, in the North, carries a virgin. The basalt organs are the finest in Europe,
- the rock of Bredons, to the south-east, whose 11th and 12th century church was built with very beautiful freestone,
- the rock of Chastel, in the North-West.

These rocks, born from the lava, have been transformed into necks thanks to erosion.

The 3 necks, composed of basanite, line up on the same eruptive fissure.

These three bare necks correspond to fillings of diatremes, as shown by the breccia sheaths surrounding their base. They are therefore the result of explosive phreatomagmatic volcanism which pulverized the basement and led to the formation of maars, which were then filled with magma. (Geological output – G.Godard / in source)

The Neck of Bredons carries the priory church of Saint-Pierre and Saint-Paul de Bredons (built in the 11th century then fortified in the 14th-15th centuries).

It has prismations arranged in sheaves clearly visible from the side of the road, oriented horizontally just above it, then almost vertically below the summit.

The necks of Bredons, Bonnevie and Chastel-sur-Murat are chronologically linked to the eruptions of the supracantalian planezes.

The Bredons nepheline basanite, highly deficient in silica, is - 4.3 +/- 0.1 Ma old (K/Ar dating). It is one of the most recent in Cantal.

Source :

Geological outing in Cantal 24 - 25 and 26 May 2015, under the direction of Gaston Godard, Lecturer at Paris-Diderot University

In the heart of the Massif Central, the volcanic massif of Cantal is the largest of the French stratovolcanoes, and one of the most important in Europe: with an area of ​​nearly 2,500 km², it has the shape of a low cone , 60 km. N-S for 70 km. E-O, at its base.

Its current maximum altitude at Plomb du Cantal culminates at 1,855 m. Projections allocate to the Cantal volcano an altitude of more than 3,000 meters before its major collapses.

It still has many spaces above 1,000 m. altitude, which gives it a strong mountain character.

The history of Cantal extends from the Miocene to the Pliocene, between -11 Ma and -3 Ma.

Over 8 million years, the eruptive activity has alternated effusive and explosive phases; eruptive materials – basaltic flows, explosion and collapse breccias, Strombolian projections and domes – piled up to form the Cantal massif, which was reshaped by glacial erosion in the Quaternary.

The town of Saint-Flour is built on a basaltic flow set up 8.8 Ma ago, prior to the construction of the Cantal stratovolcano itself (infracantal basalts) and therefore to the flows that covered the planèze of Saint-Flour ( supracantal basalts).

The Saint-Flour basaltic flow consists of an olivine basalt, rich in feldspars.

A single basaltic flow in 2 large parts, differentiated by cooling according to 2 geothermal gradients. which perpendicular to the surfaces of the flow and directed towards the heart of the flow…

The basaltic organs show a colonnade of fairly regular large prisms, surmounted by a very thick entablature of small confused prisms.

(Geological excursion in Cantal, by Gaston Godard.)

Coltines is located on the Planèze de Saint Flour, a basaltic plateau with a slight slope and approximately triangular in shape, bounded by the valleys of Lagnon, Alagnon and Epi(e).

The Planèze de Saint-Flour is made up of a supra-cantalian basalt rich in olivine.

Sources:
- Guide to the volcanoes of Europe and the Canaries / Cantal - M. Krafft and Larouzière, ed. Delachaux & Niestle
- Debris avalanche deposits in Cantal (France): witnesses
of the construction of the largest European stratovolcano of Miocene age - by Nehlig Pierre, Dardon Arnaud, Fréour Gwenael, Huguet David, Leyrit Hervé. In: Geomorphology: relief, process, environment. April-June, vol. 7, no.2. p.p. 107119.
- The volcanism of Cantal / BRGM
- Geological outing in Cantal 24 - 25 and 26 May 2015, under the direction of Gaston GODARD, Lecturer at the University of Paris-Diderot
- Geological excursion in Cantal, by Gaston Godard.

Near Issoire, the village of Usson is built on an old basalt peak, which culminates at 639 meters.

This surprising geological formation is linked to a phreatomagmatic eruption, resulting from the contact of lava with water. The region was then a vast lacustrine zone, in which the magma made its way.

This volcanism began 25 Ma ago, and is older than the volcanic events that gave rise to the volcanoes of Cantal, Mont-Dore or those of the chaîne des Puys .

The organs, located south of the village, are the remains of the deep part, the chimney of an old volcano which has now disappeared under the effect of erosion. Usson's vertical organs are cooled from above, not from the sides of the chimney. The regularity of the prisms is also associated with a rather slow cooling.

The village, classified among the most beautiful villages in France, was built thanks to the hexagonal prisms, and the arkose, visible in the walls of the church of St Maurice.

The Castle of Usson, demolished on the order of Cardinal de Richelieu, had previously been, from 1585 to 1605, the place of house arrest of Marguerite de Valois (1553-1615), nicknamed by Alexandre Dumas Queen Margot, first wife (1572-1599) of King Henry IV of France.

The Château du Sailhant, in the commune of Andelat, was built in the 13th century on a triangular basalt spur, about 20 to 25 meters high.

It is also known for the presence of the Sailhant (or Babory) waterfall nearby.

 The Babory stream flows from a height of about twenty meters at this point into a chasm whose depth is estimated between 6 and 7 meters.

Sources:
- Futura - The Exploratorium: the organs of Usson in the land of volcanoes.
- Sailhant castle - wikipedia

Before definitively stopping writing my blog, for various personal and technical reasons, I am sharing for a few days the photos of my Belgian volcanophile friends, all recent, from their visit to the Cantalien Massif (France), via La Limagne .

Some sites of La Limagne, to whet your appetite!

The cliffs of Jussat and the stromatolites:

Limagne is the region of France richest in fossil stromatolites, of ages corresponding to the Upper Oligocene / Lower Miocene.

The Jussat deposit presents a cliff made up alternately of sand with quartz levels, and others of marl and limestone, with cauliflower balls of stromatolites, created by colonies of cyanobacteria, overhanging a less resistant layer. to erosion.

The necks of Buron and Saint-Babel:

They were formed around 20 million years ago, when a rise of magma through sedimentary rocks dating from the Tertiary period formed a lava lake. This lake then cooled and solidified, then was revealed by the erosion of the sedimentary rocks that surround it, very loose and friable.

The neck of Saint-Babel overlooks an area rich in peperites, which result from the encounter between magma and water (underground or surface), during a phreatomagmatic eruption.

Sources:

- Guide to the volcanoes of Europe and the Canary Islands / La Limagne - M. Krafft and Larouzière, ed. Delachaux & Niestle.

- Planet Terre: Auvergne, a museum of stromatolites / The cliffs of Jussat (Chanonat, Puy de Dôme).