Iron pan

The iron pan is a typical sandstone Landes of Gascogne, which was formed in the sedimentary deposits brought by the wind. It is a deposit and a concretion resulting from the cementing of the grains of sand of the skeleton by oxides iron, Aluminum and Manganèse, as well as organic matter. It is formed when the physicochemical conditions are met, namely percolation of rainwater and seasonal increase of the ground water, supporting the descent of the organic compounds and the iron contribution in the horizon B.

The garluche , or hones Moors, is a Iron ore, advanced state of the Alios, which were useful a long time for construction, and as a raw material with the industry of iron in the Landes of Gascogne.

The garluches do not have any more manages use nowadays and remain a problem for certain farmers who must explode this layer of iron pan in order to be able to use their grounds suitably. The presence of iron pan was also a problem when there was some discussion about digging a whole network of Craste S (ditches of drainage of the Moor) starting from second half of XIXième century and also after the fires of the years 1950 which devastated half of the forest landaise.

Presentation

Vegetable remains (Maritime pine, Heather S…) release a Organic acid which attacks the Argile and releases the Silice, the Alumine and the Fer. The elements thus created settle on the grains of sand. When iron becomes more important it plays a part of Catalyseur and causes a Ciment, a binder in the forming sand of the hard and compact zones of iron pan. The iron pan made up of Sand, of Aluminum, Silica and Fer is impermeable. It can be deep or surface according to the quality of the ground. During work of the ground, the iron pan remains a problem: indeed, there is with difficulty fissurable and remains an obstacle to which are added the peat and sometimes thus clay.

Very present in the Moors (around the Basin of Arcachon for example), one finds plates of iron pan on the edge of the beaches, those one détérées by the force of the waves.

Etymology

“Garluche” comes from the Gascon garluisha , derived from the root prélatine kar/gar . Its literal direction is “the bad stone”.

The garluche is also known under the Gascon names of will pèira nhòga or will pèira of lana .

Uses

This stone is used since the time Gallo-Roman for the construction of the dwellings and the monuments (churches of Biscarrosse or Cazaux for example). The pyramids delimiting the Sauveté of Mimizan, set up at the beginning of the 11th century and always visible, were built in garluche.

At the 18th century, an iron and steel activity based on the use of the garluche developed, of the forging mills using the garluche were already established with Pontenx-the-Forging mills and Uza-the-Forging mills. The 19th century, the forests located along the valleys made it possible to produce the charcoal necessary to the operation of the blast furnaces. Those were installed on brooks which provided the hydraulic power necessary to actuate the power hammers which were used to crush the garluche and to work incandescent metal. There existed thus, in the valley of the Leyre, six forging mills which annually produced each one a hundred of tons of cast iron and as much of wrought iron. One counted five others of them on the Ciron, one on Estrigon and eight on the coastal brooks. They started to decline towards 1850 because of overexploitation of the forests and the exhaustion of the layers of garluche. The construction of the railroad their was fatal by allowing the diffusion of more competitive products coming from the large iron and steel basins. The iron and steel industry landaise will perdura until the beginning of the 20th century, before they are supplanted by the blast furnaces with coal.

The requirements in garluche were such, for construction, the paving of the roads and the food of the forging mills (in spite of a poor iron concentration) that all the important layers are now exhausted. Nowadays, the garluche ceased being employed like construction material. Always very required, it is used mainly at decorative ends.

Formation of the iron pans

The Landes of Gascogne are a flat country, badly drained and the grounds made up of sedimentary materials are very poor. The origin of sands landais is wind, and the limits of the sedimentary plain of the Moors are very frankly marked.

One can thus estimate that the spreading of sand on the Moors of Gascogne took place mainly with the higher Pléistocène, between 126.000 and 11.430 years BP and particularly in the 18.000 year old neighborhoods BP, which is relatively recent.

After the spreading of sand, it is a phase of modelling of the ground which followed, with the formation of dunes in the plain landaise. Later, the vegetation will fix this landscape, and there will be formation of a ferruginous sandstone in the surface layers of the sandy cover: the iron pan and garluches.

Spreading of sands landais

Modification of the hydrographic network

Pleistocene is first of all marked by an major event for the geomorphology of the Moors of Gascogne: the displacement of the Garonne towards North and Adour towards the South. A first phase of tectonic movements, which occur at the end of Pliocène and lower Pleistocene, causes to gather the flows resulting from the Massif Central and to start their inflection towards North. One second phase, which occurs at average Pleistocene, is marked by the appearance of the fault of the Garonne. It then appears a unevenness of 40 with 50m according to an axis Langon - Bordeaux and the swing of the Western compartment leads the river to the foot of the escarpment. About the same period, the various flows in the South of the Aquitanian Basin make them also the object of successive captures. The main axis of drainage is gradually off-set towards the current bed of Adour. These derivations of the rivers identify already the area of the Moors of Gascogne since this one is then drained only by small hydrographic networks which are clean for him.

Installation of the sandy cover

The origin of the sand of the Moors is wind. To the binocular magnifying glass, the sands taken in various areas, Large Moor, Marsan, Landes of Bordeaux have a great uniformity. Grains round, of 0.3mm with 0.6mm of diameter with very few very small grains, but on the other hand some more coarse grains (1 to 2 mm). The quartz which forms most of this sand is almost always white or pink clearly and the grains dulled on the surface, are rather similar to those which at the present time fall down to the East of the Dune of Pyla.

Nowadays, the blow sand taken on the shore will be fixed on the dunes of the coastal area. An good example is the Dune of Pilat with Teste of Buch.

It is a climate very different from that from today which reigned on the moors with the Quaternary superior. One needed a type of time much drier and unfavourable for any shape of vegetation of under wood or forests. It especially was necessary for winds of West very violent ones able to transport to far sand from the shore. A climate able to cause, at the time of the storms, the increase on the shore of great quantities of sorted sand, that the waves and the swell took on the detrital tablecloths of Pliocènes and the glacio-river alluvia which recovered the continental shelf. Two phenomena: mobilization and sorting of sand on the dimension and transport of quartz grains on the plate landais went hand in hand. They suppose a power of the wind actions whose we do not have any more the equivalent in Europe today.

The fact that the winds of West were violent ones and regular with the Quaternary superior is proven by the presence of enormous masses of sand on the plates Calcaire S and of Molasse of the Eastern Moors. In this area all sand is “exotic” since the tertiary Roche S which form the base of the Moors practically do not contain any. If account is held owing to the fact that the shore was then more distant towards the West, the mean level being lower with the Quaternary superior, it is at least a distance of 150 km which the sand of the Moors has of traversing of West in Is before accumulating on plates which are currently in this part of the Moors, to 150-180 m with the top of the sea level.

The accumulation of sands was by no means blocked by the valleys of the plain, whose flow was too low to disturb the progression of it. It is probable that at a certain time, all the old valleys disappeared under the uniform sandy cover. The vegetation as for it was too thin and too poor to retain sands, which could then slip without obstacle on cold grounds.

During their migration towards the East, the blow sands reached at a given time, the limits of the catchment area of the Garonne and Adour. Beyond the action of the wind and that of a possible streaming were added and sand went down quickly towards the rivers, which involved them towards the sea. Sands joined the continental platform then, the loop is buckled. Sands thus carried out closed cycles.

The formation of the iron pan and the garluche is posterior, aliotic concretions not having been able to constitute itself that after the completion of the installation of sands landais.

Formation of the continental dunes

To the Mesolithic lower, the climate will change to become very wet. All the old valleys were filled by sands. At this point in time will dig new valleys in the plain, precipitations being abundant, sands in the vicinity immediate of the bed of the rivers will be involved towards the continental shelf. A new phase comes then from wind action to the higher Mesolithic era. Precipitations decrease and sands more and more badly evacuated will accumulate in the low valleys. The wind will take again these sands to accumulate them in continental dunes which are built at the same time on the valleys and the plates. Indeed littoral sands are then not very abundant, contrary to the former phase of new sand contribution on the plain landaise, and the dunes are formed starting from the sand of the plain, brought back close to the littoral by the rivers. The continental dunes which multiply, in their turn attacked by the wind and are largely broken. These wind actions ended with the Neolithic era, and the wind becomes unable to push sands inside the grounds. The vegetation will be able to invade the dunes, and a black sand ground fossilizes then modelled to them, fixing the landscape of the current Moors of Gascogne

The Diagenesis of sand in iron pan and garluche

The Moors of Gascogne are a ground punt, where the grounds are badly drained and the average piezometric level is between 20 and 60cm in the plain.

The natural pedological evolution of the ground landais is done in the direction of a podzolisation. The most typical forms, which are often called podzols ferruginous humo, consist of three types of different horizons:

a humus-bearing horizon A1, dark gray with black, rich in organic matter coming from a0 litter, as well as roots of the plants.
a horizon washed A2, almost white ashed gray, where all the soluble elements have summers trained downwards.
One or two horizons of accumulation B where the acid humic and the biogenic salts join to modify the nature of the ground.

The humus provided gross by the vegetable sheets and remains, making the ground very acid (pH=5.5), was during millenia one of the factors determining of the podzolisation. It gave its black color to the sand of horizon A. One observes in the horizon has a general migration of the bases which disappear, while the argillaceous and ferric colloids go, with the rough humus, to fix itself in the horizon B. the scrubbing of the ground, which is at the origin of the podzolisation, is due partly to the percolation of rainwater, but it is especially abundant when the horizon has is bathed seasonally by the ground water. This one solubilizes the organomineral complex and involves it then when it goes down again in summer. The rough humus and the ferric compounds thus concentrated in the horizon B, give a compound known under the name of iron pan.

The vegetable remains release an organic acid which attacks clay and releases silica, alumina and iron. The elements thus created settle on the grains of sand. When iron becomes more important it plays a part of catalyst and causes cement, a binder in the forming sand of the hard and compact zones of iron pan. The formation of iron pan is facilitated by the side flow of the tablecloth because this one transports minerals which can go up by capillarity in the higher levels. Iron in particular when it arrives on the roof of the tablecloth, finds conditions of oxygenation which enable him to precipitate in ferric form.

The iron pan is formed near surface (one seldom finds some in on this side 1.20m), it acts of a kind of sandstone tender, a thickness varying of 20cm with 1.20m, made up of aggregates of sand and made up humo ferric more or less consolidated. It is usually very friable and breaks under the simple action of a tool or even under the pressure of the fingers.

The iron pans are usually made up of quartzose sand 96%. Ferric cement humo thus accounts for on average 4%. Their characteristic color is given by iron oxides.

One distinguishes three kinds of iron pan:

the friable iron pans of very brown color passing to the black, which are humic iron pans, simple organic matter and sand aggregates where the percentage of iron oxide is very weak: from 0.1 to 0.8%

more compact iron pans, of less dark color with brownish trails and brown yellow, which are ferruginous iron pans where the iron oxide is more abundant and better consolidated sand. Thicker and more solid mass, they are as cavernous and more irregular as the humic iron pan. They contain from 1 to 4.6% of iron.

the garluches, which are genuine sandstones with ferruginous and siliceous cement. They can be very rich in limonite and then constitute a genuine iron ore. They were exploited for their strong content of iron (12 to 16%) in forging mills at the XIXe century, as with Pontenx-the-forging mills in Country of Born. There exist also garluches of clear color (yellows) which contain only 3 to iron 4%: cement is very fine and very resistant, at the same time siliceous and ferruginous, it welds between them the grains of sand and gives a hard stone which was very largely used for construction in the Moors of Gascogne.

The layer of iron pan is not continuous in the ground, it is presented in the form of plates. Contrary to the generally accepted ideas in the Moors, the iron pan is not impermeable, but constitutes a spongy mass who can slow down the descent of water. The aggregate aliotic, very porous, and favorable to the rise of water by capillarity. If the humus using its composition could not that to go down from the horizon has, iron as for him is brought by the movements of water in the ground and is fixed in the ferric complex humo.

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