Geological map

A geological map is a representation, on a Carte, geological grounds levelling. The grounds, in general, are represented by a color according to their age.

There can be, in particular for old charts, representations rather taking into account the facies that the age.

On a geological map, it is disregarded the grounds, and often also, if they are not very thick, of the deposits Quaternaire S.

The geological maps, in addition to the knowledge of the ground in a precise point, make it possible to deduce the deep geological layers from levelling on the level from other points of the chart.

Imprimer geological maps constitutes sometimes a true prowess. Some ask for more than one score of Encre S different to be printed!

The geological map in France

In France, the geological map was born with half of the 18th century.

Since 1746, Jean-Etienne Guettard (1715-1786) carries out a first outline of the geological map of France. But 1841 should be waited until to see appearing the first geological map of France carried out on scale 1/500 000e into six layer by two mining engineers: Armand Dufrénoy and Jean-Baptiste Élie de Beaumont.

As from 1868, Napoleon {{III}} creates the Service of the Geological map, charged to raise and publish the geological map of the whole of the French territory on the scale 1/80 000e. The first layer is raised in 1875.

In 1913, a ministerial decree launches the beginning of the geological cartography of France with the 1/50 000e. The first layer appeared in 1925.

BRGM (Office of Geological and Mining Research), created by the decree n°59-1205 of October 23rd, 1959, and the Service of the Geological map gather in 1968.

The survey of the chart to the 1/80 000e is completed in 1971 with 600 published layers.

Today, the near total of France was charted to the 1/50 000, and the BRGM launches out in the cover of the territory on a more regional intermediate scale (1/250 000e). In parallel, the BRGM computerizes these charts in the form of SIG and places them free at the disposal on Internet by the means of the gate infoterre.

The BRGM, based with Orleans, has one of the largest geological map library of the world.

Various types of geological map in France

The chart with the 1/80 000e

It is the first succeeded programme of cartography of the French territory. The objective was of raising 868 charts on the funds topographic of the charts of state major (they also with the 1/80 000e).

Initiate by Napoleon in 1868, the program was to last ten years and to be carried out by the Service of the Geological map. But since 1875 the service which was initially composed of a director, under director and five engineers, evolves/moves. It calls upon the assistance of collaborator external to profit from the contributions of new concepts and the evolution of knowledge.

The geological map with the 1/80 000e presents already a technical legend to the complex symbolic system (1000 signs) with the cartography of various elements anthropic.

The chart with the 1/50 000e

The chart with the 1/50 000e is downward chart with the 1/80 000e. It allows an increased precision of geological information: it is the detailed chart.

For this program, France is divided into 1127 cuts representing on average thirty km out of twenty km. Based on the same principle as the chart with the 1/80 000e, it is pressed on the topographic chart of IGN to the 1/50 000e. The name of the geological map is the same one as that of the topographic chart.

The end of the survey of the first editions was envisaged in 2005.

These charts are available under format paper since the beginning of the program to the editions of the BRGM, but technological advances in the field of the numerical one found applications in the geological cartography.

Since 1994, the Geographical Information systems allow the sale of charts in the form of images géoréférencée (i.e. each point is related to frames of reference) scannées or vectorized.

The chart to the 1/250 000e

The chart to the 1/250 000e is a chart of regional synthesis: its less precision makes it possible to identify the great geological entities at the regional level.

The cutting of France on this scale was done in forty-four sheets, of which fourteen already were carried out and are available (figure at the 8/31/2004).

The chart to the 1/1 000.000e

The chart to the 1/1 000.000e is the chart of synthesis of current geological knowledge of France and of its edges (countries bordering and underwater platforms). It is the reference document for the ten to twenty next years.

The any last edition of this chart is the 6th revised edition which is appeared in June 2004. It is available in the forms paper and numerical (scannée and géoréférencée).

The legend of a geological map

The geological map is a geological tool of knowledge based on the representation of the levelling formations. The three principal types of formations are found there:

As well as the geological information noted by the cartographer (faults, dips, mines…). In order to find itself there, the legend of the geological map is cut out in several parts. Each type of formation is represented by a range of color and a type of code. On the other hand, the same formation present on several charts can have nuances of colors and/or the illustrated different ones according to the author from the chart and from its needs. In the same way, the codes of the formations can vary from one chart to another, but must always respect total criteria.

The programme of harmonization of the geological maps of France to the 1: 50000e tends to gum the differences of notations and colors from one chart to another for the same formation. This harmonization is in hand on a department scale and will have, in the long term, to present a coherence to the national scales.

Sedimentary formations

The sedimentary formations are classified according to their age in a anti-chronological way according to the principles of the Stratigraphie. Formation most recent (fill, alluvial surface formations…) are thus with the whole beginning of the legend. These formations are gathered according to a era or a Système of the geological Time scale.

Their stacking in successive layers is sometimes represented in edge of legend by a geological Log which includes/understands the information most representative of the layer (thickness, fossils, particular stratigraphic forms).

The code of the colors for the formation sedimentary takes again overall that of the geological Time scale. It is about the code of the colors adopted by the Commission of the geological map of the world. The code of the formation is generally composed of a first letter which indicates the time (for the formations tertiary and quaternary) or the system (for the formation former) followed figures and letters specifying the stratigraphic place of the formation.

Volcanic formations

The volcanic formations (Volcanic rock) resulting from are lava flow or projection pyroclastites. When that was possible, various castings are represented by nuances of colors.

The volcanic rocks are represented by hot colors (red, orange) and codes writes with Greek letters.

Plutonic formations

The plutonic formations (plutonic Rock) are intrusive rocks which often cause a Métamorphisme of contact while being placed.

They are often represented by colder colors and have also codes made up of letters of the alphabet Greek.

Metamorphic formations

The metamorphic formations (metamorphic Rock) are the whole of the rocks which underwent a Métamorphisme. They can be gathered in the legend partly, or dispersed according to the rock of origin (sedimentary or magmatic).

When it are gathered in the legend, they often have a particular color with a code in Greek letters. But when they are mixed with different the types of rocks, one often finds them in the form of an overload (appeared which covers whole or part of the original formation on the chart) and their code is only one light variation of that of its formation.

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