Mapping the rocks beneath our feet: an essential step for our future

Identifying rocks in the subsurface, imagining how they are organised in three dimensions and how they have evolved over time are just some of the major contributions of geological maps. Those who know how to decipher them can also deduce the existence of useful materials (minerals and water), know what natural hazards are likely to occur, or whether difficulties may arise with regard to spatial planning.

The general public is not very familiar with geological maps, which combine both scientific and economic objectives, even though they are included in school curricula and though the emblematic geological map of France is on display in virtually every class on Life and Earth sciences.

Let's find out what these documents, which are key to environmental and social issues, are all about and how they have evolved over time.

The early days of geological maps in France

Maps showing outcropping rocks on the surface appeared at the end of the 18^th century to meet practical needs. The aim was to represent the different types of rock in order to predict the location of any substances being sought.

The first geological map with colours was made by Johann Toussaint von Charpentier (1738-1805), an amateur geologist from Normandy. In 1778 he drew a map of Saxony on which eight colours corresponded to types of rock: granite, gneiss, schist, limestone, gypsum, sandstone, river sand, and clay & silt. He chose the colours according to the colour of the rocks. For example, sand is yellow.

The idea of indicating rocks by colour was developed further, next by assigning the colour according to a period rather than to the type of rock. Estimating time has always been a fundamental aspect of geology. The mauve of the Triassic period is reminiscent of Vosges sandstone, the red of the Permian of red clay, the grey of the Carboniferous of coal layers, etc.

When the Corps des Mines was created on 6 July 1794, the founding decree stipulated that its engineers were to search for mineral substances in their respective arrondissements (boroughs) and record their discoveries on maps. It is always in times of crisis that there is concern about the supply of raw materials.

Under Napoleon I, the map commissioned from Jean-Baptiste d’Omalius d’Halloy to represent the French Empire uses six colours to depict six principal geological units, almost all of which correspond to geological periods, with the exception of a single colour grouping together the basement (ancient massifs) and the young mountain chains with complex geology (the Alps and the Pyrenees). As it took a long time to produce, and the Empire had disappeared before it was ready, it was entitled "Map of France and neighbouring countries". It is worth noting in passing that Napoleon, a visionary strategist, was behind the creation of 1:80,000 scale ordnance maps to replace the less accurate Cassini maps. He was also the instigator of France's legal framework for mining (code minier), whose aim was to centrally control resources.

In the 1830s, the Director General of bridges, roads and mines asked the Prefects to start producing departmental maps to be paid for by the conseil généraux (departmental councils). The latter saw it as a great opportunity to stimulate economic and industrial development, so they called on mining engineers rather than academics. Some départements, such as the Hérault, had not waited for the request and had already produced such maps. The oldest is that of Calvados (1825).

Implementation of state-funded programmes

With the aim of centralising and coordinating surveys and publishing maps at public expense, Napoleon III created the Service de la carte géologique et des topographies souterraines (department for geological maps and underground topology) on 1 October 1868. The projected programme was for 267 maps at a scale of 1:80,000 over ten years. It was finally completed in 1925, 57 years later, and updated until 1971. It is also worth noting that the topographical backgrounds correspond to ordnance maps with cross-hatching to represent the relief. This gives a good idea of the topography, but makes it more difficult to produce geological cross-sections than if contour lines had been used.

The detailed 1:50,000 scale programme (covering an area of around 30 x 20 kilometres) with contour lines was launched by a ministerial decree in 1913. This initiative was prompted by the fact that our German neighbours had been producing accurate 1:25,000 scale maps since 1909, particularly in Alsace.

The first surveys for the 1:50,000 scale geological map programme began in 1917. They were based on precise master plans (1:10,000) drawn up close to a strategic port. The first two maps (of Toulon and La Ciotat) were published in 1925. The development of aviation during the first World War made it possible to produce aerial views, which facilitated the production of topographical maps. Despite this, only eight maps at this scale were available in 1938 due to the lack of funds.

The French geological survey was incorporated in BRGM on 1 January 1968, the year of its centenary. At the time, 120 1:50,000 scale maps had already been produced.

This emblematic programme lasted more than 100 years, mobilising around 3,000 geologists including many students, and costing around 300 million euros, or just under 300,000 euros per map. These sums should be compared with the amounts that would have had to be invested in successive studies to have sufficient knowledge of our subsurface to meet our various needs. In 2004, the Spanish geological survey estimated the cost of its geological mapping programme (122 million euros) and calculated the equivalent cost of not having a programme (2,200 million euros), giving a cost/benefit ratio of 18 to 1.

The 1:50,000 scale geological map programme now covers almost the whole of mainland France, with another series of maps at different scales, some of which are also at 1:50,000, covering France’s overseas territories. Of the 1,060 titles in the collection, 1,055 were published between 1925 and 2026, including thirty-four 2^nd editions and four 3^rd editions (Toul, Corbeil, L'Isle-Adam and Aubagne-Marseille). Five remain to be published.

Maps for the future, with many challenges to be met

Just over two centuries after they were first produced, these documents are still essential for understanding how the rocks beneath our feet are organised. Furthermore, they are now also used for other purposes. They currently concern groundwater (in terms of both resources and risks), spatial planning and natural hazards. Geological maps are therefore valuable, strategic documents.

Although geological formations evolve little except during major catastrophes and volcanic eruptions, the way in which the various elements that make up our subsurface are represented changes as more knowledge is acquired and according to the specialisations of the authors. A comparison of the two editions of the Lorient map is revealing in this respect.

Today, the oldest maps still on sale date from the 1950s, and the surveys used to produce them were carried out even earlier. Many maps also use older topographic backgrounds. They need to be updated to provide an up-to-date picture of our knowledge of the subsurface.

Geological maps are inseparable from their instructions, which come in the form of a small booklet. These are essential for understanding the maps, as it is impossible to include all the information on the map itself. For the Lorient map, the booklet increased from 19 to 202 pages between the two editions of 1972 and 2015.

Geological maps are therefore highly interpretative documents, and certainly not raw data. To illustrate this, we can cite the example of the 3^rd edition (2018) of the Marseille map booklet, in which two interpretations for the Triassic and Oligocene geological units are given, as the authors were unable to agree amongst themselves. Similarly, on the map of Saint-André-de-Valborgne, the structural diagram and the map are not in agreement: one shows a continuous sedimentary pile, while the other shows the same series superimposed twice, separated by a fault.

Geological maps today face major challenges on several fronts: the changing skills of the experts who knew how to draw them up and interpret them, the proliferation of data and the upheavals caused by digital technology.

We need to reinvent new documents for today's audiences. This is a major challenge for BRGM, the French geological survey, and for the entire geoscience community (academics, industry), as well as for secondary schools, spatial planners and the farming community, which is particularly interested in finding solutions in the context of climate change.

As a result, since 2021, Éditions du BRGM has been offering educational maps that are particularly well suited to secondary education. A key part of the educational curriculum, some geological maps have been simplified and redesigned to make them more accessible. For example, it is much easier to decipher the sequence of events.

These summary documents enable students to acquire some of the skills required in middle and high schools. They are tools for integrating knowledge, teaching people to observe and analyse, thereby encouraging reasoning. Because maps are read in two dimensions, it is often difficult for pupils to see them in three dimensions. These new maps make it easier. In addition, field outings enable pupils to get a feel for the realities on site, to understand the links between outcrops and the interpretations that can be made from a geological map.

In the future, it will be possible to go even further, with specific versions for groundwater and natural hazards. Upgraded geological maps have a bright future ahead of them.