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History of Geology

History of Geology

What rocks tell and how we came to understand it

A Concise History of Geological Maps

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March 23, 1769 marks the birthday of pioneering stratigrapher William Smith, who is also credited with creating the first useful geological map, however like many other great accomplishments also Smith's idea of depicting the distribution of rocks on a topographic map didn't materialize out of nowhere.

The German mining engineer Georgius Agricola (1494-1555) dedicated in his "De re metallica" (1556) - an early textbook on mining technologies - an entire chapter to the distribution of valuable rocks in earth's crust. The written description is correlated with various figures, showing in a sort of combined landscape - section the distribution, thickness and direction inside the mountain of the mineralized veins.

Fig.1. Veins and mineral seams, figure from "De re metallica", not a real map, but interestingly the cardinal directions are given on the borders (image in public domain).

The idea of a real map of rock-distribution was proposed first in 1684 by the British physician and naturalist Martin Lister (1639-1712). Lister suggested that the distribution of the different soil types of the British landscape could accurately be represented on a topographic map.

"The Soil might either be coloured, by variety of Lines, or Etchings; but the great care must be, very exactly to note upon the Map, where such and such Soiles are bounded…Now if it were noted, how far these extended, and the limits of each Soil appeared on a Map, something more might be comprehended from the whole, and from every part, then I can possibly foresee, which would make such a labour very well worth the pains."

As - so Lister continues - the soil types correlate with the underlying bedrock, by mapping the soils one could also map the rocks hidden in the underground.

However Lister never realized a real map based on this theoretical premise. It was the Italian Count Luigi Ferdinando Marsili (1658-1730) who made the next important step. As military engineer Marsigli traveled widely in Italy, France, Germany, the Balkans and Turkey, creating topographic maps for military use of the visited countries. As an exact representation of the landscape was essential to plan movements of an army or identify the best locations for fortifications, Marsigli became a keen observer of the landscape, sketching rock outcrops or prominent features of the landscape, and a skilled cartographer. After an unfortunate military campaign in Germany, Marsigli was accused of cowardice, his military career ruined he used his acquired skills to create maps for more peaceful applications.

In 1726 he published a map of the mining districts in Hungary and sketched the distribution of gypsum and sulfur deposits near his hometown Bologna (1717). In his sketch he connected the single gypsum quarries and outcrops along rivers with a shaded area, delimiting so the folded gypsum-bearing rocks. This map is important as it displays a first approach to the problem all geologists must face - not only documenting the visible outcrop of a rock or the position of a mine or quarry (such maps existed already), but interpolating the distribution of the not accessible part of a geological formation.

Fig.2. The map from "Atlas et Description Minéralogiques de la France" (1780), by French pharmacist and botanist Jean-Étienne Guettard, shows the distribution of outcrops with minerals, fossils or rocks. Such mineralogical maps predate true geological maps, showing sites of geological interest, however lacking the interpolation between the single "data points" (image in public domain, originally posted by BibliOdyssey).

It may surprises that despite many naturalist had already produced very detailed descriptions and maps of single outcrops, almost nobody made a connection between sites with similar rocks. But not only was the unequivocal identification of geologic formations at the time still very difficult, many naturalists considered connecting single outcrops by a presumed (not visible at the surface) extension of the rocks as unscientific speculation. This aversion of early geognosts to geological maps is exemplified by the strange behavior of naturalist Jean-Étienne Guettard (1715-1786), famous for his detailed mineralogical and volcanological maps. Guettard in 1777, after eleven years of hard work, abandoned the prestigious project by the French minister of Mining to produce a series of geological maps of France. He simply couldn't overcome the idea that a map should represent only facts (in this case outcrops) - but a blank map with just some isolated spots of color wasn't exactly what the French authorities wanted.

Maybe the first true geological map was drawn by an anonymous naval cartographer in 1757. In the outlines of the German island of Heligoland he added boundaries between four different rock types: Kreide (chalk), Muschelkalkstein (limestone), Bunter Sandstein (sandstone) and Kohle (coal beds). The map depicts the boundaries of the various geological formations even below the sea.

As the author, also the intended use of this map is unknown. The historian of geology - David. R. Oldroyd - speculates that the map maybe could be used as aid to navigation, as sailors could determine their position by evaluating the rocks and sediments dredged from the seafloor.

To be continued...

Bibliography:

FRANCESCHELLI, C. & MARABINI, S. (2006): Luigi Ferdinando Marsili (1658-1730): A pioneer in geomorphological and archaeological surveying. In VAI, G.B. ed, The origins of geology in Italy: Geological Society of America Special Paper 411: 129-139

OLDROYD, D. (2013): maps as pictures or diagrams: The early development of geological maps. In BAKER, V.R. ed, Rethinking the fabric of geology: Geological Society of America Special Paper 502: 41-101

The views expressed are those of the author and are not necessarily those of Scientific American.

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