Deciphering the Layers of Earth

?This was the man to whom all things were known;
this was the king who knew the countries of the world.
He was wise, he saw mysteries and knew secret things,
he brought us a tale of the days before the flood.
He went on a long journey, was weary, worn-out with labor,
returning he rested,
he engraved on a stone the whole story.?
?The Epic of Gilgamesh? (ca 2.000 B.C.)

Ancient philosophers and scholars had already noted and speculated about the layers found in outcrops of rocks. Recognizing fossils as remains of once living sea creatures, some Greek philosophers believed that the conformation of land and sea changes over time, forming distinct layers, and Muslim scholars described the layering of rocks as results of accumulation and deposition of rock fragments.
But these great ideas were proposed by single individuals or small groups and no consistent school of thought or even culture dedicated to the study of rocks developed. Most knowledge was soon forgotten and had to be rediscovered again and again during the following centuries.

The Italian Renaissance artist and naturalist Leonardo da Vinci studied sediments, fossils and stratification on the hills of Tuscany, Romagna and along the Po River between 1482 and 1499. From the private notes that Leonardo wrote, it appears that he understand the mechanisms of sedimentary erosion and deposition, that superimposed layers were formed at different times and that distinct layers of rocks could be traced over long distances. This empirical knowledge was ?applicated? by da Vinci in some of his paintings, when the landscapes in the background of a scene displays outcrops of rocks represented correctly with sedimentary layers. However da Vinci never published his ideas and it is even questionable if he shared his observations with other persons.

It was the work of the physician Georgius Agricola, latinized version of the German name Georg Bauer (1494-1555), which for the first time contributed to a broad diffusion of applied strata geology. His book ?De Re Metallica (On the Nature of Metals)?, posthumously published in 1556, is a systematic study of ore deposits, the order and extant of strata and description of contemporary mining technology. This book will remain the standard text on geology for the next two centuries. However Agricolas work, as remarkable as it is, followed the traditions of his time and so the content is mostly descriptive ? it offers little or only metaphysic explanations how layers or rocks form and how to study them.

It was the Danish Niels Stensen, or latinized Nicholaus Steno (1638-1686), who trained by his anatomical skills not only recognized the order of layers but actually tried to explain them and formulated rules for a general interpretation of sediments. Steno hoped that these simple and universally applicable rules would be used by other naturalists, like a sort of protocol when studying sediments, and even more important: He introduced ?time? in a stratigraphic succession.
Observing the rocks of the Italian region of Tuscany in 1667 he formulated these basic principles, on which modern stratigraphy is still based (geologist Andrew Alden discusses ?Steno?s Laws or Principles? applicated to modern stratigraphy):

-Fossils are the remains of once living creatures, comparable to modern ones and are found only in sedimentary rocks.

-Layers of rock are arranged in a time sequence, with the oldest on the bottom and the youngest on the top, unless later processes disturb this arrangement (principle of superposition).

-Layers of rocks are deposited in a horizontal position; any deviations from this position are due to the rocks being disturbed later (principle of original horizontality).

-A stratum is deposited continuously unless some other solid body stood in the way (principles of strata continuity).

-If a body or discontinuity cuts across a stratum, it must have formed after that stratum (principle of cross-cutting relationships).

Steno explained inclined strata (contradicting his principle of original horizontality) as results of the collapse of great subterranean cavities formed by erosion. It?s important to note that Steno positioned the single figures to explain this mechanism in form of a cycle of deposition, erosion and collapse ? a first glimpse of the concept of ?deep time? of the stratigraphic record.

Fig.2. Stepwise facies restoration of Tuscan geology based on sedimentation and deformation of strata (published in Niels Steno?s 1669 ?Prodromus?, image in public domain).

However Steno?s work, like the work of many others before him, was ignored for decades. Then a certain John Woodward, considered an amateur physician and naturalist by some, by others a quack, used/stole the principles formulated by Steno in his 1695 book ?An Essay toward a Natural History of the Earth?. The best part of work, thought to support the idea of the biblical sin flood as origin of the fossils, were the text passages copied from Steno.
However the book of Woodward and the principles of Steno used in it initiated a new interest in the study of sedimentary rocks.

Fig.3. In a booklet with the title ?Ragguaglio di una grotta ove vi sono molte ossa di belve diluviane nei Monti Veronesi (Description of a cave in the mountains of Verona where many bones of beasts from the deluge can be observed)? the engineer and cartographer Gregorio Piccoli del Faggiol (1680-1755) published in 1739 a topographic map of the Italian Dolomites correlated with a sort of stratigraphic column. This column shows layers only some meters thick in sequence as observed in the field. This work, nearly forgotten at its time and still today, is maybe the oldest figure of this kind (image in public domain).

With the formulation of general applicable rules first representations of stratigraphic columns appeared at the end of the 18th century; in 1760 the Italian geologist Giovanni Arduino proposed to classify the rocks of the Alps in four distinct layers ? Primary, Secondary, Tertiary and Quaternary sediments. However the term stratigraphy, as the study dealing with the processes that form sedimentary layers, was coined only in 1849 by the French Palaeontologist d?Orbigny.

Despite the recognition of the principles controlling a succession of layered rocks, the formation of the single strata remained still a mystery. During the 18th and 19th century two models prevailed, the Neptunistic approach proposed that rock strata were crystallized deposits precipitated in a distinct order from sea water, later erosion and modern deposition played a minor role in forming sediments. The Plutonistic approach in contrast stated that erosion and deposition play the major role in stratified sediments formation. According to this hypothesis all of the rocks are in principle of volcanic origin and became only later eroded and the resulting sediments deposited.
The controversy continued for years, no follower of one or the other idea could prove the stratigraphic order necessary for his model. According to the Neptunists, supporting the Neptunistic geology, the dark rock called basalt was of marine origin and the very first stratum deposited in the primordial sea ? it never was found overlying other strata. According to the Plutonists basalt was of volcanic origin and could be found in no particular stratigraphic position. Both models had to deal with the major problem of geology at these times: Geological maps depicted simply the prevailing rock type of an area, connecting single outcrops consisting of the same lithology and implying a spatial homogeneity with surprisingly little diversity and no apparent stratigraphic order.

It was the self-educated engineer William Smith (1769-1839) to discover the code hidden in the rocks. He recognized that superficial identical strata differ in their content of fossils ? fossils, regarded until them only as curiosities, beautiful, but worthless, became like the numbers on the page of a book an indispensable tool to bring order in the chaos.

Fig.4. Chronostratigraphy and collection of typical rocks and fossils of the ages of earth ? The Layers of Earth as book by Y. Fric, dealer of natural products, Prague 1861 (Collection of the Ferdinandeum in Innsbruck).

Smith applied the principle of faunal succession to publish some minor maps in 1799 and then the first large-scale geological map with profiles in 1814-1815, depicting southern England and Wales. His example was soon followed (some say more appropriately copied) by English geologists and by the French naturalists Cuvier and Brogniart. They published in 1808 the ?Essai min?raligique sur les environs de Paris?, a work dealing with the geology of the basins surrounding Paris and completed with a map and geological profiles.
However a detail of the French publication reveals that yet the revolutionary insight of Smith?s work wasn?t fully grasped by the scientific community: the legend of the geological map doesn?t show the lithologies in their stratigraphic (temporal) order like modern maps do, but the signatures are arranged by convenience.

Bibliography:

GOULD, S.J. (1988): Time?s arrow Time?s cycle Myth and Metaphor in the Discovery of geological Time. Harvard University Press: 240
KOUTSOUKOS, E.A.M. (2005): Applied Stratigraphy. Topics in Geobiology Vol.23.: 488
LAZZARI, C. (2000): Le Scienze della Terra nel Veneto dalle origini ai giorni nostri ? 8 secoli di studi, scoperte, progressi e leggende. Societ? Veneziana di Scienze Naturali: 171
VAI, G.B. (2007): A history of chronostratigraphy. Stratigraphy, Vol.4. (2/3): 83-97

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