Alessandro Grippo, stratigraphy 1

Alessandro Grippo, Ph.D.


Like a cloud drifting over, that covers the day
Time passes slowly, then fades away
(Bob Dylan)

Part 1

What is StratigraphyLast Updated  •  March 31, 2015    
As a general concept, Stratigraphy is "the study of rock successions and the correlation of geological events and processes in time and space" [1]. What does this mean?

  • First of all, what rocks are we talking about?
    Rock successions are sequences of rocks that are characterized by a relative time significance, that is, we can establish the order in which they were deposited. Mostly, we are speaking about sedimentary rocks, which are deposited according to the Principle of Superposition (with minor exceptions), but we should also include a few extrusive igneous rocks (pyroclasts, lava flows), and those metamorphic rocks where the original sequence can still be detected.

  • Second, what is correlation?
    To correlate means to establish "equivalency in time", that is to be able to say that two or more events happened at the same time, or are synchronous. In order to do that, we have to find certain indications in our rock sequences that allow us to link and match in time rocks from distant places.
    Sometimes though the world correlation can be used in a wider context, as a way to say that certain rock units cropping out at different places are the same (equivalency in the type of rock), even if they might not have formed at the same time.
    While the proper meaning of the word "correlation" can be applied over a large scale, the second only works on relatively short distances.

  • Third, what are geological events and processes in time and space?
    Anything that is recorded in rocks, from a normal, continuous deposition pattern in an undisturbed area to a "catastrophic" event such as a meteorite impact or a tsunami, constitutes an event that can be studied. These events can occur in a temporal sequence, and as a consequence they would be recorded one after the other in rocks at one location. But the same event, say for instance a tsunami, can occur at the same time in different places. So these events are studied both in time and in space.

In the end, Stratigraphy can be described as the study of layered rocks and of their temporal implications.
(The term "layered rocks" is used because it will include not only all of the sedimentary rocks but also a few igneous and metamorphic ones)

Stratigraphy allows the reconstruction of the sequence of the events in Earth history and of the evolution of life on Earth.

Today's meaning and significance of stratigraphy is much more accurate and precise than it was in the past. But when stratigraphy is interpreted as the study of the natural environment and its phenomena, we realize that it is a very old science, which originated when ancient philosophers and wise men started to observe, describe, and think Earth's natural phenomena.

Stratigraphy in Ancient TimesLast Updated  •  March 31, 2015    
While overall ancient philosophers elaborated each their own theory and speculations about Earth, the perception of the natural world was, and in part still is, different throughout the various cultures that arose in different parts of the world:

For instance, in far Eastern cultures Earth was eternal and immutable while in Indian cultures Earth changed according to infinite cycles of creation. The ancient Greeks had a more pragmatic, realistic approach: for them, Earth was subject to continuous change over time because of natural laws, without the necessity of an external divinity to modify or alter the picture.

In ancient times the approach to fossils was also variegated, and they were object of curiosity too. The word "fossil" comes from the Latin fossilis which means, literally, "that can be dug out" (modern Italian, an alteration of Latin, still uses the words "fossa" to indicate a dig, a hole, and "fosso" to indicate a ditch). Such a wide description, a fossil is anything dug out of the ground, tells us that in the past a fossil could have actually also indicated a rock, a mineral, a groups of layers, or even human artifacts. The modern meaning of the word "fossil" only applies to the remains of ancient organisms, or of their activity (as in the case of trace fossils).

The Greeks and the Romans

The major contribution of the Greeks and the Romans to the world of Earth Sciences resides in the attempt of explaining Earth and its processes through natural phenomena, as opposed to justifying everything through the intervention of a divinity, or supernatural causes in general.

Among others, Pythagoras (6th century BC) and Herodotus (5th century BC) interpreted marine shells found on mountains as remains of organisms that lived in the ocean: according to them, the ocean had covered the land and had retreated from it at least once during Earth history.
Being open to natural phenomena does not mean that these philosophers were always right in their theories: for instance, Herodotus himself, when studying the blocks of limestone used by the ancient Egyptians to build their pyramids, which were dotted by the fossils of macroscopic marine foraminifera (Nummulites), argued that these fossils had originated from lentils that were fed to the construction workers, and by them inserted into the rocks.

The Greek Xenophanes, also in the 5th century BC, argued that marine fossils found at the top of mountains, or on land in general, were a line of evidence that nothing is immutable and that physical changes actually occur on Earth, and that land and sea can "blend" together. The landscape we see is only but a moment in time of a continuously changing Earth.

Aristotle (4th century BC) thought that "fossils" were produced by a natural (not divine) "formative force" that imitated extant organisms.

During Roman times Pliny the Elder (1st century AD) was the major naturalist of the period (among other things, he is the author of Historia Naturalis, a 37-volumes encyclopedic work about Natural History, the first ever conceived). He died of asphyxia while observing a geological event, the catastrophic volcanic eruption of Mount Vesuvius in Pompeii, Italy, in the year 79 AD.

In the following years, the spreading of a new religion, Christianity, started to condition natural philosophers and theologians in their thinking. According to Christianity, there were no "natural causes" but everything was consequence of a divine "creation". Some of these thinkers, such as, for instance, the Roman Tertullian, were somehow able to maintain the idea of the organic origin of fossils (instead of a "creation"), but they still had to fit their origin within the religious framework of the biblical deluge to avoid persecution.

Later on, at first when Christianity eventually became the official religion of the Roman Empire (Edict of Constantine, 313 AD) and then when the Empire fell at the hands of invading barbaric populations from the north and the east (476 AD), the Roman culture and civilization, from which western culture derives, suffered a severe blow: the society as a whole entered a deep stagnation, and science in particular was deemed purposeless and was not cultivated any longer in the western world.

The Dark Ages and the Early Renaissance

While western civilization and society were in full decay, within a few centuries the rising Arabic world started to expand and to develop an interest for knowledge. During the 7th and the 8thcenturies AD the Arabs translated many of the Greek works that had been lost after the fall of the Roman Empire, adding some original thoughts of their own, and further integrating them with some far eastern ideas and works that were unknown in the west.

Through Avicenna (Ibn-Sina in Arabic, 980 AD - 1037 AD) the Greek Aristotelian School was eventually revived in the west and many European scholars developed again the idea that fossils could have been organic in origin, possibly representing a series of failed attempts of the divinity to originate life.

In the 1200s in Swabia (in today's Southern Germany), Albertus Magnus (Albert the Great) speculated on the nature of fossils as the remains of once-living ancient organisms.

It is eventually in the works of the Italian Leonardo Da Vinci (1452-1519) that for the first time we read about the mechanisms of sedimentary erosion and deposition in mountains and rivers, about the role that rivers play in the weathering of land and soil, and the basic principles of the Law of Superposition (which was to be demonstrated by Steno in Florence only a couple of centuries later). Leonardo also understood that different layers of sedimentary rocks, and the fossils they contain, can be traced over distances that extend beyond a single outcrop (correlation), and that these layers were formed in a sequence, at different, successive times. Unfortunately after his death, while the bulk of his major works was always well known, many of his books became scattered throughout Europe's libraries and courts, and his geological observations were ignored or forgotten for a few centuries.

Later on, an enormous contribution to the field of geology came from an essay (De Re Metallica, or "Metals") by the Saxon (Saxony is today a German region) Georg Bauer, better known by his Latin name of Georgius Agricola, which was published in 1556, a few decades after Leonardo's death. Among other observations, Agricola outlined how sedimentary layers were deposited in a consistent, sequential order. Because most of his studies were specifically devoted to ore deposits and mining industry, Agricola can be accounted, together with Steno (see next section below), as one of the founders of modern geology.

It is noteworthy to mention that, once earth-related economical interests became important in western society (the need for mineral and metal resources) enormous advances were made. Geology really became important when, in the 19th century, the industrial revolution required enormous amounts of coal as a source of energy. Something similar happened in the 20th century with the development of the oil and gas industry, whose workings provide us with a myriad information that would not be otherwise available. And it is happening today with, for instance, the hunt for "rare earths", a series of elements (Scandium, Yttrium, and the Lanthanides) that are extremely important components of our computer-based society.

In the year 1603 the Bologna, Italy naturalist Ulisse Aldrovandi, who was hailed by the Swedish Carolus (Carl) Linnaeus and by the French Comte de Buffon as the father of natural history, was the first scientist to ever use the word "geology" in its proper, scientific meaning[2]. All these advances prepared the ground for the next major step, the layout of Steno's principles.

Stratigraphy from the Renaissance to modern timesLast Updated  •  March 31, 2015    
Stratigraphy, or the art of being able to put order in a sometimes chaotic jumble of rocks, soon was at the core of every serious geologic study that took place during the Renaissance. Steno was the one who first outlined stratigraphy's principles.


Niels Stensen (1638-1686) was a Danish physician who left his birth country to move to Florence (Firenze), in central Italy, where he became the court scientist of the Medici family, rulers of Tuscany. He changed his name to Niccoló Stenone (Nicholaus Steno in Latin), and became pretty famous during his lifetime because of his naturalistic activity,his observational skills, and his deductions. He eventually became more religious during his old age, and even today his remains are buried in one of the major churches in Florence, San Lorenzo (St. Lawrence) where they are still honored.

Steno is famous because he has been the first to clearly outline the three basic principles of stratigraphy: superposition, original horizontality and lateral continuity:

  • the principle of superposition states that in a sequence of undisturbed sedimentary layer, the younger layer is found at the top and the older layer at the bottom
  • the principle of original horizontality states that sedimentary layers always form horizontally
  • the principle of lateral continuity states that sedimentary layers extend laterally to the edge of a sedimentary basin,
       or taper out and change into other, different layers

James Hutton

After Steno, one of the most significant figures for the development of geology as a science was the Scottish naturalist James Hutton (1726-1797): James Hutton was the first to speak about Uniformitarianism, or the Uniformitarian Principle.
He grasped the significance and the importance of the concept of "Uniformity" and he was the first to understand how "deep" the geologic time actually was.

The principle of Uniformitarianism states that the geologic forces and processes that shaped our planet in the geologic past were the same that are at work on Earth today, and that these forces and processes are part of our every day experience and can be observed directly by anyone.

In 1785 Hutton published his book Theory of the Earth, where he proved that the hills and mountains of today are being slowly eroded, and that the sediment produced during these erosional processes is being transported and eventually deposited as sand and mud on the sea floor to form sedimentary rocks[3].

Hutton argued that the same processes of erosion and deposition observed today must have always been at work in Earth's past. He also realized that the vast thickness of sedimentary rocks sequences that can be observed in nature implied the fact that these same processes of erosion and deposition had to have occurred throughout an inconceivably long period of time (that he called "deep time"). Such a contribution was very distinct from the mainstream ideas of the time, which were still very heavily influenced by religious beliefs. At that time, a popular theory was that Earth had formed (was created) during the month of October of 4004 BC, a date calculated by an Irish bishop using biblical genealogies.

William Smith

William Smith (1769-1839) was an English engineer that was in charge of cutting canals throughout southern Britain. These canals were required ignored to speed up transportation for the nascent industrial society of Britain. While doing his job, Smith got accustomed to the sequences of rocks he was cutting, and he soon realized there were patterns to them. He soon understood that the rocks were deposited in a regular fashion and immediately realized that the fossils found in these rock sequences always follow each other in a specific order, and that this order was exactly the same even when rocks were found as isolated layers at different locations. It was then possible to correlate these layers with each other in base to their fossil content, even if the lithology was different. Thanks to this realization, he always knew what rocks existed below or were missing above any layer of the sequence.

This is known as the Principle of Faunal and Floral Succession: fossils faunas and flora are found in a known and predictable order. Smith did not know the reason for this (it is a consequence of the evolution in time of living organisms) but he was right in his observations. Thanks to his observations, discoveries, and collection of information Smith was able to publish the first ever large-scale geologic map ever conceived between the years 1814 and 1815.

William Smith's "Strata of England and Wales" geologic map,
preserved at the headquarters of the Geological Society of London, England

George Cuvier

In contrast with the uniformitarianistic ideas of James Hutton, the French scientist Georges Cuvier (1769-1832) interpreted the geological record in terms of a series of catastrophes that succeeded each other during Earth history. He recognized extinctions in the biological record, and he interpreted them as being caused by major global changes, or natural catastrophes. He never invoked supernatural or divine causes for extinctions or catastrophes, but he simply did not think that slow, gradual change would be responsible for our rock and fossil record, or at least the one he was able to observe and know during his time.

Charles Lyell

It was in the end Charles Lyell (1797-1875) that popularized Hutton's idea of Uniformitarianism in his book Principles of Geology (1830). Lyell actually added an apparently small but very important detail, that was not in Hutton's work: according to him, it was not just that the same processes that acted in the past also acted in the present, but they also acted at the same rate (velocity, or speed) ("Gradualism").
Lyell therefore excluded temporary and local variations, or crises. We now know that this extreme approach to uniformitarianism is definitely not true. We understand that, for instance, a change in climate could speed up or slow down weathering processes, thus increasing or decreasing the amount of sediment available during the same time interval. Consequently, a sedimentary sequence can be thicker or thinner, or characterized by different textures, such as sorting and rounding, or in different colors.
The same variations apply for instance to the expansion rates of mid-oceanic ridge spreading, or other geological process whose variations Lyell could not of course have been aware of during his life.

Charles Darwin

Still, Lyell's work was fundamental: a general summary of the ideas of the time, a copy of Lyell's book travelled with Charles Darwin (1809-1882) during his scientific exploration trip aboard HMRS The Beagle in the 1830s. Darwin was very likely influenced in his thoughts by the work of Lyell, and he had chances to see Uniformitarianism at work (for instance by being a witness to an earthquake and its consequences along the Pacific coast of Chile).

Despite his significant geological contributions, Darwin is mostly remembered for his Theory of Evolution through gradual variation and natural selection, exposed and discussed in his aptly titled book On the Origin of Species by Means of Natural Selection.
The theory of Evolution was also a good explanation for William Smith's Principle of Faunal and Floral Succession. When we apply the theory of Evolution and the Principle of Faunal and Floral Succession to Steno's principle of Superposition, we realize that we can actually use fossils for stratigraphic dating and correlation.

Albert Oppel

It was the German stratigrapher Albert Oppel (1831-1865) who eventually thought of a method to divide geologic formations into different zones, based on the overlapping stratigraphic range of two fossil zones.
(The stratigraphic range of a taxon, as we will see in biostratigraphy, is the total vertical interval (in a sequence) through which that taxon occurs in the rock record).

The continuous, rapid establishment of biostratigraphic zones that followed these discoveries led to the development of a more and more refined relative time scale, which eventually allowed the construction of what we know today as the Geologic Time Scale with its subdivisions.

Stratigraphic unitsLast Updated  •  March 31, 2015    
What all these scientists have been working on is what we in the end call stratigraphy.

Stratigraphic Units include strata, or groups of adjacent strata, that are distinguished from each other by mean of physical, chemical or paleontological properties; stratigraphic units also include time units that are based on the ages of such strata.

Correlation is the establishment of equivalency between geographically separated stratigraphic units, or parts of stratigraphic units.

We can correlate in two distinct ways: while we can have lithologic correlations (that is, we say that two different outcrops are actually the same rock, or Formation) most of the times, when we speak of correlation we mean temporal correlation
(that is, we say that any two (or more) rocks are of the same age, or they formed at the same time in different locations).

[1] Koutsoukos, E. A. M., 2005. Stratigraphy: Evolution of a Concept. In: E. A. M. Koutsoukos (ed.), Applied Stratigraphy, 3-19, Springer.
[2] Vai, G. B., 2003. Aldrovandi's will: introducing the term "geology" in 1603. In: G. B. Vai, and W. Cavazza (eds.), Four centuries of the word geology: Ulisse Aldrovandi 1603 in Bologna: Minerva, Bologna, 64-111.
[3] Doyle, P., Bennett, M. R., and Baxter, A. N., 2001. The Key to Earth History: An Introduction to Stratigraphy (2nd edition). John Wiley and Sons, New York NY.

In parts 2 and 3, we will be looking at time units and time-rock units.

Go to part 2 | Go to part 3 | Go to Notes | Go to the Home Page

© Alessandro Grippo, Los Angeles, California, 1994-2015