Historical Geology/Geological column

*Spinosaurus*, a dinosaur of the Cretaceous.

In this article, I shall explain what the geological column is, how it is constructed, and what relationship it bears to the geological record. I shall also provide a rough description of the geological column summarizing some of the major trends observed in it.

The reader should already have read the article on Steno's principles, the article on the principle of faunal succession, and the article on index fossils before reading further.

Construction of the geological column

By using the principles explained in the previous articles (superposition, faunal succession, the use of index fossils) it is possible to produce an account of the order of deposition of the organisms found in the fossil record, noting that one was deposited before the other, that the deposition of such-and-such a group starts after the deposition of some other group ceases, and so forth.

This means that fossils and the sedimentary rocks that contain them can be placed in order of their deposition. The resulting table is known as the geological column.

Prolog to a sketch of the geological column

Below, I sketch out the major geological systems from the Vendian onwards. Note that it is written from the bottom upwards, so that the earliest-deposed fossils are at the bottom; the reader may therefore find it tells a more coherent story if read from the bottom upwards.

It is no more than a sketch: it records the appearance and disappearance of major groups, rather than individual species; and it has been divided into the large stratigraphic units known as systems, which geologists would divide into series, which they would further divide into stages, which they would then further subdivide into zones. I am, then, only giving the broadest outline of the geological column; those who require the finer details must look elsewhere.

I have not attached any dates to the geological systems discussed here, because as we have not yet reached our discussion of absolute dating, it would be premature to do so. All that our study of fossils and their faunal succession tells us is the order of deposition. (It is for this reason that I have used the increasingly obsolete term "geological column" rather than "geological timeline"; it is not a timeline until we get round to attaching dates to it.)

I have also avoided using terms such as "evolution" and "extinction". From a biological standpoint, it is obvious that these are the underlying cause of the patterns in the fossil record; but as with the evolutionary explanation of the principle of faunal succession this biological explanation is irrelevant to the practice of geology. For the purposes of doing stratigraphy it doesn't really matter if dinosaurs appear in the geological column because they evolved from more basal archosaurs or because they parachuted out of the sky from an alien spaceship, and it doesn't matter if they disappear from the geological column because they went extinct or because they all went to live in cities on the Moon; what matters is that we can find out where their fossils come in the sequence of deposition.

A sketch of the geological column

Quaternary

Marked by the existence and spread of modern humans and the decline and disappearance of many groups of large fauna extant in the Neogene.

Neogene

Contains recognizable horses, canids, beaver, deer, and other modern mammal groups. The Neogene also contains many large mammalian fauna no longer extant: glyptodonts, ground sloths, saber-toothed tigers, chalicotheres, etc. First hominids found in Africa.

Paleogene

Marked by the diversification of mammals and birds. Among the mammals we see the first that can be easily identified with modern mammalian orders: primates, bats, whales, et cetera. Similarly representatives of many modern bird types are identifiable in the Paleogene, including pigeons, hawks, owls, ducks, etc. Now-extinct groups of birds found in the Paleogene include the giant carnivorous birds known colloquially as "terror birds".

Cretaceous

Here we see the diversification of angiosperms (flowering plants) from beginnings around the Jurassic-Cretaceous boundary; representatives of modern groups of trees such as plane trees, fig trees, and magnolias can be identified in the Cretaceous. Here also we see the first bees, ants, termites, grasshoppers, lepidopterans. Dinosaurs reach their maximum diversity; some of the best known dinosaurs such as Triceratops and Tyrannosaurus are found in the Cretaceous. Mosasaurs appear near the end of the Cretaceous, only to disappear at the Cretaceous-Paleogene boundary, which also sees the last of the dinosaurs (excluding birds, which biologists classify as dinosaurs) and the last pterosaurs, plesiosaurs, ichthyosaurs, ammonites, rudists, and a host of other groups.

Jurassic

This system is notable for the diversification of dinosaurs. It has the first short-necked plesiosaurs (pliosaurs); first birds; first rudists and belemnites. Mammals are certainly present, but tend to be small and insignificant by comparison with reptile groups. The first placental mammals are known from the Upper Jurassic.

Triassic

The Triassic contains the first crocodiles, pterosaurs, dinosaurs, lizards, frogs, snakes, plesiosaurs, ichthyosaurs, and primitive turtles. Whether or not there were mammals in the Upper Triassic depends on what exactly one classifies as a mammal. The Triassic-Jurassic boundary sees the loss of many groups, including the last of the conodonts, most of the large amphibians, and all the marine reptiles except plesiosaurs and ichthyosaurs.

Permian

This system is noted for the diversification of reptiles: the first therapsids (mammal-like reptiles) and the first archosaurs (the group including crocodiles and dinosaurs). It also has the first metamorphic insects, including the first beetles. It has the first trees identifiable with modern groups: conifers, ginkgos and cycads. Many species and larger groups come to an end at or shortly before the Permian-Triassic boundary, including blastoids, trilobites, eurpterids, hederellids, and acanthodian fish.

Carboniferous

This system contains the first winged insects. Amphibious vertebrates diversify and specialize. The Carboniferous has the first reptiles, including, in the Upper Carboniferous, the first sauropsid, diapsid, and synapsid reptiles. Foraminifera become common. All modern classes of fungi are present by the Upper Carboniferous.

Devonian

The Devonian has the first (wingless) insects; the first ammonites; the first ray-finned and lobe-finned fish; the first amphibious vertebrates; the first forests. Terrestrial fungi become common. The first seed-bearing plants appear in the Upper Devonian. The last placoderms are found at the Devonian-Carboniferous boundary. Almost all groups of trilobite have disappeared by the Devonian-Carboniferous boundary, but one group (Proetida) survives until the Permian-Triassic boundary.

Silurian

In the Silurian, coral reefs are widespread; fish with jaws are common; it has the first freshwater fish; first placoderms (armour-plated fish); the first hederellids; the first known leeches. Diversification of land plants is seen.

Ordovician

In the Ordovician system we see the first primitive vascular plants on land; jawless fishes; some fragmentary evidence of early jawed fishes. Graptolites are common, and the first planctonic graptolites appear. Bivalves become common. The first corals appear. Nautiloids diversify and become the top marine predators. Trilobites diversify in form and habitat. The first eurypterids ("sea scorpions") appear in the Upper Ordovician. Trilobite forms such as Trinucleoidea and Agnostoidea disappear at the Ordovician-Silurian boundary, as do many groups of graptolites.

Cambrian

This system sees the first animals with hard parts (shells, armor, teeth, etc). Trace fossils reveal the origin of the first burrowing animals. Trilobites are common; chordates exist but are primitive. Archaeocyathids are common reef-forming organisms in the Lower Cambrian and then almost completely vanish by the Middle Cambrian. Conodonts are first found in the Upper Cambrian. Many groups of nautiloids and trilobites disappear at the top of the Cambrian, but some groups survive to diversify again in the Ordovician.

Vendian

This system contains the first complex life, including sponges, cnidarians, and bilaterians.

The geological column and the geological record

We should distinguish between the geological record and the geological column. The geological record is a thing: it is the actual rocks. The geological column is not a thing, it is a table of the sort given above. To ask questions such as "where can I go to see the geological column?" or "how thick is it?" is therefore a category error along the lines of asking how many people can be seated around the Periodic Table.

The relationship between the geological column and the geological record is this: when we look at a series of strata in the geological record and use the principle of superposition and way-up structures to discover the order of deposition of the fossils in it, then if we find that A < B in the strata, this will correspond to B being shown above A in the geological column. The geological column is therefore a particularly simple and neat way of recording what relationships we do and don't find in the geological record.

However, the geological column is not a picture of what we find in the geological record. There are three reasons for this.

First, as we know, the geological record is folded and faulted in some places. Recall that when we write A < B we are talking about the original order of deposition of fossils, as reconstructed by using the principle of superposition and way-up structures: it does not necessarily mean that A is actually below B; whereas the geological column is always depicted as a vertical column with A below B when A < B.

Second, by using index fossils geologists produce a single time-line for the entire planet; but clearly any particular location will only have local fossils: the column, if written out in full, would show exclusively South American Cretaceous dinosaurs above exclusively North American Jurassic dinosaurs, but these will not in fact be found in the same assemblage of strata.

The third reason is that deposition will typically not happen continuously in one place: sediment is deposited in low-lying areas; it would not be deposited on top of a mountain. What's more, an elevated area will typically undergo erosion: not only will fresh sediment not be deposited, but existing sedimentary rocks and their fossils will be destroyed. Also, marine sediment will be destroyed by subduction, so the sediment of the oceanic crust will be no older than the ocean that it's in, and even then only at the edges — it will be considerably younger near the mid-ocean rifts.

Consequently, the meaning of the geological column is not that any location in the geological record will look like the geological column: the column is merely an elegant way of representing the facts about faunal succession.

The geological column: how do we know?

As was explained at the start of this article, the geological column is constructed using ideas introduced in previous articles: the principle of superposition, the principle of faunal succession, and the use of index fossils.

Note that the geological column does not except in the weakest sense constitute a scientific theory. It does resemble one, because there is a sense in which it suggests what we are likely to observe, which is the role of a theory; but it is essentially descriptive in nature. That is, it does not really predict the sequence of fossils that we will find, it is determined by, and summarizes, the sequences of fossils that we have found. Since it is likely that what we will find tomorrow will be similar to what we have been finding for the past couple of hundred years, the column is in that sense predictive, but its predictive power goes no further than that.

So if tomorrow we found that some trilobites were deposited above the Permian system, we should simply amend the geological column to reflect this, and it would be surprising not because it contradicted the geological column as such, but because in centuries of paleontology no-one has yet made such a discovery.

Compare this with how we would feel if we consistently found violations of the principle of faunal succession. This would present a difficulty in theory, and would require us to give up on the principle of faunal succession (and to give up on using it to construct a geological column, something that would then become impossible). But finding something that contradicts the geological column as it stands is merely unlikely in practice, not in theory, and would only require us to revise the geological column in one particular detail (i.e. to take the new discovery into account) without requiring us to rethink any fundamental ideas.

So the geological column is trustworthy simply because it is no less, but no more, than an up-to-date summary of our knowledge, and so it can be taken as such. To which we might add that after all these years of looking at the fossil record it is extremely unlikely that we'll find anything so unusual as to require any major revision of the column.

← Index fossils · Unconformities →