What is stratigraphy?

The word ‘stratigraphy’ is used to describe the study of rock layers, or strata.

The most fundamental principals employed within this discipline are:

The law of superposition, which states that within any un-deformed suite of rock layers the oldest will be at the bottom and the youngest at the top: thus, a relative chronology between connected rock units can be established.

The principal of faunal succession, which states that different rock layers from different times will contain unique fossil assemblages. 

The principal of uniformitarianism, which states ‘the present is the key to the past’. To explain further: by looking at the sedimentological processes of today and the features produced by them we can draw inferences that will tell us about the conditions in which ancient rocks were laid down. For example, we’ve all seen ripple marks on beaches; these are caused by sand particles being moved about by the ebb and flow of a shallow tidal environment. It follows, therefore, that rocks from Hastings, for instance, which show fossilised ripple marks, must have been laid down in a similar environment.

The principal can also be applied to biological entities. We can compare the characteristics of extinct organisms with those of organisms living today. From these comparisons a huge amount of information regarding these ancient life forms’ lifestyles can be gleaned. One example of this can be seen in the dinosaur Edmontosaurus. Scientists believe that an inflatable sac on its snout would have been used as a resonator, enabling calls to be made to other individuals, in much the same way that Elephant seals use a similar nasal sac. 

Picture to illustrate the law of superposition. Lower Cretaceous strata at Hastings.

Pictures to show an example of sedimentological uniformitarianism - modern beach ripples


Pictures to show an example of sedimentological uniformitarianism -
A Lower Cretaceous sandstone slab at Hastings, showing 140 million year old fossil ripples. 

Pictures to show an example of biological uniformatarianism – Edmontosaurus and a male Elephant seal, with their inflatable resonance sacs. (Edmontosaurus picture by Arthur Weasley, Elephant seal picture by B. Navez) 

Types of stratigraphy

There are a variety of ways in which the rock layers around us can be split up, united into groups, or correlated. Three of the most important are set out below.

Lithostratigraphy  This is the study of the physical properties of rock layers, which include grain size, grain roundness, colour, chemical composition etc. Changes in the lithology of rock sections are the most obvious form of stratigraphy that the amateur geologist or palaeontologist can employ. If you can see distinct layers in a cliff for example, you are looking at the changing lithology of that section and can be said to be practicing lithostratigraphy.

Biostratigraphy  This is based on the fact that across sizeable distances the fossils that a rock layer contains will often be the same. Therefore locations remote from each other but with similar fossil assemblages can be shown to be of the same age. Biostratigraphy cannot always be used for correlations however, as all species are restricted by their environments as well as by time.

Chronostratigraphy.  Unlike the previous two branches of stratigraphy mentioned, which only provide relative ages for rocks and fossils, chronostratigraphy is used to assign an actual age to them. This branch of stratigraphy came into being much later than lithostratigraphy and biostratigraphy, due to the high level of technology needed to date rocks. Using chronostratigraphy in conjunction with lithostratigraphy and biostratigraphy, the world’s rocks can quite easily be dated and correlated.

A picture to show an example of lithostratigraphy - Middle Jurassic rock strata at Elgol

A picture to show an example of biostratigraphy – selected Cretaceous (Lower Greensand and Gault) ammonite zones, Folkestone.

Stratigraphic nomenclature

The vast stretch of time that preceded the present day on Earth has now been divided and sub-divided into manageable chunks. These divisions have been made according to changes in the stratigraphy of the rock column; most importantly biostratigraphical changes. Definitions of these time divisions are given below.

Eon  This is the coarsest division of geological time. The Phanerozoic eon, for example, began some 550 million years ago and continues today. This covers the time during which abundant life that readily fossilized has been present on our planet.

Era  The Phanerozoic eon is split up into the Palaeozoic, the Mesozoic and the Ceanozoic eras. These eras are divided by mass extinctions. Between the end of the Palaeozoic era and the start of the Mesozoic some 90% of marine species and 70 % of terrestrial species were wiped out. Between the end of the Mesozoic era and the start of the Caenozoic about 50% of species died out – most notably the dinosaurs and the ammonites.
Eras are sub-divided into periods.

Period  This is the division of geological time most commonly encountered by the amateur palaeontologist. If nothing else, most can confidently state that the rocks of their favorite fossil hunting ground were laid down during the Jurassic, Cretaceous, Silurian, etc. period.

Epoch  An epoch is a division of a period. Many are simply called the Lower/Early, Middle or Upper/Late part or the period they describe, e.g. the Lower Cretaceous, the Upper Triassic, etc. Some, however, have names relating to where the type sections of the epoch are found, e.g. the Ludlow, Wenlock and Llandovery epochs of the Silurian period, which are named after locations in Shropshire, England and Dyfed, Wales.
Interestingly, the Palaeocene, Eocoene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene are not periods as many think but epochs of the Palaeogene (Palaeocene to Oligocene) and Neogene (Miocene to Holocene) periods.  

Stage  The stage sub-division is used by palaeontologists to denote a suite of rock layers within an epoch that all contain a certain easily identifiable and abundant index fossil or fossils. One stage may be represented by numerous different environments around the world however, so stage index fossils are regional in nature.

Zone  Stages can be divided into zones. These are named after fossil species that are abundant, often short lived and relatively restricted to that particular zone.  

Bed  This is a specific layer or stratum of rock that that can be separated from the proceeding and succeeding by some definite change in stratigraphy.