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Transcript of Stratigraphy
Stratigraphy It is the interpretation of sequence of materials such as rocks sediments as a series of events in the evolution of the Earth
It is the key to Earth history and environmental change on many different timescales
It is the art of detection as every layer or stratum has a tale to tell of its geography, climate, ecology at a particular time
What does the stratigrapher do? Observes
In terms of processes and events
Stratigraphic tools Establishes the sequence of natural materials present
Works out the chronological order of these materials
Interprets the environment in which each unit was deposited
Establishes the changing environmental conditions of the site
How do we establish the sequence of events? Use scientific principles
Not laws as there can be exceptions to them but they are good guidelines
First recognised and developed by Steno in the 17th Century (a Dane working in Italy)
What are Steno’s Principles? Principle of Superposition
Principle of original horizontality
Principle of original lateral continuity
Principle of original lateral continuity
The layer of sediment continues over an area infinitely unless interrupted.
The stratigrapher’s job is to explain the absence of continuity
Other Principles used Principle of Uniformitarianism “Present is the key to the past”
Principle of inclusions
Principle of Cross cutting relationships
Principle of Cross cutting relationships Whatever cuts across something else must be younger than that through which it cuts.
i.e. The new knife cuts through the older layers of cake
Stratigraphy application for forensics It shows us change from the normal natural events within forensic biology/science
It is a method of relative dating events
Analysis gives a view in 4 dimensions Strata layed down over time Most archaeological events can be seen as aggregate of small scale events
Each layer of strata can be viewed as a frozen moment in time
Biostratigraphy – Fossils can be found in the layer of strata making it possible to show the relative date of that particular layer.
Chronostratigraphy – Seeks to derive the absolute date of the stratigraphic layer
Practical use of the principal of inclusions An event such as an earthquake or volcano can cause strata to be deposited ontop of a piece of evidence. Previous and subsequent layers of strata may be dated and be used to date the disaster.
e.g. Temple at Archanes – Anemospilia caught in the pyroclastic flows of Vesuvius.
An event such as the sinking of a ship will lay dormant as layers of strata build up around it showing the time at which it sank.
e.g. the Mary rose
This principal can apply to any item, as anything may be a piece of evidence Burial Practical application of crosscutting In situ items Items within graves are more likely to remain there and may be dated easily by cronostratigraphy.
Some items may be more preserved than if they were exposed.
e.g. Parasite eggs were found in the hair, clothes and tissue of bodies in Korean graves, that were not found in the surrounding soil.
Grave robbing The instance of grave robbing may be seen as a crosscutting event also, but in a more complex way. Grave robbing The cross cutting from the burial is, itself cut by the crosscutting of the grave robber Burial earth The earth used in the burial can itself be used as evidence and also hold imprints providing evidence of what is no longer there. Burial earth If earth has been transported, it may be possible to trace its original location.
Earth may contain pollen or other plant matter that can not only be used to trace the location of the earth but also give an idea of the time of year of the burial.
Impressions Impressions left by digging material can give an idea of what tools where used
e.g. bulldozer tracks left in soil laid between layers of bodies in a Bosnian mass grave.
Impressions left by items or bodies in soil give an idea of what was once there.
The type of soil can hold impressions to a greater or lesser extent depending mostly upon grain size. Sandy or clay soils hold a good impression, gravel does not.
Fluids such as fats, leache from the bodies and will enter the soil making it possible in some cases to determine how long the body has been in the ground by comparing the soil composition close to and far from the body along one strata. Mass graves Mass graves provide their own strata of bodies laid one on top of another, and through computer simulation it is possible to recreate the order of events, however the pressure from above can cause displacement of limbs as bodies lower in the grave decay.
Decay is slower in Mass graves, making it possible to identify items for a longer period of time.
Bioturbation Top strata can be merged by action of ploughing, burrowing animals and worms.
e.g. In some Guatemalan graves the uppermost 40 cm of stratification is indistinct due to its active organic nature.
References Brookfield M.L. 2004 Principles of Stratigraphy, Part II Chs 5,-7
Goldberg P & Macphail R.I, 2006, Practical & theoretical Geoarchaeology, Ch 2, 29-41
Hanson I.D. 2004, The importance of stratigraphy in forensic investigation in Pye K & Croft D.J., 2004, Forensic Geoscience: Principles, techniques & applications, 39-47