The stratigraphic principle consists of three parts:
- 1) the principle of lateral expansion of rock strata , or the principle of horizon stability , that is, rocks that occur in different places but have exactly the same properties are also the same rock strata.
- 2) the principle of original horizontality , or the principle of horizontal deposition of sediments
- 3) the principle of superposition , or the principle of the storage sequence, that is, sediment layers are deposited in a chronological order, from the older in the horizontal ("below") to the younger in the hanging wall ("above").
These principles were formulated for the first time in 1669 by the Danish naturalist Nicolaus Steno (1638–1686) in his work De solido intra solidum (“From the feast within the feast”) after he had examined the rocks of the Apennines near Florence .
The stratigraphic principle allows statements about the relative age of the rocks, but not about their absolute age. In principle, the younger layer could have formed a few days after the older one, or it could not have formed until millions of years later.
Steno's train of thought
Nicolaus Steno assumed that all rocks and minerals were once liquid and that rock layers are formed as particles gradually fall out of an emulsion in water and sink to the bottom. This process would therefore horizontal ( söhlige generate) layers, it is in the layers at the base of the oldest and youngest to in those above.
Steno was already aware, however, that other geological processes could lead to apparent exceptions to his deposit laws. He attributed these deviations to later disturbances in the rock compound. He argued that subterranean rivers may have eroded deep layers and that the collapse of the cave then carried large parts of the higher layers into the depths. He also suspected that rocks could be lifted up by underground forces.
Like his contemporaries, Steno attributed the formation of most rocks to the biblical flood . Nevertheless, he noticed that the lower layers of the two most important rock types in the Apennines contained no fossils , while the higher layers were very fossil-rich. He therefore suggested that the upper strata formed during the Flood, after life was created, while the lower strata formed before there was life on earth. This represents the first known attempt in the history of geology to distinguish between certain periods in the history of the earth .
Today it is known that there are indeed exceptions to the principle of horizontal deposits. The most famous example are sand dunes . There are slopes with a gradient of up to 15 °, where the internal friction between the individual grains prevents the embankment from sliding down to a flatter angle. Similarly, sediments can cover a pre-existing sloping surface. In addition, sediment layers do not extend infinitely on all sides, but rather wedge out sooner or later, which also requires slight deviations from the horizontal during the deposition.
Molten rock can work its way through the surrounding rock and sometimes penetrate between two older layers, which is also an exception to the stratigraphic principle.
On the other hand, the stratigraphic principle basically even applies to rocks that are not separated from water, but from igneous eruptions or from the air, such as volcanic effluent rocks and ashes, and aeolian sediments transported by the air .
However, misalignments of the rocks through folding and faults can make the investigation of a stratigraphic sequence considerably more difficult. Due to tectonic thrusts , for example, it can happen that, contrary to expectations, older rocks are found on top of younger ones. The reconstruction of the actual bedding conditions is made more difficult by the fact that thrusts often only have a small angle to the stratification of the thrust rocks or are sometimes even formed parallel to the strata. In fact, apparently violations of the stratigraphic principle in thrust zones are often cited by creationists and creation scientists as evidence to cast doubt on the general fundamentals of geology. Despite all of this, such processes leave visible traces that can be deciphered. For example, the rocks along fault lines are mostly cracked, broken or even mineralogically transformed .
- Hermann Wieh: Niels Stensen - His life in documents and pictures . Wuerzburg 1988
- Alan Cutler: The shell on the mountain. About Nicolaus Steno and the beginnings of geology. Albrecht Knaus Verlag, Munich, 2004, ISBN 3-8135-0188-4
- W. Kenneth Hamblin: The Earth's Dynamic Systems, A Textbook in Physical Geology. Burgess Publishing Company, Minneapolis, Minnesota, 1978, p. 115: "The Principle of Superposition and Original Horizontality"
- Principles of Archaeological Stratigraphy. London & New York: Academic Press. ISBN 0-12-326650-5