Law of superposition

The geological stratigraphic principle (also called " Stratigraphic Basic Law " or storage rule) is the fundamental principle of stratigraphy in geology. It consists of three parts:

• 2) the principle of original horizontality, or principle of horizontal deposition of sediments

• 3) the principle of superposition, or principle of the storage sequence, that is, layers of sediment are deposited in a time sequence, the older in the footwall ( "down" ) to the younger ones in the hanging wall ( "up" ).

These principles in 1669 for the first time by the Danish naturalist Nicolaus Steno ( 1638-1686 ) in his De solido intra solidum formulated ( "From festivals in Festen" ) 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 on their absolute age. The younger layer could in principle have formed a few days after the older or even until millions of years later.

Steno's reasoning

Nicolaus Steno was assumed that all rocks and minerals had once been fluid and that form layers of rock particles by turn gradually from an emulsion in water and sink to the ground. This process would thus generate horizontal ( söhlige ) layers, it is in the layers at the base of the oldest and among those above the youngest.

Steno was, however, already aware that other geological processes could lead to apparent exceptions of his deposit laws. These deviations, he led back to later disturbances of the rock association. He argued that underground rivers could have eroded deep -lying layers and that then the collapse of the cave have transported large parts of higher layers in depth. He also suspected that rocks could be lifted by underground forces in the air.

Like his contemporaries Steno led the formation of most rocks on the biblical flood back. However, he noticed that the deeper layers of the two main rock types in the Apennines contained no fossils, while the higher-lying were very rich in fossils. He suggested, therefore, that the upper layers would have formed during the flood, after the creation of life, while the lower had arisen before there was life on Earth. This represents the first known attempt in the history of geology, to distinguish certain periods in the history of the earth from one another.

Present State

It is now known that there are indeed exceptions to the principle of horizontal deposition. The most common example is sand dunes. There are relationships of up to 15 ° slope, in which the inner friction between the grains prevents the embankment from slipping to a shallower angle. Similarly, sediments can cover an existing sloped surface. In addition, sediment layers do not extend infinitely in all directions, but wedges out sooner or later, which also requires slight deviations from the horizontal during deposition.

Molten rock can make their way through the surrounding rock and sometimes penetrate between two older layers, making it also an exception to the stratigraphic principle.

On the other hand, the stratigraphic principle applies in principle even for rocks that are not deposited from water, but from magmatic eruptions or from the air, such as volcanic igneous rocks and ash, and transported over the air aeolian sediments.

However, adjustments of rocks by folding and faults can significantly complicate the investigation of a stratigraphic sequence. Due to tectonic thrust faults may occur, for example, that contrary to expectations, older rocks are found on younger. The reconstruction of the real storage conditions is complicated by the fact that thrust faults often have only a small angle to the layering of the overthrust rocks or occasionally even layer formed parallel. In fact, just apparent violations of the stratigraphic principle in thrust zones of creationists and creation scientists are often cited as evidence to draw the general principles of geology in doubt. Despite all this leave such processes visible traces that can be decrypted. For example, the rocks along fault lines are usually cracked, broken or even converted mineralogical.