From the geological point of view, stratigraphy is all about layering, sequencing, composition, age and distribution of sediments and layered rocks. Stratigraphy can give us information about the sequence of the development of life, glacial history, landscape development, and much more. The main principle is that younger layers remain piled over older lays, assuming they have not been disturbed. The layers can be identified and dated according to their properties using with different methods.
The subdivision of layer series will be based on different properties and attributes of the layers. The classification of rock units on the basis of their physical and mineralogical properties and relationships to surrounding rocks is called litostratigraphy. Biostratigraphy is used to divide layers or successions of layers into units (biozone) based on the presence of one or more fossils that are characteristic of the zone. In the field of quaternary stratigraphy, it has been the tradition to define stratigraphical units and derived units of time using paleoclimatic criteria. A climastratigraphic unit is a stratigraphic unit whose boundaries are defined by geological evidence of climate change. The climastratigraphic units have formed the physical reference basis for the chronostratigraphic units in the Quaternary stratigraphy.
Quaternary Geology is a discipline that deals with the youngest period in Earth's history -- circa the past 2.6 million years. This period is characterized by major climate fluctuations and transitions between glacial and inter-glacial periods. The last glacial period, known as the "Weichselian" in Northern Europe, started c.117, 000 years ago and ended c.11,700 years ago.
After this began our current inter-glacial interval called Holocene. During both glacial age and interglacial periods, there have been greater and lesser climate fluctuations. Cold phases within the ice ages are called stadials, while the milder stages are called interstadials. These has cold phases also have name and chronologic position in the stratigraphical hierarchy. At the very end of the last ice age (Weichsel) glaciers began to grow in a cold period that lasted about 1000 years, a period now called the Younger Dryas.
The age of these climate events, and many others, can be determined by a range of methods: Radiocarbon dating, Optic Stimulated Luminescence, Tephrostratigraphy and Paleomagmetism and Exposure dating:
- 14C dating (Radiocarbon dating) can be applied to plant and animal fossils that are under 50,000 years old. The method is based on the fact that all living organisms take up the radioactive 14C isotopes in a certain amount relative to non-radioactive 12C isotopes. When the organism dies and carbon absorption stops, the radioactive breakdown of 14C isotope results in the decrease of istope14C. By measuring the difference between the two it is possible to determine when the organism lived.
- Optically Stimulated Luminescence (OSL) can be used to date sandy sediments that were exposed to sunlight at the time of deposition. While sediments have been covered by other sediments, they are exposed to radioactive background radiation that affects the atoms in some of the minerals. During optical stimulation in the laboratory the atoms will return to their normal state and send out a light that can be used to calculate the amount of time the sediment has been buried. By this method, one can date sediments that are hundreds of thousands of years old.
- Exposure dating can be used to date both bedrock, and large blocks, to find out how long they have been exposed to cosmic rays. Rock surfaces have been exposed in nature to high energy cosmic radiation, which has resulted in the formation of a variety of radioactive isotopes in the mineral structure. The amount formed depends on how long the surface has been exposed to cosmic radiation. This group of methods has about the same range in age as the OSL method.
- Tefrostratigraphy is based on the fact that volcanic eruptions lead to the distribution of ash throughout the atmosphere, which become deposited over large areas. These tephra layers provide time-synchronous marker horizons. Once the age of a particular layer of ash in a sediment is determined with other dating methods, the age may be applied wherever this ash layer is present. In this way, researchers are able to correlate data from ice cores, marine sediment cores and sediments on land.
- Paleomagnetism involves using magnetic properties of a sediment to provide a relative age; that is, the age in relation to something else. Magnetic mineral grains that fall down and deposit on the seabed, for example, will orient themselves to Earth's existing magnetic field. The geographical position of the magnetic poles, however, continuously move, and the North and South Pole have switched places several times. Only in the course of the last 5 million years, the Earth's magnetic field was reversed almost 30 times. Data on the polar wandering and polar reversals of the Earth's magnetic field are both used to calculate the age of the sediments.