A technique used in the analysis of data from oceanic sediments in which the material retrieved by the core yields information on temperature changes in the ocean through time. These changes, suggestive of climatic variation, help to chart the progress of glaciation and, since they can be dated, the technique assists in the establishment of a chronology for the Quaternary. The cores, some 5 cm. in diameter and up to 25 m. deep, are extracted from the ocean floor. The sediments they contain have a high percentage of calcium carbonate content made up of the shells of small marine organisms and these sediments build up very slowly, from 10-50 mm per 1000 years, but their sequence is uninterrupted. Since these organisms have different temperature preferences depending on species, the relative abundance of the various species changes as the temperature alters. Variations in the ratio of two oxygen isotopes in the calcium carbonate of these shells give a sensitive indicator of sea temperature at the time the organisms were alive. Through the identification of the species, and by the use of oxygen isotope analysis, a picture can be built up of variations in temperature over the millennia. Since various forms of dating (radiocarbon dating, ionium dating, uranium series dating, palaeomagnetism, protactinium/ionium dating) can be used on the carbonate in the shells, absolute dates can be given to the different levels in the core. Thus dates emerge for glaciations and interglacial periods, which can assist in the age determination of archaeological material found in association with these glacial phases. Problems with the technique are the difficulty of correlating oceanic temperature changes with continental glacial and interglacial phases, and the disturbance by animals living on the ocean bottom. The piston corer was developed in 1947.