A discipline involving the description and interpretation of layered sediments and rocks, and especially their correlation and dating. Correlation is a procedure for determining the relative age of one deposit with respect to another. The term “dating” refers to any technique employed to obtain a numerical age, for example, by making use of the decay of radioactive isotopes found in some minerals in sedimentary rocks or, more commonly, in associated igneous rocks. To a large extent, layered rocks are ones that accumulated through sedimentary processes beneath the sea, within lakes, or by the action of rivers, the wind, or glaciers; but in places such deposits contain significant amounts of volcanic material emplaced as lava flows or as ash ejected from volcanoes during explosive eruptions.