A dating method based on the examination of tephra (volcanic ejecta); in areas of repeated activity it is often possible to recognize distinctive events within a pyroclastic succession, and to use such markers for local correlation (see marker bed). The succession, so established, provides useful data on the history of the volcano as well as a guide to magmatic and geochemical changes operating below ground. In many cases, well removed from the volcanic activity, thin ash falls may be found within sedimentary successions. Such thin horizons are regarded as isochronous and can be used in correlation. The ash is usually altered to form bentonitic clays (see bentonite), and because these clays still contain traces of volcanic material they can be dated by radiometric means. This is especially important where the intervening strata contain fossil remains (e.g. of early humans in Africa) which can be calibrated by using the dates of the ash falls. Good examples of this application in regional stratigraphy occur in the Lower Palaeozoic strata of southern Scandinavia and the mid-Cretaceous sediments of the western interior of the USA.