Volcanic ash as kaolin tonstein, cast in synthetic resin

Kaolin tonsteins are formed from volcanic ashes. They can be radiometrically dated. In the Ruhr area they occur at different levels of the Upper Carboniferous strata, mostly in coal seams. They were also deposited in an area of up to 400 kilometers. Given these characteristics, they serve, among other things, as isochronous time markers and to subdivide the geological layers.