So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks. Radiopotassium dating is used for establishing chronologies of ancient hominin sites. Helens Y tephra ash dated at years BP, and the lower white line is from the Mount Mazama eruption that took place almost years ago courtesy Jerome Cybulski. The process goes on continuously, and the fluorine-content of the bone or tooth increases in course of time.
Radioactive elements are common only in rocks with a volcanic origin, so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers. Relative Dating Stratigraphy and biostratigraphy can in general provide only relative dating A was before B , which is often sufficient for studying evolution. This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks.