||There are basically three lines of investigation that are involved in geochronology ? the dating of rocks and other earth materials. First, there are the physical and chemical dating methods which give us numerical estimates of age. Second, there is the reconstruction of the order of events in sections of sedimentary rocks ? those laid down by water and wind. What is done is to find rocks which can be dated within a sequence of strata in order to establish the ages of fossils found within it, since the fossils themselves are not directly datable. The third line of investigation is called geologic correlation, in which one establishes that two or more events happened at the same time, or very nearly so.