It is also an ash deposit, one that can be found globally. In the southern US and Mexico it includes small bits of silicate glass called tektites. Tektites are flash frozen bits of ejecta produced by meteorite impacts. Say you have identified time-stratigraphic systems in two widely separated areas. One way, the only way available to geologists until late this century, is through the use of fossils.
Fossils are the remains of ancient organisms found in the rock record.
The ongoing evolution of fossil life forms provides a relative geologic clock. The last means of relative dating exploits cross-cutting relationships in the rock record. Ditto faults, which must be younger than the strata they displace. The Geologic Time Scale European geologists used the stratigraphic succession as inferred from superposition, fossils and cross-cutting relationships to assemble a relative time scale. It is an amazing accomplishment that allowed geologists everywhere to put local rocks into a global sequence.