It was produced during an enormous explosive volcanic eruption which probably lasted only a few days. Ash within the layer may have taken several weeks to months to fall, but in terms of geologic time, it was deposited instantaneously. It represents an isochron in the stratigraphic record. 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.