There are no sedimentary strata to record what happened during the intervening interval.
organisms, geologists employ a variety of techniques.
These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events.
Another essential concept in stratigraphy is the unconformity.
An unconformity is a surface upon which no new sediments were deposited for a long geologic interval.
In the example below, the contact between the conglomerate and the granite beneath it appears likely to be a nonconformity.
However, it is possible that the granite may have intruded as a magma within the crust, beneath conglomerate, after the conglomerate formed.Unconformities separate chapters in the geologic history of a given region.For instance, an orogenic episode (a long geologic episode of mountain building) may finally come to end and the eroded mountains may be buried beneath a new sequence of sediments.They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.In the 20th century, radiometric methods of absolute age determination were developed.After being tilted or folded, the older layers of sediment were eroded.