We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.
Geologists use this type of method all the time to establish relative ages of rocks.
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co.
Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.
Learn how inclusions and unconformities can tell us stories about the geologic past.
We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!
Other times, geologists discover patterns in rock layers that give them confusing information.
There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.
In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. sediments, which are deposited and compacted in one place over time.