NSF Postdoctoral Research Fellow at Stony Brook University.
I study how bone tissue, growth, and metabolism evolve at macroevolutionary time scales.
Major boundaries in Earth's time scale happen when there were major extinction events that wiped certain kinds of fossils out of the fossil record.
When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.
This all has to do with describing how long ago something happened. There are several ways we figure out relative ages.
The simplest is the law of superposition: if thing A is deposited on top of (or cuts across, or obliterates) thing B, then thing B must have been there already when thing A happened, so thing B is older than thing A.
Venus, Io, Europa, Titan, and Triton have a similar problem.
On almost all the other solid-surfaced planets in the solar system, impact craters are everywhere. We use craters to establish relative age dates in two ways.
In fact, I have sitting in front of me on my desk a two-volume work on is not light reading, but I think that every Earth or space scientist should have a copy in his or her library -- and make that the latest edition.