Geologic Time & Radioisotope Dating

We live on an amazing planet with a 4.5 billion year history of life evolving from a single cell to multicellular life to the sheer volume of diversity of species we see through time and walking the Earth today.  How do we know the timeline for this? How do we date the rock units and mountains and stones beneath our feet?
We use simple observation out in the field to look at rock and observe that generally speaking, older rock units tend to be deeper than the younger rock on top. We use index fossils like ammonites or Triassic paperclams to help give us a better understanding. We also turn to chemistry and use the decay rates of radioisotopes to help give us a time stamp on a rock unit. We use Carbon-14 for rock younger than 50 thousand years or Potassium-40 or Uranium for rock older than 50 thousand years. It was through the decay rate of uranium that we arrived at a relative age of the Earth of 4.5 billion years.
We have new techniques evolving out of various fields of science to help us gain a deeper understanding of the Earth. Donald Prothero has been taking deep-sea drilling cores which tell us of shifts in the magnetic field of the Earth. His work will let us date the fossils we find to within 100,000 years — a significant insight over the plus or minus 2-million-year dating that radioisotopes give us.