In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.
By 1904 physicist Ernest Rutherford showed how this decay process could act as a clock for dating old rocks.
When it comes to dating archaeological samples, several timescale problems arise.
For example, Christian time counts the birth of Christ as the beginning, AD 1 (Anno Domini); everything that occurred before Christ is counted backwards from AD as BC (Before Christ).
In a stratigraphical context objects closer to the surface are more recent in time relative to items deeper in the ground.