Both the physical geologists and paleontologists could point to evidence that much more time was needed to produce what they saw in the stratigraphic and fossil records.As one answer to his critics, Kelvin produced a completely independent estimate -- this time for the age of the Sun.For example, the element Uranium exists as one of several isotopes, some of which are unstable.When an unstable Uranium (U) isotope decays, it turns into an isotope of the element Lead (Pb).The methods work because radioactive elements are unstable, and they are always trying to move to a more stable state. This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay.
Different methods of radiometric dating can be used to estimate the age of a variety of natural and even man-made materials.
There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.
As one example, the first minerals to crystallize (condense) from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to 4568 plus or minus 2 million years....!! Other events on earth can be dated equally well given the right minerals.
This is an enormous branch of geochemistry called Geochronology.
It is an accurate way to date specific geologic events.