Where the rocks are not strongly folded or tilted it is possible to work out the order in which the layers were formed.
The oldest rocks and fossils are at the bottom and the youngest are on top.
The number of tracks increases over time at a rate that depends on the uranium content.
Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.
Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.
The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.
This newer method converts a stable form of potassium (potassium-39) into argon-39.
Measuring the proportions of argon-39 and argon-40 within a sample allows the age of the sample to be determined.This technique is, however, useful for providing relative dates for objects found at the same site.Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone.The technique can, however, provide the relative ages of bones from the same site.Most fossils are found in sedimentary rocks deposited in layers.This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission.