Relative dating of geological events
Sandstone and shale, much of it red in color, with river channel cross-beds, stream ripples, and pond desiccation cracks.Also contains layers of white gypsum where ponds, lakes or coastal lagoons underwent extensive evaporation. Fossils of plants, mostly ferns, and quadruped footprints have been found. The upper part of the group is non-marine and tracks of quadrupeds are found on bed tops.There is a definite mathematical relationship between the ratio of the amount of daughter product isotope to the amount of radioactive parent isotope, and the number of half-lives that have elapsed.
Also, the formations that appear on the simplified stratrigraphy diagram above are marked in the table with the number that corresponds to the diagram.By the definition of a half-life, the amount of parent isotope at each half-life is half of what it was before the half-life elapsed.As the amount of parent isotope decreases by radioactive decay, the amount of the daughter isotope increases commensurately.The table below tracks the decay, half-life by half-life, of a radioactive isotope, and the accumulation of the daughter product isotope that the parent changes into once it decays. There are several different radioactive isotope systems that are used for measuring ages of geologic materials.
For more information on these systems, see the isotopes and half-lives section of the Geologic Time Basics page.
For example, if the age is based on measurements of the amount of uranium-238 ( yr.