Coral and absolute dating
To serve as geochronometers, the records must be complete and the accumulation rates known.
The fossiliferous part of the geologic column includes perhaps 122,000 metres of sedimentary rock if maximum thicknesses are selected from throughout the world.
This idea is sound in principle, provided that all the other natural variables remain constant.
Soil permeability, rainfall, temperature, and the concentration of fluorine in groundwater all vary with time and location, however. In a restricted locality where there is uniformity of climate and soil, the extent of fluorine addition is at least a measure of relative age and has been so used with notable success in dating certain hominid remains.
Sometimes human observation can be maintained long enough to measure present rates of change, but it is not at all certain on a priori grounds whether such rates are representative of the past.
Specimens that have been exposed to fire or to severe abrasion must be avoided.Our editors will review what you’ve submitted and determine whether to revise the article.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work!In spite of this tremendous spread, most geologists felt that time in the hundreds of millions of years was necessary to explain the sedimentary record.Castile Formation, for example, consists of alternating layers of gypsum and calcite that may reflect an annual temperature cycle in the hypersaline water from which the minerals precipitated.
In the thick oil shales of Wyoming and Colorado in the United States, the flora is not so well defined, but layers alternating in organic richness seem to communicate the same seasonal cycle.