Fluoride ions are present in trace amounts in most soils and groundwaters.Over time, buried bones pick up fluoride ions from soil moisture or exposure to groundwater.The slope of the line determines the date, and the closeness of fit is a measure of the statistical reliability of the resulting date.

Aspartic acid is the compound most often used because it has a of 15,000-20,000 years and allows dates from 5,000-100,000 years to be calculated.From the amount of absorbed fluoride in the item, the time that the item has been in the soil can be estimated.Many instances of this dating method compare the amount of fluorine and uranium in the bones to nitrogen dating to create more accurate estimation of date.Here is one example of an isochron, based on measurements of basaltic meteorites (in this case the resulting date is 4.4 billion years) [Basaltic1981, pg. Skeptics of old-earth geology make great hay of these examples.For example, creationist writer Henry Morris [Morris2000, pg.As we pointed out in these two articles, radiometric dates are based on known rates of radioactivity, a phenomenon that is rooted in fundamental laws of physics and follows simple mathematical formulas.Dating schemes based on rates of radioactivity have been refined and scrutinized for several decades.