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.
In a related article on geologic ages (Ages), we presented a chart with the various geologic eras and their ages.
In a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating techniques.
When I first got involved in the creationism/evolution controversy, back in early 1995, I looked around for an article or book that explained radiometric dating in a way that nonscientists could understand. Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.
All these methods point to Earth being very, very old -- several billions of years old.
No matter when Kennewick Man died, his bones were exposed to the flooding of the Columbia River, plus the bones were exposed to rain, humidity, moisture in the soil (i.e.