Article about dating
This problem period may be even larger because: (1) some dating authorities believe that the effective range for K-Ar doesn't begin until about 400,000 ya, and (2) many of the older fossils are found at sites that lack the volcanic rocks necessary for K-Ar dating and hence cannot be dated by this method at all.
Although young-earth creationists challenge the legitimacy of all of the dates obtained by the long-term radiometric methods, even evolutionists are beginning to admit that this dating gap presents a problem for them.
Relative dating does not provide actual numerical dates for the rocks.
Fossils are important for working out the relative ages of sedimentary rocks.
At least 406 human-fossil individuals are placed by evolutionists in this 40,000-to-200,000 ya time-period gap and hence are questionably dated. The inability of the radiocarbon and the K-Ar methods to cover this time period explains why many alternate dating methods have been devised to attempt to give coverage in this area.
Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.