It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.

Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.

For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.

Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.

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It is clear that the sedimentary rock was deposited and folded before the dyke was squeezed into place.

By looking at other outcrops in the area, our geologist is able to draw a geological map which records how the rocks are related to each other in the field.The atoms of some chemical elements have different forms, called isotopes.These break down over time in a process scientists call radioactive Tas Walker A geologist works out the relative age of a rock by carefully studying where the rock is found in the field.The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.As an example of how they are used, radiometric dates from geologically simple, fossiliferous Cretaceous rocks in western North America are compared to the geological time scale.