From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
The geologist may have found some fossils in Sedimentary Rocks A and discovered that they are similar to fossils found in some other rocks in the region.
He assumes therefore that Sedimentary Rocks A are the same age as the other rocks in the region, which have already been dated by other geologists.
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.
In other words, the age should lie between 197.2 million years and 203.6 million years.
However, this error is not the real error on the date.
In fact, he would have been equally happy with any date a bit less than 200 million years or a bit more than 30 million years.
They would all have fitted nicely into the field relationships that he had observed and his interpretation of them.
by Tas Walker A geologist works out the relative age of a rock by carefully studying where the rock is found in the field.