On his return, he sends his sample to the laboratory for dating, and after a few weeks receives the lab report.Let us imagine that the date reported by the lab was 150.7 ± 2.8 million years.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.In the same way, by identifying fossils, he may have related Sedimentary Rocks B with some other rocks.These were then eroded and Sedimentary Rocks B were deposited.
In other words, the age should lie between 197.2 million years and 203.6 million years.Thus, he already ‘knows’ that the igneous dyke must be younger than 200 million years and older than 30 million years.(Creationists do not agree with these ages of millions of years because of the assumptions they are based on.) Because of his interest in the volcanic dyke, he collects a sample, being careful to select rock that looks fresh and unaltered.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.
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.
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