Final answer:
Scientists infer the age of the rock when they find trilobite and brachiopod index fossils in distant cliffs, as these fossils suggest the rocks were formed during the same time period due to biostratigraphic correlation.
Step-by-step explanation:
When scientists find trilobite and brachiopod index fossils in cliffs that are a great distance apart from each other, they can infer A the age of the rock. These index fossils are known to have existed during a specific geologic time, which makes them highly useful in the process called biostratigraphic correlation. By identifying the presence of these fossils, scientists can conclude that the rocks in which the fossils were found are likely from the same time period, even if the rocks are located far apart.
Index fossils are helpful in establishing geologic dating because they lived through narrow time frames, were widespread, abundant, and easy to identify. So, regardless of the sediment type, matching index fossils indicate that the rocks were deposited at the same time. This kind of dating technique is part of using relative dating methods, which do not determine the exact age in years but establish a sequence of events and relative ages of rock layers and the fossils contained within them.