Final answer:
Geologists use radiometric dating of igneous rocks above and below a sedimentary layer to bracket their age. Other methods, such as biostratigraphic correlation using fossils, also support the dating of the sedimentary layers.
Step-by-step explanation:
To date a sedimentary rock layer between two igneous rock units, geologists use radiometric dating and assume that the sedimentary layer's age is bracketed by the ages of the igneous rocks above and below it. Using radiometric dating, they measure the decay of radioactive materials within the igneous rocks to determine their ages. Since sedimentary rocks can't be dated as precisely using radiometric dating—because this method dates the time of crystallization of minerals, not the time of sedimentation—geologists apply the principles of relative dating. These principles include the law of superposition, which states that in undisturbed stratigraphic sequences, the sedimentary layer in question must be younger than the rock beneath it and older than the rock above it.
Geologists use other methods such as examining fossil content for biostratigraphic correlation to help support the dating of the sedimentary layer. Fossils can be particularly useful in correlating sedimentary rock layers across different areas when they contain the same species known to have lived during a specific period.