Final answer:
In geology, the oldest rock layer in an undeformed sequence is usually at the bottom, following the Law of Superposition. Stratigraphy and the fossil record help establish the relative ages of rock layers, while geochronology offers precise absolute dating using radioactive isotopes.
Step-by-step explanation:
In the context of a geologist examining different rock layers visible on a cliffside, the oldest rock layer is typically the one that is located at the bottom. This principle is part of a broader geological concept known as the Law of Superposition, which states that in an unreformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it. Additionally, through the study of stratigraphy, geologists can piece together the Earth's history by examining both the rock layers and the fossils they contain, known as the fossil record. The fossil record provides insights into the evolution of species, their existence on Earth, and instances of extinction.
Furthermore, the age of rocks can be determined more precisely using geochronology, which involves the use of radioactive isotopes found in some rocks. By measuring the relative amounts of parent and daughter isotopes, the absolute age of a rock can be calculated, supplementing information obtained from relative dating techniques. This helps reinforce our understanding of the geological timescale and the extensive age of the Earth.