Final answer:
Geologists determine the stratigraphy of a layer when using a sequence of layers to find its age. Stratigraphy involves studying the sequence and structure of soil and rock layers, which helps to establish relative ages.
Step-by-step explanation:
When geologists use a sequence of layers to find an age, they are finding the stratigraphy of the layer. Stratigraphy refers to the study of rock layers and layering within the geological field. This concept is critical in understanding and determining the relative ages of rock layers and the artifacts found within them.
Geologists and archaeologists can use stratigraphic information gathered from the sequence and structure of the soil and rock layers to ascertain the relative ages of phenomena or artifacts observed in a particular area. They employ methods such as stratigraphic superposition and sequence stratigraphy which rely on the interpretation of layer sequences and the principle that older layers are generally situated below younger ones.
By identifying the order and relative positions of strata, as well as using techniques in lithostratigraphy and biostratigraphy, geologists can draw conclusions about the depositional environment, geological events, and the temporal context of any given set of strata. This field of study encapsulates the broader study of historical geology, revealing much about Earth's history and the development of natural and cultural layers over time.