What role do fossils play in geological mapping?

Fossils are essential in geological mapping because they offer clues about the age and environmental conditions of rock layers. The presence of specific fossils can help geologists establish the relative age of the rock formations, as certain organisms lived during specific geologic time periods. This information, known as biostratigraphy, allows for the correlation of rock layers across different regions.

Additionally, fossils provide insights into the paleoenvironment, helping geologists understand the conditions under which the rocks were formed. For example, fossils of marine organisms indicate that the area was once underwater, while land organism fossils suggest terrestrial environments. This context helps to reconstruct the geological history of an area, making fossils a vital tool for unraveling the complexities of Earth's past.

The other options, while relevant to geology, do not capture the primary function of fossils in geological mapping as effectively. For instance, while fossils might indirectly inform on water sources or soil composition, their most significant contribution lies in age determination and environmental context.