Geologists use and fossils to infer what the environment of a place was like when a layer of it was forming.

rocks

Explanation:

A geologist is a person who studies the rocks and the earth's surface to find different phenomenon. The study of rocks and he fossils found there helps the geologist to understand and  conclude the type of climate and the environment it formed the fossils of a place on earth. It provides large amount of information about the climatic conditions of the place when the layer of it was forming.