Knowing the differences between one layer and the next helps archaeologists understand how the past came to be buried. Geologists are also helpful when it comes to identifying the different building stones used by ancient people. Knowing where the stone ancient people used to build their buildings helps archaeologists better understand ancient industry and trade. For instance, the ancient Romans used many beautiful colored marble to decorate their villas, but it isn't until the marble has been identified by geologists that we learn that some of the marble the Romans used had to be transported from thousands of miles away. From that we can begin to understand how well organized and powerful Roman economy and trade used to be.
Geologists can also help identify the components of clay used in making pottery.
This helps archaeologists know whether the clay came from deposits near where the potters made their pots, or whether the pots were imported from faraway places. Geologists also help archaeologists understand how the movement of the earth or the build up of alluvial sands near river deltas affects archaeological sites.