Through looking at layers of rock you can tell what kind of things transpired. If you find Fish fossils you can assume that at one point in time, there may have been a body of water of some sort. if you find certain plants, you can deduce that there was a forest or a grassy pasture. Or if there are variations in the layer you may be able to tell what kind of soil was there at one point in time, or maybe if there is volcanic rock, you could rightly assume there was a volcanic eruption at some point.
