Running water has materials such as dirt, sand, and dead plants and animals in it. When this water ends up in a lake, the materials it was carrying fall to the bottom of the lake and form layers. A layer is thicker when more water enters the lake. For example, thick layers form during times of heavy rain, and thin layers form during times of little rain. Sometimes lakes dry up. The bottoms of dry lakes can change into rock. This rock will still have layers. A geologist studied one of these rocks made from the bottom of a lake. Which rock layer formed during the wettest season? Layer