Roberto Pincente, age 10, of Toronto, Ontario, Canada, for his question:

How do rocks form in layers?

Some of the nicest looking rocks are arranged in neat, flat layers that remind you of the pages in a book. Almost always, these rocks are formed under water that contained a lot of mud or other floating debris. Dust, as we know, tends to settle over everything in flat layers. In somewhat the same way, floating debris tends to sink down through the water and settle in flat layers on the bottom.

Seas and lakes contain all sorts of floating debris, such as silty clay and specks from tiny shellfish. These fragments filter down and in time they spread themselves in flat layers on the bottom. Later, the region may rise up above the water. Then the moist flat layers dry out and become flat layers of solid rock. We call them sedimentary rocks because they are formed from dregs or sediments that settled down through the water.