Rose Marie Russell, age 12, of Brooks, Alta., Canada, for her question:

Why are most sedimentary rocks in flat layers?

Any type of rock can be bent or creased by the earth to form mountains or valleys. These crustal ups and downs contain layers of igneous, metamorphic and sedimentary rocks. However, the major part of the flat areas is composed of limestone, sandstone, slates, shales and other sedimentary rocks, though other types are present.

Sedimentary rocks begin to form as silty sediments of sand and mud that fall to the floor of the sea. As a rule, they form there like thick, flat, cozy carpets. Later, when the water above them dries up, they become solid layers of sedimentary rock and often remain as flat areas of the land for countless ages. In time they are buried under newer layers, and some of them become twisted and bent to form valleys and heaving mountains.