Layers Represent Changes: Because the conditions on the Earth's surface may change often and can change rapidly (considering the length of geologic time), different types of rocks may be successively deposited on top of one another. For example, an area next to a major river may be covered by mud deposits during a small flood, but get covered by sand and gravel during a really big flood, every thousand years or so. If the climate changes, the same area may get overrun by sand dunes. If sea level were to rise, the same area may later be covered by beach or marine deposits. Sedimentary rocks therefore generally have lots of layers, with each layer representing one event or time period. Boundaries between layers represent the end of an event (like a flood) or a change in the environmental conditions.