Sedimentary rocks form when layers of sediment (tiny pieces of rock, sand, mud, shells, or plant material) pile up over long periods and eventually harden into solid rock. Think of it like making a peanut butter and jelly sandwich: each layer you add stays separate and visible, just like the stripes you can see in sedimentary rocks.

These rocks tell stories about Earth's past. When rivers carry sand and mud to the ocean, the particles settle on the bottom in layers. Over millions of years, the weight of new layers pressing down turns the old ones into rock. Scientists can read these layers like pages in a history book: a layer of sandstone might show where an ancient beach once existed, while a layer full of tiny shells reveals an ancient sea.

The most famous sedimentary rocks include sandstone, limestone, and shale. The Grand Canyon's colorful stripes are sedimentary layers, each one representing a different era in Earth's history. Fossils form almost exclusively in sedimentary rock because the gentle layering process can preserve delicate bones and shells, unlike the intense heat of volcanic rocks or the crushing pressure that creates other rock types.