Sedimentary Rock Layers Sedimentary rocks start out as grains and pieces of material. Sand, mud, and gravel are sediments that drop to the bottom of lakes and rivers.
Sediments fall to the bottom of the ocean, too.
The shells of small ocean animals form a large part of some ocean sediments. As time passes, and more and more material is deposited, sediments are squeezed. Sediments often are cemented together into rock. Rock layers at the surface come from the most recently deposited sediments, and each layer down is older than the one above it. Rock layers at the surface come from the most recently deposited sediments, and each layer as you cut down is older than the one above it.