Sedimentary rock is the great record-keeper of Earth's history. Here's how it's made, step by step:

1. Weathering — wind, water, and ice break existing rock into small bits: sand, silt, and mud.
2. Transport & deposition — rivers and wind carry those bits and drop ("deposit") them, often underwater, where they settle into flat layers.
3. Burial & hardening — more layers pile on top; their weight presses the bits together while natural cement glues them. Over long times this hardens into rock: sandstone from sand, shale from mud, limestone from shells and sea life.

Those layers are called strata (a single one is a stratum), or simply beds — the bands you see in the wall of a canyon.

How layers reveal age

Here's the powerful idea, called the law of superposition: in a stack of undisturbed layers, the oldest layer is at the bottom and the youngest is on top — because the lower layers were laid down first. Reading layers from bottom to top is like reading history from oldest to newest.

Fossils — the preserved traces of ancient life — form almost entirely in sedimentary rock, because gentle burial in layers protects them. Matching fossils between far-apart rocks helps geologists line up their ages.

Just how old are we talking?

Geologists use a geologic time scale stretching back about 4.5 billion years, divided into long chunks (eons, eras, and periods). A couple of anchors to give you a feel: the famous dinosaurs died out about 66 million years ago, while our own species is only a few hundred thousand years old — a blink right at the end of the story.

(spacing is not to scale — 66 million years is really only the last 1.5% of Earth's history)