Here's why:

* Formation: Sedimentary rocks form from the accumulation and cementation of sediments (like sand, mud, or organic matter). These sediments are deposited in layers over time.

* Compression: As more layers are deposited, the weight of the overlying material compresses the lower layers, forming solid rock.

* Evidence of Time: Each layer represents a different period of time and environment, offering valuable insights into Earth's history.

Examples:

* Sandstone: Shows distinct layers of sand grains.

* Shale: Fine-grained layers of clay.

* Limestone: May have layers of fossils or different types of calcium carbonate.

While not all sedimentary rocks have perfectly defined layers, the layered structure is a defining characteristic.