Here's a breakdown:

* Sediment: Loose materials like sand, mud, or shells that settle at the bottom of bodies of water.

* Pressure: Over time, more and more sediment accumulates, creating immense pressure on the lower layers.

* Lithification: This pressure, along with chemical changes and the cementation of minerals, transforms the loose sediment into solid rock.

So, the correct answer is not a single word but a process: lithification.