Sedimentary rocks form when:

* Layers of small particles (sediments) are deposited: These sediments can come from a variety of sources, like weathered rocks, the remains of organisms, or volcanic ash.

* These layers are compacted: Over time, the weight of overlying layers presses down on the sediments, squeezing out water and air.

* The sediments are cemented together: Minerals dissolved in groundwater precipitate between the sediment particles, acting like glue to bind them together.

So, it's not just compression, but also cementation that plays a crucial role in forming sedimentary rocks.