The type of rock formed when layers of loose materials are compacted and cemented together is called sedimentary rock.

Here's a breakdown:

* Loose materials: These can include sand, silt, clay, pebbles, shells, or even fragments of other rocks.

* Compaction: Over time, the weight of overlying layers presses down on the loose materials, squeezing out water and air, causing the particles to become tightly packed.

* Cementation: Minerals dissolved in groundwater precipitate between the particles, acting like glue to bind them together.

This process is called lithification, and it is how sedimentary rocks are formed.