The sedimentary rocks that form when existing rocks are broken down by weathering and erosion, then compacted together under pressure are called clastic sedimentary rocks.

Here's a breakdown:

* Weathering: This is the process where rocks are broken down into smaller pieces by forces like wind, rain, ice, or chemical reactions.

* Erosion: This is the process of transporting those broken-down rock fragments (sediments) away from their original location by wind, water, or ice.

* Compaction: As sediments accumulate in a basin, the weight of the overlying sediments presses down on the lower layers, squeezing out water and air. This process compacts the sediments together.

* Cementation: Minerals dissolved in the water within the sediments crystallize and act like glue, binding the sediment particles together.

Some examples of clastic sedimentary rocks include:

* Sandstone: Formed from sand-sized grains.

* Conglomerate: Formed from gravel-sized fragments.

* Shale: Formed from very fine-grained clay particles.

Let me know if you'd like to learn more about any of these rocks!