Here's how it happens:

* Sediment Accumulation: Clay and mud particles settle at the bottom of bodies of water like lakes, oceans, or swamps.

* Compaction: Over time, the weight of overlying sediment compresses the clay and mud, squeezing out water and reducing the volume of the sediment.

* Cementation: Minerals dissolved in groundwater precipitate between the sediment particles, acting like glue to bind them together. This process is called cementation.

* Lithification: The combination of compaction and cementation transforms loose sediment into solid rock.

Shales are fine-grained sedimentary rocks that are fissile, meaning they split easily into thin layers. They are primarily composed of clay minerals, but can also contain some silt and organic matter.

Mudstones are also fine-grained sedimentary rocks, but they are not fissile. They are primarily composed of clay and silt particles, with a higher percentage of silt than shales.

The specific type of rock that forms (shale or mudstone) depends on the exact composition of the original sediment and the conditions of lithification.