Mudstone forms at the bottom of water bodies such oceans, lakes and rivers, particularly in quiet sections. The stones form when fine-grained clay particles settle in water.
Layers of overlying sediment bury and compact the mud in water, eventually squeezing the water out and forming mudstone. Mudstone and shale share similar compositions, with both made of silt and clay particles. However, mudstone eventually disintegrates into blocky pieces while shale comes apart into slim chips. While calm water conditions with only intermittent weak turbulence can facilitate formation of mudstone, it can also form when current speeds are fast enough to move and deposit sand.