A fine-grained, soft and crumbly sedimentary rock is termed as a mudstone.
The formation of mudstone occurs by the settlement of fine-grained clay particles at the bottom of the water bodies such as oceans, lagoons, lakes or rivers.
The mud gets compacted by the overlying sediment and there is dehydration of the mud that takes place due to the overlying pressure and this leads to the formation of the mudstone.
So compression of fine-grained clay particles leads to the formation of mudstones and this is why it looks like hardened clay.
