Mudrocks are a family of fine-grained siliciclastic sedimentary rocks deriving from the consolidation of clay- and silt-sized sediment (< 0.0625 mm), collectively known as mud. Mudrocks are also known as pelites or lutites, based on their Greek and Latin root names. The distinction of silt (0.004 – 0.0625 mm) from clay (< 0.004 mm) is possible only with laboratory or microscopy techniques. In terms of composition, mudrocks contain very fine-grained quartz and feldspar (constituting the silt-sized particles) and clay minerals such a kaolinite, montmorillonite, and illite, cemented by iron oxides or carbonatic cement. As other clastic sedimentary rocks, the name of mudrocks derives from the grain size of the corresponding loose sediment. There are several classifications, but in general, following Prothero (1996), we can classify a mudrock as:
– siltstone, if they contain more than 2/3 silt over clay
– mudstone, when they contain between 1/3 and 2/3 clay with the rest is silt
– claystone, when clay constitutes more than 2/3 of the total.

In the field, we cannot distinguish silt from clay with the hand lens or the unaided eye. Thus, we can loosely classify these rocks as mudrocks. However, we can touch them to obtain a rough estimate of their grain size. According to Prothero (1996), siltstones are gritty on the touch, whereas mudstones and claystones feel loamier. Claystone is slick.