Shales, mudstones and their unconsolidated equivalents, constitute the vast majority of all sediments on Earth. The composition of such rocks may be roughly characterized (ignoring variation in clay mineralogy) by only two parameters: the water content (porosity) and the fraction of silt relative to total solid fraction (hereafter called the solid fraction of silt). The water content will decrease during compaction of the rock while the solid fraction of silt is unchanged. For shales and mudrocks the silt grains are mainly unconnected and the load-bearing component is the clay.

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