Sediment is natural earth material. It consists of soil particles of all sizes ranging from the smallest, “silt (mud) and sand”, to larger sizes which include the gravels, cobbles and boulders. Sediment also includes extremely hard and very large materials (bedrock) and very small tightly compacted materials (clay). All of these materials succumb to the forces of erosion from water, wind and pressures caused by expanding and contracting forces. A stream contains a certain variety of sediment based upon its geology, topography, precipitation rates and other factors in the watershed. The study of the land and water interactions is called “fluvial geomorphology”.
An extreme version of scoria occurs when volatile-rich lava is very quickly quenched and becomes a meringue-like froth of glass called pumice. Some pumice is so full of vesicles that the density of the rock drops low enough that it will float
