Argillite   Argillite is fine-grained clay particles that have been hardened by metamorphic processes (heat and pressure) and shows very little slaty cleavage (if it shows this cleavage it is referred to as shale). Both argillite and shale contain an abundance of clay particles, and both were formed in a deep-water environment, either fresh water lakes or marine. Weathering processes erode away rocks on the surface and sediment from these rocks is transported in streams and rivers to lakes or the ocean. The coarse particles settle out quickly along beaches, but very fine-grained clays and mud are carried into deep water where they settle and form mud on the bottom. Heat and pressure changes the mud to shale or argillite. These rocks can also contain fossils of plants and animals that once lived in the mud. Frontier Red