Cross-bedding can be used to interpret the agent's energy, direction, and environment of deposition. For example, cross-beds inclined in opposite directions one on top of another may indicate deposition by tidal currents. Interpreting the environment of deposition is an important goal in the study of sedimentary rocks, because the environment of deposition is an important aspect of the geologic history of an area. Metamorphic rocks Metamorphic rocks form by some sort of change in a parent rock , brought on by various combinations of elevated temperature, pressure , and the presence of hot chemical fluids . Because these agents of metamorphism are generally present at depth in the earth's crust, metamorphism usually occurs in deeper crustal settings.
Regional metamorphism requires elevated temperature together with pressure .
Pressure results in the formation of layering or foliation in the newly formed metamorphic rock. The layering may either be defined by micas arranged in closely spaced planes, or defined by light and dark color bands of non-ferromagnesian and ferromagnesian minerals respectively. Gneiss is a metamorphic rock with color banding. Regional metamorphism occurs at convergent plate margins where plates are colliding with one another. These rocks may be produced by regional metamorphism , but no foliation is developed because platy or sheet-like minerals do not form, neither do alternating light and dark bands of silicate minerals.