|Many rocks contain flat, tabular, flaky, elongate, or needle-like grains. If such grains are arranged parallel to one another, the rock texture is described as 'oriented' rather than 'random'. Oriented and random textures are found in both crystalline and clastic rocks.|
|Parallel orientation of flat or needle-like grains may be achieved in a number of ways. Consider these experiments and observations:
Deposition: A mixture of round and flaky grains is poured into a container of water. The flaky grains arrange themselves parallel to the floor of the container.
Shaking, Compaction and Rotation: A container holds a mixture of round grains and randomly oriented flaky grains. The mixture is packed very loosely, with lots of pore spaces between the grains. Upon being shaken, the mixture compacts, and the flaky grains rotate to become more parallel to the floor of the container.
Compression and Rotation: Matchsticks are embedded randomly in a mass of clay. The clay is compressed. The matchsticks rotate to become more perpendicular to the compressional forces.
Compression and Selective Growth: A crystalline mixture containing randomly arranged needle-shaped grains is heated up and subjected to compressional forces. Grains that are oriented parallel to the compressional forces dissolve while grains that are oriented perpendicular to the compressional forces grow.
Flow: Logs that float in the still waters of a lake are arranged randomly. Logs that float in a flowing stream arrange themselves parallel to the direction of flow of the water. In the winter, the surface water turns to ice and the arrangements of the logs in the lake and the stream are locked into place.