The arrangement of particles in distinct layers results in a variety of structures referred to as bedding. In some cases, distinct sedimentary structures are apparent within rock beds, which are treated separately. In other cases, sedimentary structures are absent or indistinct. Three categories are defined in the Ferm classification for bedding without distinct sedimentary structures on the basis of the thickness, orientation, and percentage of different materials:
- 5X4 3X4 1X4 Massive or homogeneous:No evidence of internal layering
- 5X3 3X3 1X3 Streaked: One component (sand, clay or coal) is less than 20% of the rock mass
- 3X2 Layered: Individual components compose between 20% and 80% of the rock mass
Massive rocks show no obvious lines, layers, or streaks that divide the rock into parts. Grains may be aligned (parallel) or randomly oriented.
Streaked rocks contain a subordinate constituent of either sand, clay, or coaly material. Streaks are thin (1 to 5 millimeters) and discontinuous.
Layered rocks have thicker units of both materials, and the individual layers tend to go through the entire specimen. Layers can be wavy (rippled) or flat (graded).
Streaked and layered rocks may have sedimentary structures produced by flowing water. These structures are generally recorded in the decimal digit in the rock code. Go to Sedimentary Structures to complete the classification of bedded rocks.
Rocks with primary bedding structures may be subsequently modified by biogenic processes (root penetration or burrowing). In some cases, the bioturbation completely obliterates the original structure. These features are recorded in the third or decimal digit of the rock code, depending on the degree of bioturbation. Go to Biogenic Structures for more information.