GeologyRocks

Bed

A centimetre to decimetre layer in a stratigraphy. Beds can be massive or contain structures, such as cross bedding, ripple marks or laminations. Beds are grouped into formations.

Biological species concept

Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. See also morphospecies concept and phylogenetic species concept.

Blueschist

A low temperature, high pressure regionally metamorphic rock, containing abundant glaucophane.

Bouma Sequence

The five part deposit produced by a waning turbidite flow. The deposit is split into:

• Structureless massive sand with gravel-rich erosional base.
• Parallel bedded sands produced by the upper flow regime.
• Cross laminated sands with ripple marks.
• Deposited in the lower flow regime.
• Laminated silts.Fine pelagic muds.

A typical Bouma sequence is illustrated below, although all five units may not be developed in each flow:

Breccia

A clastic rock composed of a matrix and angular clasts. A fine example of a breccia are the Devonian deposits at Siccar Point.