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Rock
of organic matter. There is no subsoil. The topsoil rests directly on the rock. (See also: Alkaline soil.)
Reniform
Kidney-shaped (for example, haematite).
Reservoir rock
A permeable rock in which petroleum accumulates. It is overlain by a cap rock that prevents the petroleum from rising further.
Reversed fault
A fault where one slab of the Earth’s crust rides up over another. Reversed faults are only common during tectonic plate collision. (See also: Normal fault.)
Rhyolite
An acidic lava whose mineral content is similar to granite. It is
a very sticky lava, associated with explosive eruptions. It does not flow broadly and quickly solidifies, often before extensive crystals can form. It has a number of glassy variants that emerge on the surface, including obsidian and pumice. Rhyolite may show flow banding and may even have some visible crystals in it.
Pumice is often formed at the surface of rhyolitic lava, as gases
try to escape, but become trapped by the stickiness of the lava. The large number of cavities in pumice gives it the appearance of a brittle, grey-coloured sponge.
Rill
A small channel formed in soil by running water. It is small enough
to be ploughed out. If allowed to grow, a rill will develop into a gully (see: Gulleying).
Rock
A naturally occurring solid material containing one or more minerals.
 Rock – Limestone, a sedimentary rock.
(For the main rock types see: Acid rock; Agglomerate; Alkaline rock; Arenaceous; Argillaceous; Arkose; Basic rock; Breccia; Calcareous rock; Chemical rock; Clastic rock; Coarse-grained rocks; Conglomerate; Extrusive rock; Fine-grained rocks; Igneous rock; Impermeable; Intrusive rock; Layered rock; Metamorphic rock; Ore; Permeable rock; Plutonic rock; Porphyritic rock; Sedimentary rock.)
 Rock – Marble, a metamorphic rock.
 Rhyolite – Rhyolite often develops a frothy surface material called pumice. It is grey.
 Rock – Gabbro, an igneous rock.
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