Shale

Part of speech: noun

Pronunciation: /ʃeɪl/

Definitions

1. A sedimentary rock that is primarily made up of clay particles and exhibits the ability to break into thin layers along its bedding planes
2. This type of rock is formed from the compaction of fine particles, especially clay, and is known for its ability to cleave into sheets
3. A type of sedimentary rock formed from compacted clay and silt particles, recognized for its tendency to split into thin, flat layers

Etymology: Shale is a close cousin of "scale" and "shell", and all three trace back to a single prehistoric root meaning "to split or cut." The Old English "sċealu" meant shell, husk or pod, the outer covering that splits away from a seed or nut. Middle English "schale" kept that sense of a casing, and the rock took its name from the way it breaks apart: it separates easily into thin, flat layers, much like a husk peeling away. The connection to "scale" is the surprising part. The two words are doublets, meaning they descend from the same ancestor but reached English by different routes, one through the native line that gave us "shell" and the other through a related form that came to mean a thin flake, as on a fish or a reptile. Their Dutch relatives show the same pattern: "schaal" means shell, and "schalie" is the Dutch word for shale, so the rock-name appears in a sister language with the same root.

Synonyms: slate, stone, rock, sedimentary rock, claystone