Costean

A trial pit or trench, dug down through surface soil and loose debris to solid rock, used to search for and trace the line of a mineral vein before any full-scale mine was sunk.

Historical usage

Costeaning is an old Cornish prospecting method — the word itself is thought to come from Cornish costēn, from codha (to fall/happen) and stēn (tin) — and was in everyday use wherever British miners needed a quick, cheap way to read the geology hidden under a covering of soil, subsoil or scree before committing to sink an actual shaft.

How it worked

A prospector sank a series of small pits down to bedrock across the suspected line of a vein, then dug connecting trenches between them at right angles to that line, cutting across every vein or lode running through the ground in between. Where a trench crossed a mineralised vein, the outcrop would be exposed and its width, direction and apparent richness could be judged directly, without the time and expense of sinking a full shaft on unproven ground.

Why it was used

Costeaning let a mine adventurer test ground cheaply and quickly, gathering real geological information — not just surface float or guesswork — before risking capital on a shaft that might strike nothing worthwhile. It remained a standard early step in exploration wherever shallow cover masked the bedrock beneath.

Risks & limitations

Costeaning only worked where the loose surface cover was shallow enough to dig through by hand; it told a prospector nothing about how a vein behaved once it ran deeper than a pit could easily reach, so promising costean results still had to be followed up with real underground working before a mine's true worth was known.

Regional variation

The same basic method appears well beyond Cornwall under other names — in Australia it survives in modern mineral exploration regulation, where costeaning is still a recognised, regulated technique for exposing shallow bedrock with minimal ground disturbance.

Sources

1. (primary) Definition of costean — Mindat
2. (secondary) Costean — Wikipedia
3. (secondary) Glossary of Mining Terms — Open University Geological Society (South West Branch)

Record created: 20 August 2026 · Last researched: 20 August 2026