Shale

Fissility is the whole identification

The test takes three seconds. Pick up the specimen and try to split it with a knife blade parallel to the visible lamination. Shale comes apart in sheets less than five millimetres thick with almost no resistance, and the split surfaces are dull and slightly rough. Mudstone has the same composition and the same grain size but no fissility — it breaks in blocky, irregular chunks. The difference is not chemistry; it is whether the clay flakes had time to settle and align quietly, or were dumped quickly in a slurry that set in a chaotically oriented mass.

Colour as an environmental indicator

Shale colour is a redox indicator and it is more informative than most field properties.

• Black: organic carbon above about 1% preserved because the sediment sat below the oxygenated zone. Black shale is the source rock for a large share of the world's oil and gas.
• Grey to green: moderate organic content, deposited in dysoxic conditions; green reflects ferrous iron in clay.
• Red or maroon: fully oxidised, no preserved organic matter; the sediment was exposed to oxygen throughout deposition.
• Banded red and grey: fluctuating oxygen levels, often in a marginal marine or lacustrine setting.

Why it matters far beyond a field guide

Fine-grained mudrock is where the geological record is most complete, because fine sediment accumulates slowly and buries things gently. The Burgess Shale in British Columbia preserved the soft tissues of Cambrian animals — legs, gills, guts — that no other rock type recorded, and that single formation rewrote the history of early animal life. Black shales also hold the world's largest unconventional hydrocarbon resource and are the focus of most current carbon-capture research, because clay-rich rock adsorbs carbon dioxide well. Whether you are interested in fossils, energy or climate, shale is where the important problems concentrate.

Collecting and handling

Two practical rules. First, split in the field: shale that dries and re-wets will crumble along internal planes, and the clean split you want is only available while the rock is damp. Second, know the site rules. Many of the best shale localities are protected — Burgess Shale collecting is prohibited outright and a permit is required even to visit — and black shale outcrops can be unstable, with a tendency to slab off in plates at head height. Helmets and caution are appropriate, and pyritic black shale will rust and stain anything stored next to it.

Reading bedding in a shale sequence

Because shale splits along bedding, a fresh road cut or coastal section gives you a continuous readable surface that sandstone cannot match. Measure laminate thickness in millimetres, note whether the sequence is rhythmically banded, and look for graded beds and sole marks on the underside of siltier laminae — those record individual turbidity currents and give you a current direction. A season of measuring a shale section produces a far more detailed environmental history than the same effort spent on coarser rocks.

Sources

Hardness, streak, cleavage and specific-gravity values quoted on this sheet are checked against the references below.

• Geology.com Rocks — rock-type reference — descriptions and photographs of igneous, sedimentary and metamorphic rocks
• British Geological Survey — UK national geological survey — the three rock types, the rock cycle and mineral definitions