SedimentarySoil pH 5.5–7.5, depending on the beds

Shale

Mud from still, deep water — the finest-grained common rock, and the one that holds the most fossils.

How it forms

Fine sediment stays suspended until the water stops moving almost entirely — a deep sea floor, a lake bottom, a lagoon. It settles a fraction of a millimetre a year, sealing whatever fell into it, and burial squeezes the water out until the clay plates lock together.

Sedimentary — laid down in layers

Sediment settling through water in layers, then buried and compacted into rockwaterweight above squeezes the water out and cements the grains

Rock built from the wreckage of older rock, or from the remains of things that lived — sand, mud, shell and skeleton, settled in flat beds and buried until the weight turned them to stone. The only rocks that hold fossils.

What it is
Compressed clay and silt, laid down where the water was too quiet to move anything larger. The particles are flat and settle flat, which is why shale splits into sheets.
How to know it
Splits into thin flat pieces, feels smooth rather than gritty, and often smells faintly of earth when freshly broken or wetted. Dark shale is dark because of the organic matter buried with it.
How it wears
Into chips, then into clay. It is weak, so it forms valleys and slumping banks rather than hills, and road cuts through it keep sliding.
What it means above ground
Heavy clay soil that holds water and drains badly. Shale is impermeable, so water runs off it rather than into it, and streams on shale rise and fall fast after rain.
Where you stand on it
The Appalachian valleys, the Great Plains from Texas to Saskatchewan, the Michigan and Illinois basins, and the Utica and Marcellus beds of the northeast.
What lives on it
Wet meadow and swamp forest in the hollows, because the water cannot get away. Springs are rare; seeps are everywhere.
What it's used for
Brick, tile, and cement. Some shale beds are the source rock for oil and gas.
What it does to the soil
Neither sweetens nor sours — the soil above takes its pH from elsewhere.

Next to it in the cycle

  • Slate

    shale squeezed hard enough to split cleanly

  • Sandstone

    the coarser sediment that settled nearer the current

  • Marl

    shale with a large share of lime mud in it

Is this what you're standing on?

Find the rock under your town

Enter a town or postal code and we'll show you the rock beneath it and how old it is, the soil it makes, and the chain of rivers your water joins on the way to the sea.

General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.