MetamorphicSoil pH usually 4.5–5.5

Slate

Shale squeezed hard enough to grow new minerals in a single plane — which is why it has roofed houses for six hundred years.

How it forms

Shale is compressed sideways during mountain building. New mica grows perpendicular to the squeeze, all in one plane, and the rock acquires a splitting direction that can cut across the original layers at any angle.

Metamorphic — changed by heat and pressure

Rock buried and squeezed from both sides, its minerals regrowing aligned across the direction of pressureheat and pressure, but never meltinggrains pointing every wayall in one plane

Rock taken deep, squeezed and heated until its minerals regrew in new forms, without ever melting. The new minerals line up under the pressure, which is why so many of these split in sheets or come out in stripes.

What it is
Fine-grained metamorphic rock with a perfect cleavage that has nothing to do with the bedding it inherited. It splits into flat sheets in whatever direction the pressure ran.
How to know it
Dull grey, green or purple, splits into flat plates that ring when tapped, and feels smooth and cold. Look for the original bedding running at an angle to the splitting plane.
How it wears
Into thin flat chips, then into heavy clay.
What it means above ground
Cold, wet, acid clay soil on steep ground. Good ground for moss and conifers, awkward for a garden.
Where you stand on it
The slate belt of Vermont and New York, the Appalachian ridge and valley, and the Eastern Townships of Quebec.
What lives on it
Hemlock and spruce, moss and fern; the wet acid soil suits very little else.
What it's used for
Roofing, flooring, billiard tables and school blackboards — all of them because it splits flat and stays flat.
What it does to the soil
Sours the soil above it — nothing in it neutralises rain, so the ground stays acid.

Next to it in the cycle

  • Shale

    what slate was before it was squeezed

  • Schist

    what slate becomes with more heat

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.