For Kids · The land and the sky · The ground under you

The ground under Oulu

North Ostrobothnia · Nordic & Baltic Boreal · shale country

Fall · October 7, 2026

Oulu, North Ostrobothnia

Nordic & Baltic Boreal·Hardiness zone 5a

Try it

Dig down under Oulu

Drag the spade down, or tap Dig deeper, through the leaves, the living topsoil and the subsoil, all the way to the bedrock that is really down there.

About 0 cm down

The grass

the top of the ground

Under the grass is a whole hidden world. Drag the spade down — or press Dig deeper — to find out what is down there.

Layer 0 of 5

It takes hundreds of years to make one centimetre of topsoil. The depths are typical — every place is different — and the ruler is squashed at the bottom so the rock fits on the screen.

Say it like this

Age 2“Dig, dig, dig! Who lives down there? A wiggly worm!”
Age 4“Leaves, dark soil, lighter soil, broken rock, solid rock. Can you count the layers? Five!”
Age 6“Topsoil is made of rotted leaves and tiny bits of broken rock. Why do you think it takes hundreds of years to make?”

Go outside: Dig a small hole in the garden and count how many worms live in one shovelful — then put them all back and fill it in.

The river right now

Water near Oulu is running high

242%

of the ten-year median for this date · 520.2 m³/s against 214.5 normally

high

drynormal for the dateflood

Running high for the date. Water will be pushy and coloured, fish will be tucked into the slack water at the edges and behind obstructions, and crossings that are normally easy are not. It is steady through the week ahead.

Under your feet

Oulu sits on shale

About 1.6 billion years old — laid down before anything with a shell or a backbone existed.

All the kinds of rock, and how each one forms →

At the surface1.6 billion years oldNeither way
Shale
Mesoproterozoic sedimentary rocks · Mesoproterozoic
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 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.
Below, layer 22.5 billion years oldSours the soil
Granite
Proterozoic I granite · Paleoproterozoic
What it is
Magma that never reached the surface. It cooled kilometres down over tens of thousands of years, and that slowness is why its crystals are large enough to see without a lens: glassy grey quartz, blocky pink or white feldspar, and flecks of black mica.
How it wears
Feldspar turns to clay and lets go, so the rock crumbles grain by grain into coarse sharp sand. Rounded corners come first: a granite boulder field is a set of spheres because the edges weathered fastest.
What it means above ground
Sour, sandy, quick-draining soil that holds neither water nor nutrients well, over rounded outcrops and boulder fields. The water off it is soft, clear and acid, with almost nothing dissolved in it.
Below, layer 32.0 billion years oldSours the soil
Granite
Igneous: intrusive; Intrusive: granitic · Orosirian
What it is
Magma that never reached the surface. It cooled kilometres down over tens of thousands of years, and that slowness is why its crystals are large enough to see without a lens: glassy grey quartz, blocky pink or white feldspar, and flecks of black mica.
How it wears
Feldspar turns to clay and lets go, so the rock crumbles grain by grain into coarse sharp sand. Rounded corners come first: a granite boulder field is a set of spheres because the edges weathered fastest.
What it means above ground
Sour, sandy, quick-draining soil that holds neither water nor nutrients well, over rounded outcrops and boulder fields. The water off it is soft, clear and acid, with almost nothing dissolved in it.

Bedrock and soil figures are modelled from national geological surveys and global soil mapping, at a resolution of a few hundred metres — right for the shape of a district, not for a particular garden. River discharge is modelled, not gauged. For anything that matters, dig a hole and look.

The field note for Oulu, every week

One email, Sunday evening. Unsubscribe anytime.

The ground under Oulu

North Ostrobothnia · Nordic & Baltic Boreal · shale country

Try it

Dig down under Oulu

Drag the spade down, or tap Dig deeper, through the leaves, the living topsoil and the subsoil, all the way to the bedrock that is really down there.

About 0 cm down

The grass

the top of the ground

Under the grass is a whole hidden world. Drag the spade down — or press Dig deeper — to find out what is down there.

Layer 0 of 5

It takes hundreds of years to make one centimetre of topsoil. The depths are typical — every place is different — and the ruler is squashed at the bottom so the rock fits on the screen.

Say it like this

Age 2“Dig, dig, dig! Who lives down there? A wiggly worm!”
Age 4“Leaves, dark soil, lighter soil, broken rock, solid rock. Can you count the layers? Five!”
Age 6“Topsoil is made of rotted leaves and tiny bits of broken rock. Why do you think it takes hundreds of years to make?”

Go outside: Dig a small hole in the garden and count how many worms live in one shovelful — then put them all back and fill it in.

Try it

Dig down under Oulu

Drag the spade down, or tap Dig deeper, through the leaves, the living topsoil and the subsoil, all the way to the bedrock that is really down there.

About 0 cm down

The grass

the top of the ground

Under the grass is a whole hidden world. Drag the spade down — or press Dig deeper — to find out what is down there.

Layer 0 of 5

It takes hundreds of years to make one centimetre of topsoil. The depths are typical — every place is different — and the ruler is squashed at the bottom so the rock fits on the screen.

Say it like this

Age 2“Dig, dig, dig! Who lives down there? A wiggly worm!”
Age 4“Leaves, dark soil, lighter soil, broken rock, solid rock. Can you count the layers? Five!”
Age 6“Topsoil is made of rotted leaves and tiny bits of broken rock. Why do you think it takes hundreds of years to make?”

Go outside: Dig a small hole in the garden and count how many worms live in one shovelful — then put them all back and fill it in.

The river right now

Water near Oulu is running high

242%

of the ten-year median for this date · 520.2 m³/s against 214.5 normally

high

drynormal for the dateflood

Under your feet

Oulu sits on shale

About 1.6 billion years old — laid down before anything with a shell or a backbone existed.

All the kinds of rock, and how each one forms →

At the surface1.6 billion years oldNeither way
Shale
Mesoproterozoic sedimentary rocks · Mesoproterozoic
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 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.
Below, layer 22.5 billion years oldSours the soil
Granite
Proterozoic I granite · Paleoproterozoic
What it is
Magma that never reached the surface. It cooled kilometres down over tens of thousands of years, and that slowness is why its crystals are large enough to see without a lens: glassy grey quartz, blocky pink or white feldspar, and flecks of black mica.
How it wears
Feldspar turns to clay and lets go, so the rock crumbles grain by grain into coarse sharp sand. Rounded corners come first: a granite boulder field is a set of spheres because the edges weathered fastest.
What it means above ground
Sour, sandy, quick-draining soil that holds neither water nor nutrients well, over rounded outcrops and boulder fields. The water off it is soft, clear and acid, with almost nothing dissolved in it.
Below, layer 32.0 billion years oldSours the soil
Granite
Igneous: intrusive; Intrusive: granitic · Orosirian
What it is
Magma that never reached the surface. It cooled kilometres down over tens of thousands of years, and that slowness is why its crystals are large enough to see without a lens: glassy grey quartz, blocky pink or white feldspar, and flecks of black mica.
How it wears
Feldspar turns to clay and lets go, so the rock crumbles grain by grain into coarse sharp sand. Rounded corners come first: a granite boulder field is a set of spheres because the edges weathered fastest.
What it means above ground
Sour, sandy, quick-draining soil that holds neither water nor nutrients well, over rounded outcrops and boulder fields. The water off it is soft, clear and acid, with almost nothing dissolved in it.

The soil it made

Oulu is very acid sandy loam

4.8pH in the root zone

4 · sour6.58.5 · limey

Clay: 14%
Silt: 27%
Sand: 59%

What that means

Well below what most vegetables want. Phosphorus and molybdenum get locked away at this pH, and aluminium becomes soluble enough to hurt roots — so plants can sit in fed soil and still go hungry.

Happy here without a fight

Blueberries, cranberries, rhododendrons, azaleas, heathers, potatoes, conifers and most woodland ferns.

If you want the rest

Lime raises it, slowly — autumn is the time, and it takes a season to move. Test before and after rather than guessing; overshooting is harder to undo than undershooting.

Working it

Water runs straight through, so it dries fast and warms early — you can work it days after rain and sow ahead of heavier ground, but it needs watering and feeding more often because both wash out.

The field note for Oulu, every week

One email, Sunday evening. Unsubscribe anytime.

The ground under Oulu

Dig down under Oulu

Water near Oulu is running high

Oulu sits on shale

Shale

Granite

Granite

The field note for Oulu, every week

The ground under Oulu

Dig down under Oulu

Dig down under Oulu

Water near Oulu is running high

Oulu sits on shale

Shale

Granite

Granite

Oulu is very acid sandy loam

The field note for Oulu, every week

Dig down under Oulu

Oulu is very acid sandy loam