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

The ground under London

England · Atlantic Europe · river deposits country

Fall · October 6, 2026

London, England

Try it

Dig down under London

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 London is running very low

23%

of the ten-year median for this date · 7.5 m³/s against 32.3 normally

very low

drynormal for the dateflood

Well below normal for the date. Fish crowd into whatever deep, cool water is left, which makes them findable and also makes them fragile — a fish played hard in warm shallow water often does not recover. It is rising — about 186% more water by the end of the week.

Under your feet

London sits on river deposits

About 56 million years old — after the dinosaurs, as mammals and grasslands spread.

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

At the surface56 million years oldNeither way
River deposits
Thames Group · Eocene
What it is
Alluvium: loose sediment laid down by moving water on a floodplain, a delta or a bar. Some of it is decades old rather than millions of years.
How it wears
It is already weathered; that is what it is. The river sorts it, moves it, and puts it down somewhere else.
What it means above ground
The deepest, most fertile and most level soil in any landscape, which is why floodplains are farmed and towns get built on them. Also the ground that floods, and the ground that liquefies in an earthquake.
Below, layer 256 million years oldNeither way
Shale
Eocene claystone · Eocene
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 356 million years oldNeither way
Sedimentary rock
Cenozoic sedimentary rocks · Eocene
What it is
The family that covers about three quarters of the land surface, in a thin skin over everything else. Some geological maps say only this much, which still tells you it will be layered and may hold fossils.
How it wears
Along its bedding planes, layer by layer, at a rate set by whatever the layers are made of — which is why sedimentary country weathers into steps.
What it means above ground
Layered country with flat-topped hills and stepped valley sides. What grows on it depends entirely on whether the beds are limey or sandy, which is the question worth asking next.

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 London, every week

One email, Sunday evening. Unsubscribe anytime.

The ground under London

England · Atlantic Europe · river deposits country

Try it

Dig down under London

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 London

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 London is running very low

23%

of the ten-year median for this date · 7.5 m³/s against 32.3 normally

very low

drynormal for the dateflood

Under your feet

London sits on river deposits

About 56 million years old — after the dinosaurs, as mammals and grasslands spread.

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

At the surface56 million years oldNeither way
River deposits
Thames Group · Eocene
What it is
Alluvium: loose sediment laid down by moving water on a floodplain, a delta or a bar. Some of it is decades old rather than millions of years.
How it wears
It is already weathered; that is what it is. The river sorts it, moves it, and puts it down somewhere else.
What it means above ground
The deepest, most fertile and most level soil in any landscape, which is why floodplains are farmed and towns get built on them. Also the ground that floods, and the ground that liquefies in an earthquake.
Below, layer 256 million years oldNeither way
Shale
Eocene claystone · Eocene
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 356 million years oldNeither way
Sedimentary rock
Cenozoic sedimentary rocks · Eocene
What it is
The family that covers about three quarters of the land surface, in a thin skin over everything else. Some geological maps say only this much, which still tells you it will be layered and may hold fossils.
How it wears
Along its bedding planes, layer by layer, at a rate set by whatever the layers are made of — which is why sedimentary country weathers into steps.
What it means above ground
Layered country with flat-topped hills and stepped valley sides. What grows on it depends entirely on whether the beds are limey or sandy, which is the question worth asking next.

The field note for London, every week

One email, Sunday evening. Unsubscribe anytime.

The ground under London

Dig down under London

Water near London is running very low

London sits on river deposits

River deposits

Shale

Sedimentary rock

The field note for London, every week

The ground under London

Dig down under London

Dig down under London

Water near London is running very low

London sits on river deposits

River deposits

Shale

Sedimentary rock

The field note for London, every week

Dig down under London