Every rock is somewhere in the same loop
There are only three things that can happen to rock: it can melt and cool, it can be worn down and laid back out in layers, or it can be buried and squeezed until its minerals regrow. Every rock you will ever pick up is one of those three, caught partway round. Which one decides how it wears, what soil it leaves, and whether that soil is sweet or sour — which is most of what a garden, a river and a wood are arguing about.
The rock cycle
Nothing in this loop is permanent. The granite of a mountain range becomes the sand of a beach becomes the sandstone of a cliff becomes the quartzite of a ridge — and, given enough burial, molten rock again.
Igneous — cooled from molten
Rock that was liquid. Cooled slowly at depth it grows crystals big enough to see; cooled fast at the surface the crystals never get the chance, and the rock comes out fine and dark. Everything else on this page started as one of these.
- Acid soilpH usually 4.5–5.5
Granite
The root of a mountain range, cooled slowly enough to grow crystals you can pick out by eye.
How it forms → - NeutralpH usually 6.0–7.0
Basalt
Lava that made it to the surface and cooled fast — dark, fine-grained, and unusually generous to whatever grows on it.
How it forms → - NeutralpH usually 5.5–6.8
Volcanic rock
Everything a volcano throws out — lava, ash and fragments — from eruptions a few thousand or a few billion years ago.
How it forms → - Acid soilpH 5.0–6.5, depending on the member
Igneous rock
The family of rocks that were once molten, whether they cooled underground or in the open air.
How it forms →
Sedimentary — laid down in layers
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.
- Sweet soilpH usually 7.0–8.0
Limestone
A shallow tropical sea, compressed — and the one common rock that rain dissolves.
How it forms → - Sweet soilpH usually 7.0–8.0
Dolostone
Limestone whose calcium was partly traded for magnesium — tougher, and the reason escarpments stand up.
How it forms → - Acid soilpH usually 5.0–6.0
Sandstone
A beach, a dune field or a river bar, buried and cemented — and visibly still made of sand.
How it forms → - NeutralpH 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 → - Sweet soilpH usually 7.5–8.3
Chalk
Plankton skeletons, kilometres from any land that might have muddied the water. Soft enough to cut with a spade, yet it stands as cliffs.
How it forms → - Sweet soilpH usually 7.0–8.0
Marl
Halfway between limestone and shale — lime mud and clay from a quiet sea floor, and historically dug to sweeten sour fields.
How it forms → - NeutralpH 5.0–8.0, depending entirely on the beds
Sedimentary rock
Rock built from the debris of older rock, or from the remains of things that lived — laid flat, and buried.
How it forms →
Metamorphic — changed by heat and pressure
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.
- Sweet soilpH usually 7.0–8.0
Marble
Limestone recrystallised — the fossils gone, and in acid country a narrow stripe of sweet ground that grows something different.
How it forms → - Acid soilpH usually 4.5–5.5
Gneiss
Ancient crust taken deep enough to nearly melt, then banded by pressure into light and dark stripes.
How it forms → - Acid soilpH usually 5.0–6.0
Paragneiss
Gneiss that used to be mud and sand. The banding is the ghost of the bedding it started with.
How it forms → - Acid soilpH usually 5.0–6.0
Schist
Mudstone cooked and squeezed until its clays regrew as mica, all lined up. It splits into sheets and glitters.
How it forms → - Acid soilpH 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 → - Acid soilpH usually 4.0–5.0
Quartzite
Sandstone cooked until the grains fused into one mass. Nearly pure silica, and one of the hardest rocks there is.
How it forms → - Acid soilpH 4.5–6.0, depending on the member
Metamorphic rock
Older rock changed in place by heat and pressure, deep in the crust, without ever melting.
How it forms →
Surface deposits — not yet rock
Not rock at all — the loose material lying on top of it, dropped by ice or rivers within the last couple of million years. It is what most people actually dig in, and in a lot of North America it is metres thick.
- NeutralpH 6.0–7.5, inherited from whatever is upstream
River deposits
Sand, silt and gravel dropped by a river and still being dropped — the youngest ground there is, and the most fertile.
How it forms → - NeutralpH 5.5–7.5, inherited from wherever the ice came from
Glacial till
Everything a glacier carried, dumped where it melted — clay, sand, gravel and boulders, mixed with no sorting at all.
How it forms → - NeutralpH 5.0–6.5
Glacial outwash
Sand and gravel sorted by meltwater rivers pouring off a retreating ice sheet — droughty ground, and excellent aquifers.
How it forms →
Find out what you're standing on
Which of these is under your town?
Enter a town and we'll show you the rock beneath it, how old it is, the soil it weathers into and where your water goes from there.