Limestone
A shallow tropical sea, compressed — and the one common rock that rain dissolves.
How it forms
Corals, shellfish and plankton build carbonate skeletons out of seawater. They die, the pieces settle, and over millions of years the weight of everything above compacts and cements them. A metre of limestone is a very long time of dying.
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.
- What it is
- Nearly pure calcium carbonate, most of it the shells and skeletons of animals that lived in warm shallow water where this place now sits. Break a fresh face and you can often see them.
- How to know it
- Pale grey to buff, and it fizzes in vinegar — that is the definitive test and it works in the field. Fossils are common. The surface is often sculpted into rounded runnels and deep fissures rather than being broken into blocks.
- How it wears
- It dissolves rather than crumbles. Rain is faintly acid and limestone is the one common rock that reacts with it, so the ground develops fissures, sinkholes, caves and springs, and rivers can vanish underground and come out somewhere else.
- What it means above ground
- Thin, sweet, fast-draining soil over flat pavements of rock, with plants rooted straight into the cracks. Water that has run through it comes out hard and full of dissolved calcium, which is why limestone country grows so much fish food.
- Where you stand on it
- Eastern Ontario and the Ottawa and St. Lawrence valleys, the Niagara Escarpment, Kentucky and Tennessee, the Ozarks, central Texas, and most of Florida.
- What lives on it
- Maidenhair fern, hepatica, walking fern and orchids that appear nowhere else nearby. Hard-water lakes and rivers thick with snails, crayfish and mayflies — and therefore with fish.
- What it's used for
- Cement, agricultural lime, building stone, and the flux that makes steelmaking possible.
- What it does to the soil
- Sweetens the soil above it — raises pH, and grows a lime-loving flora.
Next to it in the cycle
Is this what you're standing on?
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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.