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
Carbonate settling out of the water and clay washing in from land arrive in the same place at the same rate, and get buried together.
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
- A mixture of carbonate mud and clay, in roughly comparable amounts. Neither pure enough to be limestone nor clean enough to be shale.
- How to know it
- Soft, pale grey to buff, fizzes in vinegar but leaves a muddy residue behind. Sticky when wet.
- How it wears
- Softens and slumps, shedding a sticky calcareous clay that holds water well.
- What it means above ground
- Heavy, sweet, water-holding soil. Farmers dug marl pits for centuries and spread it on acid fields, which is where the practice of liming comes from.
- Where you stand on it
- The Atlantic and Gulf coastal plains from New Jersey to Texas, and lake-bottom marls across the glaciated Midwest and Ontario.
- What lives on it
- Rich fen and calcareous wetland — some of the most botanically diverse ground on the continent, with orchids and sedges found nowhere else.
- What it's used for
- Agricultural lime, cement, and historically fertiliser.
- 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.