Dolostone
Limestone whose calcium was partly traded for magnesium — tougher, and the reason escarpments stand up.
How it forms
Limestone is buried and seawater or brine percolates through it. Magnesium in that water replaces some of the calcium, atom for atom, and the rock recrystallises slightly denser and harder than it was.
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
- Chemically limestone's sibling: calcium magnesium carbonate instead of calcium carbonate. The swap usually happened long after the original mud was laid down, as magnesium-rich water moved through it.
- How to know it
- Looks like limestone but fizzes only weakly in vinegar, and often not at all unless you powder it first. Tends to break into blocks rather than dissolving into runnels, and weathers to a buff or slightly pinkish grey.
- How it wears
- Slowly, and more by fracture than dissolution — which is why it stands as cliffs where softer limestone beside it has worn away.
- What it means above ground
- Sweet, shallow, stony soil and hard water, on ridges that are often the only high ground for a long way.
- Where you stand on it
- The Niagara Escarpment from New York through Ontario to Wisconsin, the Manitoba escarpment, and much of the upper Midwest.
- What lives on it
- Cliff-edge cedar — some of the oldest trees in eastern North America grow on Niagara Escarpment dolostone, stunted and a thousand years old.
- What it's used for
- Aggregate, refractory brick, and a source of magnesium.
- 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
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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.