A coarse granite or feldspathic rock found in decomposition; weathered granite debris.
From Cornish 'grouan' or related to granite. It's a technical geological term that entered English through Cornish miners working with granitic materials in southwest England.
Growan is what happens when mountains get tired—it's the crumbly, weathered granite you find on hilltops, and it's basically the Earth slowly turning rock into sand.
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