A large, elongated depression in the Earth's crust where sediment accumulates in thick layers over geologic time, often becoming the site of future mountain formation.
From 'geo-' (earth) plus 'syncline' (from Greek 'syn-', together, plus 'klinein', to lean), literally meaning 'earth leaning together'—an area where crust sinks.
Geosynclines help explain how the Appalachian Mountains, Rockies, and other mountain ranges formed—thick piles of sediment got buried, compressed, and then thrust upward by colliding continents.
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