Linear trough of subsidence of the Earth's crust, in which vast amounts of sediment accumulate. The filling of a geosyncline with thousands or tens of thousands of feet of sediment is accompanied by folding, crumpling, and faulting of the deposits. Intrusion of crystalline igneous rock and regional uplift complete the transformation into a belt of folded mountains. The concept was introduced by James Hall in 1859 and is basic to the theory of mountain building. See alsoAndean Geosyncline; Appalachian Geosyncline; Cordilleran Geosyncline.