linear trough of subsidence of the Earth's crust within which vast amounts of sediment accumulate. The filling of a geosyncline with thousands or tens of thousands of feet of sediment is accompanied in the late stages of deposition by folding, crumpling, and faulting of the deposits. Intrusion of crystalline igneous rock and regional uplift along the axis of the trough generally complete the history of a particular geosyncline, which is thus transformed to a belt of folded mountains. The concept of the geosyncline was introduced by the American geologist James Hall (Hall, James) in 1859 and is basic to the concept of mountain building.

Two segments of a geosyncline are recognizable in the rock strata of many of the world's mountain systems today. Thick volcanic sequences, together with graywackes (sandstones rich in rock fragments with a muddy matrix), cherts, and various sediments reflecting deepwater deposition or processes, were deposited in eugeosynclines, the outer, deepwater segment of geosynclines. The occurrence of limestones and well-sorted quartzose sandstones, on the other hand, is considered to be evidence of shallow-water formation, and such rocks form in the inner segment of a geosyncline, termed a miogeosyncline.

Aside from the parts or segments of a geosyncline, several types of mobile zones have been recognized and named. Among the more common of these are the taphrogeosyncline, a depressed block of the Earth's crust that is bounded by one or more high-angle faults and that serves as a site of sediment accumulation, and the paraliageosyncline, a deep geosyncline that passes into coastal plains along continental margins.