The definition of a geosyncline is a long trough in the surface of the earth where sediments and deposits collect, thought of in the late 19th century and early 20th century as the origin of most mountains.
An example of a geosyncline is the Adelaide Rift Complex in South Australia.
a very large, troughlike depression in the earth's surface containing masses of sedimentary and volcanic rocks
Origin of geosynclinegeo- + syncline
A usually elongate, basinlike depression along the edge of a continent, in which a thick sequence of sediments and volcanic deposits has accumulated.
Back-formation from geosynclinal.