A geosyncline is the name for a trough shaped fold in sedimentary rock layers.
Before the advent of the theory of plate tectonic, geosynclines were thought to be the sources of fossils on the continents and mountain building. Today they are thought to be the result of plate movements that cause the layers to bend.
A geosyncline is a downward curve in the sedimentary rock layers that can be observed where the roads have been cut into hills.
Sedimentary layers are normally horizontal. In a geosyncline the normally horizontal layers have been bent into curved shapes.