syncline

A syncline is a trough-shaped fold in layered rocks formed by compressional forces. In a syncline, the rock beds dip toward the hinge line, and the fold is bounded by an axial plane that roughly divides it into two limbs. The youngest strata are typically located near the center of the fold, with progressively older rocks toward the limbs.

Synclines occur in a range of forms. A symmetric syncline has limbs dipping at similar angles toward the hinge, while an asymmetric syncline features one steeper limb. A plunging syncline has a hinge line that is inclined and intersects the surface away from the fold’s center, giving the fold a tilted appearance. An overturned syncline has limbs that dip in the same direction, and one limb may be steeper than the other, sometimes making the age relationships appear inverted in exposure.

Formation and significance: synclines arise from regional compressional tectonics, such as during mountain-building events, and are common in sedimentary sequences. They influence the geometry of subsurface rocks, affecting erosion patterns, groundwater flow, and the distribution of natural resources. Understanding synclines and their axial planes helps geologists interpret structural maps and cross-sections, distinguish them from anticlines (the opposite arch-shaped folds), and assess potential traps for hydrocarbons in fold-thrust belts.