What is a downward fold in rock caused by compression in the Earth's crust called?

A downward fold in rock formed by compression in the Earth's crust is referred to as a syncline. This geological structure occurs when tectonic forces push rocks together, causing them to bend and create a trough-like feature. In a syncline, the youngest rock layers are typically found at the core, while older rocks are positioned on the flanks, reflecting the folding process.

Understanding the nature of a syncline is important in geology because it helps scientists interpret the history of geological formations and the dynamics of Earth's crust. By recognizing synclinal structures, geologists can infer past compressive forces and the overall behavior of the tectonic plates involved.

In contrast, an anticline, which is the opposite structure, is characterized by an upward fold where the oldest rocks are found at the center. Fault lines refer to fractures in the Earth's crust where displacement has occurred, rather than folds. Stratum describes a layer of sedimentary rock or soil, which doesn't specifically correspond to the folding processes associated with synclines or anticlines.