What is formed by an upward fold in rock due to compression of Earth's crust?

An upward fold in rock due to the compression of Earth's crust is called an anticline. This geological structure occurs when tectonic forces push rock layers together, causing them to bend and arch upward. The characteristic shape of an anticline resembles an "A" or a dome with the oldest layers of rock typically found at the core of the fold, while progressively younger layers are found on the flanks. This is significant in understanding geological formations and can help in locating natural resources, such as oil and gas, which often accumulate in these folds.

In contrast, a syncline is a downward fold where rock layers are bent downward, forming a trough-like structure. A monocline involves a step-like fold in rock layers, where one section is raised relative to another. The term "fold arch" is not a standard geological term used to describe folds in rock layers. Understanding these distinctions is key to grasping the dynamics of Earth's geological processes.