The downfold (trough) in a folded rock layer is known as a:

When layers of rock are pushed together by compressional forces inside the Earth, usually due to the movement of tectonic plates, they bend into wave-like shapes called folds. Folding is one of the main processes that form mountains.

A fold has two main parts:

Anticline: the upfold or arch-shaped part of the fold, where the rock layers bend upwards. The oldest rocks are found at the centre of an anticline.

Syncline: the downfold or trough-shaped part of the fold, where the rock layers bend downwards. The youngest rocks are found at the centre of a syncline.

The sloping sides of a fold are called limbs.

Folds can be of several types, such as symmetrical folds (both limbs slope equally), asymmetrical folds, overturned folds (one limb pushed over the other), recumbent folds (lying almost horizontal) and nappes (when the fold breaks and one part slides far over the other).

Fold mountains are formed when huge masses of rock are folded on a large scale. The Himalaya, the Alps, the Andes and the Rockies are young fold mountains, formed during the Alpine orogeny (mountain-building period) in the Tertiary era. The Himalaya were formed by the collision of the Indian plate with the Eurasian plate. The Aravallis in India and the Appalachians in North America are old fold mountains, which have been worn down by erosion over millions of years.

Interestingly, anticlines are often good traps for petroleum and natural gas, which collect at the top of the arch below an impermeable layer.

Why not the other options?

• ✗An anticline is the upfold or arch.
• ✗A horst is a raised block of land between two faults (block mountain).
• ✗A graben is a sunken block of land between two faults (rift valley), such as the Narmada valley.