A fold is a rock formation that’s been made by flat rocks becoming deformed due to stress and pressure. Folds are created when two plates that make up the earth’s crust collide. As the plates are forced together they bend, curve, or make jagged patterns in the rock.
Although there are many types of folds, we’ll discuss three of the most basic: monoclines, anticlines, and synclines.
Monoclines are the simplest types of folds. They consist of a dip or drop in an area of flat or sloping rock, and look like a single step. Older rocks are found at the bottom of a monocline, and the newer ones on its top.
If a fold arches away from the earth, it’s called an anticline. The top of the arch is called the crest of the anticline. The oldest rocks in an anticline are found at its core, or center. Sometimes these older rocks become exposed because the crest erodes away more quickly than the slopes of the arch. If an anticline’s shape is fairly round, it’s called a dome.
Finally, if the fold bends down towards the earth, it’s known as a syncline. This kind of fold’s center is called its hinge. Unlike anticlines, synclines have their youngest rocks at their centers, and older rocks at the outsides. The depressions formed by synclines are sometimes called basins.
Structures formed by geological folding are found all over the earth, and can be of any size. The Alps in Europe, the Andes in South America, and America’s Rocky Mountains are all examples of mountains that have been formed by folding.