Folds <ul><li>Fold (geology), in geology, bend in a rock layer caused by forces within the crust of the earth. The forces that cause folds range from slight differences in pressure in the earth’s crust, to large collisions of the crust’s tectonic plates. As a result, a fold may be only a few centimeters in width, or it may cover several kilometers. Rock layers can also break in response to these forces, in which case a fault occurs. Folds usually occur in a series and look like waves. If the rocks have not been turned upside down, then the crests of the waves are called anticlines and the troughs are called synclines (see Anticline and Syncline). </li></ul>
What is a Fold? <ul><li>A fold is a bend in the rock layers of Earth's crust. Folds usually occur in sedimentary rocks, which were originally deposited in flat layers. Pressure from crustal movements deep within the planet causes the buried strata to bend. Later, erosion and uplifting of the rocks caused by plate movements bring the rock layers back to the surface. An upward bend in a fold is called an anticline, and a downward bend is a syncline. Folds are also classified according to their shape and angle. If the axial plane along which a fold occurs is vertical, the resulting fold is a symmetrical fold. If the axial plane is tilted, meaning that the force causing the bend was stronger on one side, the fold is asymmetrical. Sometimes the force on one side of the fold is so strong that the fold is pushed over on its side in an overturn, or recumbent, fold. These types of folds figured prominently in the formation of mountain systems such as the Andes, the Alps, and the Himalayas. </li></ul>
Fold Mountains <ul><li>Movements in the earth’s crust have compressed these layers of sedimentary rock into a fold called an anticline. Geologists study features of the earth’s surface, such as this anticline, to understand the various processes that change the landscape. For example when large pieces of the earth’s crust move laterally, they create huge compressional forces that can bend or even break rocks. </li></ul>
35.00 million years ago Alpine Mountain System Forms Africa collides with Europe, folding up layers of rock to form the Alpine mountain system.