Solid rock can bend like a rug, from tiny crinkles to mountains
Rock layers laid down flat can end up curled into arches, troughs and zigzags. Geologists call these folds, and they range from microscopic crinkles to structures the size of mountains. Reading them is a kind of detective work: the pattern of old and young layers reveals which way the crust was squeezed.
Most folds form when layered rock is shortened parallel to its bedding, but they can also arise where a fault bends or stops propagating, where sediments compact unevenly over buried reefs, or above a shallow igneous intrusion that bulges the rock above it, as over a laccolith. Their presence in soft sediment, in every grade of metamorphic rock and even as flow structures in some igneous rock shows they form across a huge range of temperatures and pressures. Regional sets of folds make up fold belts, typical of mountain-building zones.
A fold has flanks called limbs that meet at a hinge, the line of sharpest curvature, and a plane joining the hinges of stacked layers is the axial surface. Tightness is measured by the angle between the limbs: gentle folds span 180 to 120 degrees, open ones 120 to 70, close ones 70 to 30 and tight ones under 30, while isoclinal folds, below 10 degrees, have almost parallel limbs. Chevron folds have straight limbs and sharp hinges; similar folds thin on their limbs and thicken at the hinge, whereas concentric folds keep uniform layer thickness.
Age tells arches from troughs. An anticline has its oldest layers in the core, a syncline its youngest, whichever way up it now lies. Where the age sequence is unknown, geologists use the neutral terms antiform and synform. Domes and basins are the circular versions, and a recumbent fold lies nearly flat, flipping one limb upside down.
The rock's strength, or competence, controls the style. A thick, strong layer embedded in weaker material buckles into rounded folds, while alternating sandstone and shale tends to form kinks, box folds and chevrons. Very weak rock such as salt, or rock buried deeply enough, simply flows, passively taking whatever shape its stiffer neighbours impose, much like glacier ice. In the Jura Mountains, thrusting over a slippery layer of Middle Triassic evaporites produced box-style detachment folds. Even fresh sediment slumping down a slope folds at its leading edge, and the asymmetry of those slump folds reveals which way the ancient seabed tilted.
Source: Fold (geology)
