Geological Folds

Geological folds are bends in a layer of rocks. It is a tectonic structure caused by deformation where several layers of rocks have been warped into wavelike, angular, and cylindrical patterns. They affect the horizontal layers of geological material as they are the result of the distortion of the Earth’s crust by powerful forces. Folds range in scale from microscopic in some fine-grained metamorphic rock to hundreds of kilometers across in old epeirogenic warps, with the Michigan basin being an example of it.

Overall, there are two kinds of this structural deformation: parallel fold, which keeps approximately constant layer thickness around the fold measured perpendicular to bedding; non-parallel fold, which has a variable layer thickness measured also perpendicular to bedding. Parallel folds are very common in Triassic limestone and shales of the Mesozoic era.

Parallel folds can further be divided into: 1) cylindrical parallel fold, whose layers of rocks completely curve around into a circular or cylinder-like pattern; 2) curved parallel fold, with the rock layers that compose it being disposed in the form of a curve, like segments of circular arcs; 3) angular parallel fold, whose layers of rocks are arranged at angles as they are more common and widespread than curved parallel folds. Most of the angular parallel folds date back to the carboniferous period of the Paleozoic era.

Curved parallel fold can even be further divided into synclines and anticlines. A Syncline is a downfold; in other words, it is a downward fold in which the strata of rocks convex downwards, in a trough-like pattern. An anticline, on the other hand, is a curved parallel fold that looks like an arch, with the curve being upwards.