Structural Geography Overview: Synclines
In structural geology, a syncline is a fold structure in which the sides of the fold slope together. To geologists, folds are defined by waves in stratified layers of rock of the Earth's crust. Stratified layers of rock are generally formed by sediments layered over time on a flat, horizontal surface. But as the Earth's crust continues to move through their natural phases of destruction and rebuilding, these formerly flat layers of rock become warped and can create folds. The type of tension typically associated with the formation of folds is the compressional stress that occurs with the forces that build the Earth's mountainous landscapes, however, folds are rarely formed by a single process. In nature, the Earth's crust is constantly responding to a combinations or forces, pressures, and tensions.
There are two primary types of these folds: synclines and anticlines. Technically, until the relative ages of the rocks are confirmed, the structures of these folds can be called syncforms for the downward closing fold and antiforms (which are generally also anticlines) for the upward closing form. The terms syncline and anticline refer to these structures and the relative ages of the layers of rock involved. A tip for remembering the typical shapes of these structures is that antiforms generally form an "A" shape, whereas syncforms form the shape of the bottom of an "S."
In a syncline, the fold forms a concave upward shape with the layers of younger rock (or rock that had been originally deposited on the top-most layer of the horizontal surface) closer to the center or hinge of the structure.
Synclines in Nature: Ellesmere Island
Most natural synclines do not form as perfectly as the teaching diagrams illustrate in geology textbooks. In fact, geologists generally determine locations of anticlines and synclines only after erosion has exposed the rock layers. Additionally, the forces shaping the rock can be so gentle (in relative geological terms) that that the folds are be barely perceptible. Synclines have been discovered to be as small as a hand specimen. However, a geologist's syncline dream can be realized in the Canadian Arctic on Ellesmere Island, where an extremely well-formed and large syncline exists. Composed of fossiliferous limestone, shale, and sandstone, the Ellesmere Island syncline is one of the most famous. To make the syncline all the more unique, the Ellesmere Island syncline is actually an antiformal syncline, or one that has occurred in overturned strata (sedimentary rock that has been turned upside down after the fold was created).
Synclines and Typography
Geologic structures like synclines do not always affect the topography of the surrounding landscape. As such, a syncline does not necessarily create a valley or basin landscape. Synclines tend to have a greater effect on topography in very young landscapes, but generally the expression of a syncline is dependent on the rock layers themselves. If softer rocks are in the core of the syncline, they will erode to form a valley, but if the inner layers are harder, erosion carves them into a peak instead.