Folded Rocks

Although the earth we stand on seems quite stable, in reality the earth is a dynamic system and undergoes many changes. Some of these changes result in the fracturing and faulting of the earth. I will discuss the faulting and the results of faults (earthquakes) in a later article. At other times, forces are applied to the earth that alter the rocks without breaking them. The result is rocks that bend and fold but do not break. Folded rocks come in many sizes and types depending on the pressure applied to the rocks. Some folds are like Devils Slide (at right) near Yellowstone National Park. These rocks of the Chugwater Formation have been folded so much that they are nearly vertical! This article looks at the different types of folds and a little of how they are formed.

All folds have a basic structure to them. This structure is important to understand as fold types are named based on the orientation of the basic components of a fold. The hinge line is an imaginary line along which the fold bends. Imagine taking a piece of paper and bending the ends of the paper down. If you draw a line along the hump that forms in the middle of the paper you have drawn a hinge line. A separate hinge line is located at each layer of rock. Another imaginary line can be drawn connecting all the individual hinge lines, this line is called the axial plane. The sides of the fold to the left and right of the hinge line is known as the limb.

There are two basic types of folds. An anticline is a classic arched fold, where the limbs of the fold dip away from the hinge line. You can imagine the anticline as an upfold, with the open end of the fold pointing downwards. A syncline is the opposite of an anticline. In a syncline, the limbs of the fold dip toward the hinge line. The syncline is a downfold, where the open ends of the fold point upward. Anticlines and synclines are generally found in groups, a syncline sandwiched between two anticlines. The folds are the result of steady compressional pressure applied to the rocks over millions of years.

Nature does not generally provide us with classic examples of anticlines and synclines. Generally, folds dip into the ground at an angle, known as a plunging fold. Folds by their nature are hard to see on the surface unless a man-made or natural feature cuts through the fold. In a horizontal fold or non-plunging fold, the underlying rock layers are exposed on the surface as horizontal stripes. These might simply be confused as tilted rocks and not a fold without other information about the rocks under the ground. In a plunging fold, by their nature of dipping into the ground, the layers of form a "V" on the surface. This can make the fold a bit easier to identify on the surface.

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