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The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature - hydrothermal gradient, as evidenced by their presence in softsediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature oforogenic zones.
Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms:
Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata which, altogether, result in deformation. The best analog is bending a phone book, where volume preservation is accommodated by slip between the pages of the book.
Buckling - Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness. Folding under this mechanism is typically of the similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically.
Mass Displacement - If the folding deformation cannot be accommodated by flexural slip or volume-change shortening (buckling), the rocks are generally removed from the path of the stress. This is achieved bypressure dissolution, a form of metamorphic reaction, in which rocks shorten by dissolving constituents which move to areas of lower strain. Folds created in this way include examples in migmatites, and areas with a strong axial planar cleavage.
Folds are classified by their size, fold shape, tightness, symmetry:
Fold shape - It is necessary to convey a sense of the shape of the fold. A fold can be shaped as a chevron, with planar limbs meeting at an angular axis, as cuspate with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength.
Fold tightness is defined by the angle between the fold's limbs, called the interlimb angle. Gentle folds have an interlimb angle of between 170° and 180° , open folds range from 170° to 90°, tight folds from 90° to 10°, and isoclinal folds have an interlimb angle of between 10° and zero, with essentially parallel limbs.
Fold symmetry - Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length are termed symmetrical, and those with highly unequal limbs asymmetrical. Asymmetrical folds will generally have an axis which is at an angle to the original, unfolded surface which they formed upon.
Some types of folds include
Anticline: linear, strata dip away from axial center, oldest strata in center.
Syncline: linear, strata dip toward axial center, youngest strata in center.
Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.
Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
Monocline: linear, strata dip in one direction between horizontal layers on each side.
I think this location will surprise you. When I think of folding, I see the slow curves of synclines and anticlines along road cuts along highways (Rt. 22/322)
To gain credit for this EarthCache, please email the owner the answers to the following (do not put answers in your log):
1. Describe the rocks and the folding that you are viewing.
2. Using the classification presented above, classify the folding you see here.
3. What type of folding do you see here?
4. What do you think happened here to cause this formation?
5. Take a photo of yourself with your GPSr in hand in front of the outcroppings in the area and post it with your log.
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