This roadside pullout provides an excellent spot to see a Niehart Quartzite Fold. Look across the road from the parking area you will see the fold.
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Between the observation onsite and the material presented you should be able to answer the following questions.
- Describe the fold: Such as the angle(s) of the fold and number of layers involved?
- Touching the rocks are they generally quite soft or hard and brittle?
- What would you say is the thickness of the largest layer?
- What type of fold do you think this is and why?
What are folds?
Fold occurs when rock deforms in such a way that it bends instead of breaking. The rocks, like any other material, deform to the action of external efforts. We do not grasp this deformation, but we know when a rock is deformed. Folds come from pressure on the rocks that occur over very long periods of time and vary in size from microscopic crinkles to mountain-sized folds.
Geological folds are the curvatures in rocks or strata containing them. Most visible stratified rocks are located in the rivers, quarries or coasts were, originally, sediment layers deposited in horizontal or near horizontal beds. However, when we observe today they are not only solidified but usually inclined in one direction or another.
Sometimes when the layers come to the surface, you can see a bow or climb to descend into a breast. The folds are measured in terms of wavelength (peak to peak or breast to breast) and height (peak-to-trough). They may be microscopic or have lengths of miles.
An isolated fold is defined by the maximum undulation curvature of the strata. The hinge is the line connecting the points of maximum folding in each layer. The axial plane defined meets these lines in successive layers. The shaft is any line parallel to the line bed ripple. When the shaft is tilted from the horizontal is said to be immersed.
According to Wikipedia,
“A geological fold occurs when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds.”
They can be classified according to various factors independently.
Linear
- Anticline: linear, strata normally dip away from the axial center, oldest strata in center irrespective of orientation.
- Syncline: linear, strata normally dip toward the axial center, youngest strata in center irrespective of orientation.
- Antiform: linear, strata dip away from the axial center, age unknown, or inverted.
- Synform: linear, strata dip toward the axial center, age unknown, or inverted.
- Monocline: linear, strata dip in one direction between horizontal layers on each side.
- Recumbent: linear, fold axial plane oriented at a low angle resulting in overturned strata in one limb of the fold.
Other
- 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
- Chevron: angular fold with straight limbs and small hinges
- Slump: typically monoclinal, the result of differential compaction or dissolution during sedimentation and lithification.
- Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.
- Parasitic: short-wavelength folds formed within a larger wavelength fold structure - normally associated with differences in bed thickness
- Disharmonic: Folds in adjacent layers with different wavelengths and shapes