Folds
Sediment is deposited in horizontal layers, called "beds". The oldest sediment is on the bottom and the youngest is on the top (unless the beds are overturned). After deposition, compressional stress applied to a rock will cause it to fold. There are two main types of folds. An anticline is a fold that bends downward, creating a hill-like structure. A syncline is a fold that bends upward in the shape of a "U".
Folds are considered either cylindrical or non-cylindrical. If you were to slice a cylindrical fold (like bread), the sections would appear similar. If you could to take a pencil, without lifting it from the surface of the fold, and bring it from one limb to the other, the fold is cylindrical. Looking at the diagram of the non-cylindrical fold, it is evident that the pencil could not remain on the surface
Cylindrical non-cylindrical
Concept of cylindrical fold
The plunge of a structure is measured as an angle with respect to its position from the horizontal.
Folds can be plunging or non-plunging, according to the inclination of its fold axis. (see below diagrams)
Non-pluging Fold Pluging Folds
The above photograph represents a fold that occurs in Unit 2. Strikes and dips were taken on both limbs and the data was plotted on a stereonet in order to determine the orientation of the fold axis.
Using a Stereographic Net
A stereograph is used by geologists to plot the strike and dip of folds, faults and bed layers uncovered in the field. A Wulff Net stereograph consists of divided areas of equal angle. This will be the type of stereonet used for demonstrations shown here.
All longitudinal lines and the equatorial line are considered Great Circles. A plane that intersects the sphere and passes through the center, lies on a Great Circle. All latitude lines (except the equatorial line) are Small Circles. A plane that intersects the sphere without passing through the center, lies on a Small Circle. Lines are divided into 2 degree intervals, with the darker lines representing 10 degree marks.
Preparing the stereonet:
Place a thumb tack through the the center of the stereonet. Next, place a sheet of tracing paper over the net, pushing through the tack. Put a small pencil eraser on top of the tack (to avoid injuries). Trace over the circumference of the net and mark "North" with a tick mark and "N" on the tracing paper.
Plotting a line: ( coordinate: (120,20SE)
1.Count 120 degrees (clockwise) east from North. Put a tick mark on this spot.
2.Turn the tracing paper until this mark aligns with the E-W equatorial Great Circle.
3.From the tick mark, count toward the center 20 degrees. Put a dot on this spot.
Plotting a plane: ( coordinate: (040,45SE)
1.Count 40 degrees (clockwise) east from North. Put a tick mark on this spot.
2.Turn the tracing paper so the "40 degree tick mark" is aligned with North.
3.From the East end of the equatorial line, count in 45 degrees. Mark with a dot.
4.With the dot lined up on one of the longitudinal Great Circles, trace along the Great Circle from top to bottom. Rotate the tracing paper back where both "North's" line up to view the orientation of the plane.
Plotting a fold: ( coordinate: (020,40W) and (325,70NE)
1.
Count 200 degrees (clockwise) from North. Put a tick mark on this spot.
2.Turn the tracing paper so the "200 degree tick mark" is aligned with North.
3.From the East end of the equatorial line, count in 40 degrees. Mark with a dot.
4.With the dot lined up on one of the longitudinal Great Circles, trace along the Great Circle from top to bottom.
5.Repeat the same procedures for the second coordinate.
A bedding plane is measured in the field and the following coordinate is established: (040,45) The strike of the plane is N40E and the plane dips 45 degrees SE. The dip direction is N130E.
To represent this plane on a stereonet, count (clockwise) 40 degrees east from North. The red dashed line represents the strike of the plane. The solid red line represents the dip of the plane. All points on this line are the apparent dip, other than the 45 degree mark (which is considered the true dip). The pole to the plane is "normal" to the plane and can be located on the stereonet by counting 90 degrees from the "true dip" mark on the solid red line.
See the above instructions on how to accurately plot these points on a stereonet.
Plotting a fold axis
A fold axis is measured in the field with the following coordinate: (120,20). The strike of the fold axis is N120E and it is dipping 20 degrees SE.
The fold axis is represented on the stereonet by counting (clockwise) 120 degrees east of North. (red "tick mark") The red dot represents the 20 degree dip in the fold axis.
Note: A fold axis is a "line", which is represented on a stereonet as a dot.
Plotting a fold and finding its axis
A fold is measured in the field by obtaining the strike and dip of each of the limbs. The fold represented on the stereonet to the left has one limb orientated at 325 degrees and dips 70 degrees to the NE. The other limb is orientated at 334 degrees and dips 30 degrees west.
The fold axis is a line that represents maximum curvature of the fold and separates the two limbs. On a stereonet, this can be found where the two limbs cross each other. In this case, it would be at: (334,30W).
