-Stage 2.1-

Folds

There are many types of fold that a geologist could study. These include:

Anticline

Syncline

Plunging Folds

and Non-symmetrical Folds

You should also be aware of how to name folds.

But before going into those, you need to understand some basic geologic map skills.

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Geologic Rock Units

Geology Legend

  • The list on the right is representative of a geologic map legend
  • Rocks on maps are listed in age order
  • The rocks on the top of the list are the youngest
  • The one's on the bottom are the oldest
  • This resembles real-life where rocks are deposited, then the "younger rocks" are deposited later on top of them
  • These rock units will be used in the proceeding pictures.

Anticline

Anticline

  • A fold in the rocks resulting in an "A" shape (A for Anticline)
  • The top side of the box on the left represents the Earth's surface
  • The front of the box shows the type of fold
  • The dotted line represents the "fold axis"
  • The resulting fold has the oldest rocks in the center
  • Then rocks repeat on both side of the center symmetrically
  • The width of the units on the surface are equal on both sides of the center unit.

Syncline

Syncline

  • A fold in the rocks resulting in a "U" shape
  • The top side of the box on the left represents the Earth's surface
  • The front of the box shows the type of fold
  • The dotted line represents the "fold axis"
  • The resulting fold has the youngest rocks in the center
  • Then rocks repeat on both side of the center symmetrically
  • The width of the units on the surface are equal on both sides of the center unit

 

Plunging Folds

Plunging Syncline

Plunging SynclinePlunging Anticline

Plunging Anticline 

 

  • The direction of plunge is visible along the side of the blocks
  • This changes the symmetrical pattern seen along the surface
  • The new pattern is giant curves on the earths surface
  • The "arrow" pattern points in the direction of plunge in an anticline
  • The "arrow" points in the opposite direction in a syncline

Non-Symmetrical Folds

Non-Symmetrical Anticline

Non-symmetrical Anticline

  • Non-symmetrical folds cause the same rock units to be different widths on either side of the fold axis
  • On the surface, Synclines and Anticlines will both have the unequal unit widths in non-symmetrical folds
  • On the surface, the side with the thinner outcroppings overlie the steeper portion of the fold

Identifying Folds

When identifying folds there are 3 types of qualifiers that would normally be used for a complete description:

 

Anticline or Syncline

Symmetrical or Non-symmetrical

Plunging or Non-plunging

(typically you don't say "non-plunging", you would just leave off that term)

Examples:

Plunging Symmetrical Anticline

Non-symmetrical Syncline