Structures In Rocks Topics

Geologic Structures (Part 5)

Anticlines and Synclines

Anticlines and synclines are the up and down folds that usually occur together and are caused by compressional stress.

Fold diagram
Anticline and syncline (Diagram by Phyllis Newbill)

Anticlines are folds in which each half of the fold dips away from the crest.  Synclines are folds in which each half of the fold dips toward the trough of the fold.  You can remember the difference by noting that anticlines form an “A” shape, and synclines form the bottom of an “S.” 

Fold block diagram
Folding and surface rock patterns (Diagram by Phyllis Newbill)

After erosion has occurred, geologists can use the patterns of rocks on the surface to determine where anticlines and synclines exist.  In the block diagram above, the top of the block represents the ground surface and what would be shown on a geologic map. The sides of the block show the underground geology. In anticlines, as seen on the ground, the oldest rocks are in the center of the fold.  In synclines, the youngest rocks are in the center of the fold.

Folds in the fieldFolds in limestone (Photograph by Stan Johnson)

MapThis intensely folded limestone from Highland County shows how anticlines and synclines typically occur together.  These sharp folds are called "chevron" folds.