Fold Facts

  • Rocks usually form in flat layers called strata. Tectonic plates can collide (see converging plates) with such force that they crumple up these strata.
  • Sometimes the folds are just tiny wrinkles a few centimeters long. Sometimes they are gigantic, with hundreds of kilometers between crests (the highest points on a fold).
  • The shape of a fold depends on the force that is squeezing it and on the resistance of the rock.
  • The slope of a fold is called the dip. The direction of the dip is the direction in which it is sloping.
  • The strike of the fold is at right angles to the dip. It is the horizontal alignment of the fold.
  • Some folds turn right over on themselves to form upturned folds called nappes.
  • As nappes fold on top of other nappes, the crumpled strata may pile up into mountains.
  • A downfold is called a syncline; an upfolded arch of strata is called an anticline.
  • The axial plane of a fold divides the fold into halves.