Folds

Rocks usually form in flat layers called strata. Tectonic plates can collide with such force that they crumple up these strata. Sometimes the folds are just tiny wrinkles a few centimetres long. Sometimes they are gigantic, with hundreds of kilometres between crests ( the highest point on a fold ).

The shape of a fold depends on the force that is squeezing it and on the resistence 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.

fig. 5 - The main features of a fold