Fig.1 Earth’s crust before folding Fig.2 Earth’s crust after folding

The upfolded waves are called anticlines and the troughs or downfolds are called synclines. Due to the complexity of the compressional forces, thefolds may develop much more complicated forms. When the crest of a fold is pushed too far, an overfold is formed (Fig.3). If it is pushed still further, it becomes a recumbent fold. In extreme cases, fractures may occur in the

crust, so that the upper part of the recumbent fold slides forward over the lower part along a thrust plane forming an over thrust fold. The over-riding portion of the thrust fold is termed a nappe.