A syncline is a fold in which the rock layers dip towards each other, and the younger layers are in the centre. It is called a synform if the strata dip towards each other, but the older layers are in the centre. When a series of minor folds is superimposed on a large syncline, it is called a synclinorium. In cross-section, the secondary folds have the shape of inverted fans. Both synclines and anticlines are major components of fold belts or fold systems. There are different types of synclines and different causes of formation. Large synclines tend to become structural basins, which are ideal locations for oil- and gas-bearing strata and coal deposition.