An anticline is an example of rocks which were previously flat, but have been bent into an arch. It is thus an arch-shaped fold in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. Oil that finds its way into a reservoir rock that has been bent into an arch will flow to the crest of the arch and get stuck (provided of course that there is a trap rock above the arch to seal the oil in place). Opposite of syncline.