 At least one rock type, which is regarded as the starting point, must be repeated.Based on the processes that generate the cyclic deposits, two types of sedimentary cyclic successions can be distinguished.Allocycles are sedimentary cycles caused by processes outside of the depositional system and that involve forced oscillations of the sedimentary system; in this case the cyclic succession registers some features of the forcing process (Sea level fluctuations, climate oscillations or tectonic activity).