Successive formations are in most cases separated from each other by blank intervals of time of great length, for fossiliferous formations thick enough to resist future degradation can, as a general rule, be accumulated only where much sediment is deposited on the subsiding bed of the sea.
Jarraikako formazioak, kasurik gehienetan, iraupen handiko hutsartez bereizirik agertzen dira sarri; izan ere geroko erosioari eusteko moduko formazio fosildun lodirik, arau orokor gisa, ez daiteke forma beheraka doan itsas hondar batean sedimentu asko metatzen den tokian bestetan.