They cite the Cambrian explosion — the appearance of abundant fossils in the upper Ediacaran (Vendian) Period and lower Cambrian Period — as the pre-Flood/Flood boundary, the presence in such sediments of fossils that do not occur later in the geological record as part of a pre–flood biota that perished and the absence of fossilized organisms that appear later (such as angiosperms and mammals) as due to erosion of sediments deposited by the flood as waters receded off the land.