Geologically speaking, the Delta is a relatively recent phenomenon, created roughly 6,000 years ago when advancing sea levels at the end of the Ice Age backed up the Sacramento and San Joaquin rivers into a maze of sloughs, channels and wetlands that ebbed and flowed with the tides. Natural levees, some fairly high and crowned with riparian forest, some low and thick with willow and grasses, existed, all subject to flooding on a regular basis as the seasons turned. Over the centuries, the vegetation filling these shallow, changeable waters decomposed into rich peat, in some areas sixty feet deep, whose incredible fertility made an enticing target when European settlers arrived.