The first chapter here describes a complete collapse of a civilization and can apply to several scenarios including the 'fall from Eden' itself. So we will not try to pin it down, but recall just how often collapses have happened to antique empires and that they all led to a massive loss of life.
We also get several additional versions of the comet passage of the Pleistocene nonconformity.
Obviously this event was as expects a dominant meme for the survivors. More importantly it shows us that there was ample survival of even some form of organized society.
Add in the excellent description of the organization of irrigation in perhaps Sumeria where they do have a famous reed swamp.
THE BOOK OF MANUSCRIPTS
Great boulders will be hurled down upon men, crushing them into red powder.
The Earth was troubled and shook, the hills and mountains moved and rocked. The dark smoke-filled Heavens bowed over Earth and a great howl came to the ears of men, borne to them upon the wings of the wind. It was the cry of the Dark Lord, the Master of Dread. Thick clouds of fiery smoke passed before him and there was an awful hail of hot stones and coals of fire. The Doomshape thundered sharply in the Heavens and shot out bright lightings. The channels of water were turned back unto themselves when the land tilted, and great trees were tossed about and snapped like twigs. Then a voice like ten thousand trumpets was heard over the wilderness, and before its burning breath the flames parted. The whole of the land moved and mountains melted. The sky itself roared like ten thousand lions in agony, and bright arrows of blood sped back and forth across its face. Earth swelled up like bread upon the hearth.