The Younger Dryas impacts, and subsequent sustained cataclysm, changed the face of the earth completely and wrought particularly significant havoc ac… - Graham Hancock

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The Younger Dryas impacts, and subsequent sustained cataclysm, changed the face of the earth completely and wrought particularly significant havoc across North America. We have considered the question of huge volumes of meltwater released into the Arctic and Atlantic Oceans from the destabilized ice sheet and looked at the effects on global climate. But keep in mind that those enormous floods also devastated the rich North American mainland to the south, perhaps the best and most bounteous real estate then available anywhere.
This immense and extraordinary deluge, 'possibly the largest flood in the history of the world,' swept away and utterly demolished everything that lay in its path. Jostling with icebergs, choked by whole forests ripped up by their roots, turbulent with mud and boulders swirling in the depths of the current, what the deluge left behind can still be seen in something of its raw form in the Channeled Scablands of the state of Washington today — a devastated blank slate [...] littered with 10,000-ton 'glacial erratics,' immense fossilized waterfalls, and 'current ripples' hundreds of feet long and dozens of feet high.
If there were cities there, before the deluge, they would be gone.
If there was any evidence of anything that we would recognize as technology there, before the deluge, it would be gone.