He explains how Manhattan's foundation was literally built by catastrophic meltwater floods at the end of the last ice age. When the ice sheets retreated, the Hudson River wasn't just flowing, it was conveying orders of magnitude more water than it does today. These weren't normal seasonal floods. They were massive pulses of glacial meltwater carving and reshaping the landscape in real time.
First, glacial masses carved the valley itself. Then came the floods, washing debris downstream and depositing it in what geologists call a fan delta. That's the ground Manhattan now sits on: sediment laid down by Ice Age megafloods. It's a reminder that the landscape we take for granted was shaped by forces far more violent than anything we see in the historical record.