He traces the mechanics of ice sheet formation during glacial periods, focusing on two primary nucleation centers: one near Hudson Bay in the East and another over the Western Rocky Mountains. The key distinction he draws is in how precipitation cycles differently during glaciation versus interglacial periods. In warmer climates, rainfall returns to the ocean through watersheds, either quickly via surface runoff or slowly through subterranean aquifers over decades or even centuries. But during ice sheet growth, that same precipitation, evaporated from the oceans, falls as snow and becomes locked in ice rather than cycling back. The implication is clear: as ice mass accumulates, the oceans lose volume, and the hydrological cycle fundamentally changes. This is not speculation, it is basic mass balance. The water has to come from somewhere, and when it stops returning to the ocean, sea levels drop accordingly.