He walks through the scale of North America's ice age geography with precision: two massive ice sheets, the Laurentide comparable in size to Antarctica today, the Cordilleran matching modern Greenland. Around 30,000 to 40,000 years ago, the ice extent remains debated, but the evidence shows it was vastly reduced compared to its maximum. Then, centered roughly around Hudson Bay, the Laurentide begins its relentless expansion eastward and southward. Simultaneously, the Cordilleran grows from the western mountains. By approximately 18,000 years ago, these two colossal ice masses merge, forming a continuous frozen barrier across the continent. The timing and mechanics of this coalescence carry profound implications for understanding climate forcing, sea level dynamics, and the routes available for human migration into the Americas.