120 Million Years Ago—Mountains Rise. The northern continent extends laterally for more than 60 miles (100 kilometers) beneath northern Alaska, thickening the crust.
The position of the coastline is sensitive to changes in sea level. During the peak of the last ice age, about 18,000 years ago, the coastline was near the edge of the continental shelf. It would have been poss More
Near the beginning of the Paleozoic Era, 500 million years ago, western North America was smaller than it is today, and near the equator. The Colorado Plateau region was along a passive continental margin. The More
Passive continental margins abutting the Atlantic Ocean and Gulf of Mexico developed as a large continent, Pangea, ripped apart about 200 million years ago.
100 Million Years Ago—High Mountains. The mountains reach heights similar to those of the modern-day Alps, as the hard crust of northern Alaska breaks and uplifts.
150 Million Years Ago—Thick Sedimentary Layers. Continents collide but continue to converge.
400 Million Years Ago—Iapetus Ocean Closes. A passive continental margin of ancient North America approaches a subduction zone on the edge of Gondwanaland.