Western emigrants didn’t feel they had “crossed the Rockies” until they’d crossed the Continental Divide. To do that, the westward bound wagons had to traverse rugged and challenging mountains. And at that point, they were about half-way between their start in Missouri and Oregon or California. But what is the Continental Divide? It’s the mountaintop line that governs the waters of the oceans: this is what separates the waters that flow toward the Pacific from those that drain into the Atlantic.