" crossing the Divide and down into the bright and terrible desert "
The Continental Divide of the Americas
, or Great Divide, is the hydrological divide that separates the watersheds draining into the Pacific Ocean from those that drain into the Arctic and Atlantic Oceans.
The Divide runs from Cape Prince of Wales in Alaska down through Mexico and South America, culminating in Patagonia. It largely follows the line of the American and Canadian Rocky Mountains.