Now we are prepared to understand why it was that the western portion of North America remained for so long a time a mysterious and unknown region. There were no waterways by which it could be explored, while snow-clad mountains and deserts made access to it doubly difficult.
By the beginning of the last century the Americans had overcome the natural obstacles in their westward progress, and their settlements reached as far into the wilderness as the Mississippi River. Hunters and traders were soon pushing far beyond, spreading over the Great Plains and up to the very base of the Rocky, or Stony Mountains, as they were then called. The Missouri River became the great highway into the Northwest, for the adventurers took advantage of the streams wherever possible. Many other rivers were discovered flowing from the western mountains, but with the exception of the Platte and Arkansas they were generally too shallow for navigation even with a light canoe.
Starting in the early spring from the mouth of the Missouri, the hardy trappers sailed and paddled up the river, taking several months to reach the head of navigation at the Great Falls. In the autumn, when the boats were loaded with furs, it was a comparatively easy matter to drop down the river with the current. It would have been almost impossible to transport the loads of goods on pack-horses across the thousand miles of prairie, where the traders would be subject to attack from hostile Indians.
Adventurous men pushed farther and farther west through the passes in the mountains and began trapping upon the waters which flow into the Pacific. It had long been supposed that the Rocky Mountains formed a barrier beyond which our country could not be extended, and that the Pacific slope was made up of mountains and deserts not worth securing.
The explorers showed that the Rocky Mountains were not continuous, but consisted of partly detached ranges, and that while their eastern fronts were indeed almost impassable for long distances, there were places so low that it was difficult to locate the exact spot where the waters parted to seek the Pacific Ocean and the Gulf of Mexico. In southwestern Wyoming the continental divide, known as the Great Divide mesa, though more than a mile above the sea, is but a continuation of the long, gentle slope of the Great Plains.