To some minds, that which is unknown is simply invisible. To a mind like Thomas Jefferson's, the unknown is infinite in its possibilities. How little was known about the Louisiana Territory? The president, whose encyclopedic interests included paleontology, thought there was a chance that the woolly mammoth and the giant sloth might still exist in the West. He had heard stories about active volcanoes along the Missouri, a mountain made of salt, and a tribe of white Indians descended from Welsh explorers who reached America before Columbus. There was no good reason to discount any of these things.

Lewis and Clark made the land real for Jefferson, but what they found was perhaps no less fantastic than what he imagined. No mammoths roamed the prairies, but there was a monster much more ferocious, the "verry large and turrible" grizzly bear.There were no mountains of salt, but the Rocky Mountain range turned out to be a sight more spectacular than anyone dreamed. It was also the hard fact that dashed the best hopes for the expedition.

Jefferson pictured the Rockies as similar to the low mountains he could see from Monticello. It was a misconception at the very heart of his plans for the future of the nation. He assumed that the Missouri and Columbia rivers, if not connected, were separated by a short and easy portage. This "water communication" might become an expressway to Asian markets for American fur traders.

The actual Rockies presented, as Lewis put it, a "snowey barrier" to the Pacific. The party's pack horses skidded and rolled down the precipices. Food supplies ran so low in the barren alpine heights that the men began eating the horses.

The story of the Lewis and Clark expedition takes on an immediacy when read in the original form of daily entries. When the authors are blind to what lies around the next bend--when even the Rocky Mountains come as a great surprise--we cannot help sharing their apprehensions and their sense of wonder.