No explorers were more persevering than Meriwether Lewis and William Clark. When they boated up the Missouri River in 1804, they had no idea the Big Muddy was 2,700 miles long. They were completely ignorant of the existence of the Rocky Mountains, a seccession of barrier ranges 180 miles wide (60 miles covered with eternal snow) at the point where the expedition reached them. They were fools enough to shoot the narrows of the Columbia in dugout canoes, where the river hurtles through the Cascade Mountains. But they got where they were supposed to go. They reached the Pacific Ocean at the Columbia's mouth. They got there because, by God, Thomas Jefferson told them to go there.

David Freeman Hawke in his "Those Tremendous Mountains" lays the historian's wreath of tribute: Because Jefferson "issued the most demanding set of instructions any explorer . . . had ever been burdened with" and because Lewis "obeyed them to the letter, what might have been little more than an adventurous trek that got where it was going was transformed into one of the greatest expeditions in [the] history of exploration."

When the exploring party, a couple of dozen U.S. Army men, returned to settled territory on the Mississippi in 1806 after a 28-month absence, the infant American republic, having given them up for dead, was electrified. Years later Jefferson recalled that "never did a similar event excite more joy thro' the United States."

The expedition's story has been told many times, most impressively by Bernard DeVoto in "The Course of Empire." The prime source for any narrative remains Lewis and Clark's journals, deposited by Jefferson with the American Philosophical Society and printed in eight volumes. As literature the journals are hard to beat, full of dramatic incident (grizzly bears, floods, starvation, rattlesnakes, chance encounters with Indian braves in the wilderness) and constructed around a central "quest" motif, the search for an all-water route from the Mississippi to the Pacific.

Hawke's history has the merits of conciseness and the presentation of new information. He explains the equipment the explorers used. He directs attention to the expedition's extensive scientific work -- the collection of botanical and wildlife specimens, weather data and ethnographical facts about the western Indian tribes. He examines the psychological relationship between the co-commanders.

To round out his narrative, Hawke quotes extensively. The extracts from the journals are satisfying. We hear the sound of fiddle music around the campfire in the midst of the awful loneliness. Clark's description of a 600-pound, 7 1/2-foot high grizzly as a "verry large and terrible looking" animal is wonderfully childlike in its accuracy. Jefferson's instructions to Lewis are printed in full, a state paper of commanding vision and precise detail. As with all accounts of this fabulous journey, we are awed as the travelers move slowly through scenery of incomparable natural grandeur, an earthly paradise not yet marred, a demi-continent inhabited by gigantic herds of buffalo and a few shy, sometimes sullen, rarely hostile Indians.

With "Sacagewea of the Lewis and Clark Expedition," we leave history for the shadowy world of legend, although the authors, Ella E. Clark and Margot Edmonds, valiantly attempt to record what definitely is known about Sacagewea, or "Bird Woman." This Shoshone woman who accompanied the explorers to the Pacific and back again from the upper Missouri, figured in the climactic first ascent of the Continental Divide when the explorers were aided by a band of Snake tribesmen led, mirabile dictu, by her long-lost brother.

The temptation to turn Sacagawea into a Pocahontas West was long irresistible to some chroniclers. The historian James Truslow Adams asserted she was one of the six most important American women. In fact, her role as the expedition's "guide" rests solely on her recognition of a few landmarks, and she was not even present at the crucial first encounter with the Snake tribesmen. Clark and Edmonds firmly debunk the Indian-Princess-Leading-the-White-Men-Over-the-Shining-Mountains legend. They also print the annals of a woman who died on the Wind River Reservation in western Wyoming in 1884 whom they accept as the historical Sacagawea. Unhappily, the oral history on which their acceptance is based, collected for the most part in 1925, seems too suspect to be altogether convincing.