There for a while I really hadn't felt the need for another book about the opening of the American West. James Alexander Thom obviously disagreed. And now that I've read this splendid, if endless, new novel about the westering of America, I have to admit I was wrong and he was right.

It's true that nobody, including Thom, can improve on such classics as the fiction and nonfiction of Allan W. Eckert, Bruce Lancaster, John Bakeless and Kenneth Roberts. But a time when blossoming American patriotism threatens to root no more deeply than in shared television shows is a time that cries out for rediscovering some of the deeds and ideas, humanly flawed as they may have been, upon which this country was built. By gloriously engaging the reader in the first, and implying the second, "From Sea to Shining Sea" turns out to be essential after all, for all those readers who look to current fiction for knowledge of their world and missed this subject the last time around. For the rest of us, it's a rousing refresher course.

Back when achievement rather than sensationalism won public recognition, George Rogers Clark and his brother William were household names. George, the "father of Kentucky," pushed the frontier to the Mississippi before the end of the Revolution. Billy, who was so young during his older brother's exploits that he baby-talks through the first 350 pages of the book, later pushed it the rest of the way to the Pacific as a partner in that ultimate American vacation, the Lewis and Clark Expedition.

They emerged from what seems to have been the legendary colonial family, strong, silent father, wise, loving mother (notable here for pithy axioms, like "Give a man an army, and he'll use it," and "One can do whatever he doesn't know he can't do"), six brave sons and four resourceful daughters. The first half of the novel advances along several leading edges as Thom alternates the stories of all the sons and their services in the Revolution with family events back home.

At first this diffusion of subject and narrative seems unwieldy. The occasionally erratic frontier dialect dismays (900 pages of apostrophes ahead?), and the domestic accounts sound artless. Before long, though, everything falls into place around two natural centers, George Rogers Clark's stunning 1779 capture of the British fort at Vincennes in the Indiana Territory, and the westbound half of Lewis and Clark's pilgrimage for science and empire.

In the doing, the three-week Vincennes campaign over 150 miles of flooded plains and river bottoms must have been a feat worse than death. Clark's men, frontier "tavern-wreckin', eye-gougin', horse-stealin', bear-rapin' outlaws and general ring-tailed roarer s " were just the kind to respond to his prodigious leadership, based on "Songs. Jokes. And a dream o' glory." Days they spent in "water ankle deep and sometimes calf deep," or "under the perpetual rain and sleet, crossing these flooded valleys, swimming the horses across, rafting the barrels and bundles over to reload on the pack animals on the other side, still in waist-deep floodwater." In the end, "Hair-Buyer" Hamilton's fort fell without the loss of a single American life.

Equally miraculous, 25 years later only one American died during the Voyage of Discovery, possibly from appendicitis. This mind-boggling 2 1/2-year trek by some 30 white men, a black slave, an Indian woman and her baby, and a Newfoundland dog over a wilderness full of strange plants and people is the epitome of all human fantasies from Homer to Spielberg.

Thom, author of two other books on similar subjects, traveled the entire route of the expedition and knows the feeling: "Range after range of immense, purple-sided, snow-capped mountains stretched away to the west until they were lost in each other . . . Over and through those incredible gleaming towers and dark valleys moved the shadows of clouds, and a silence so deep it seemed to make the ears ring within."

Except perhaps for George, who may be credited with finer feeling and fewer bloody deeds than history hints, Thom sticks close to the facts. He never soft-pedals the blood and violence, the treachery and greed of Indian and white alike, but he resists cheap thrills as well as the Hollywood temptation: Sacajawea as Raquel Welch. He is clear about the ingratitude of a government that kept voting swords of honor to George but never reimbursed him for his financial self-sacrifice on behalf of his country. (William fared somewhat better.)

"No one family wrought more change on this country than did the Clarks o' Virginia." As historical fiction, "Sea" makes no new waves. But Thom tells the story with humor and eloquence, and a thumping good tale it is, too.