LONG before Europeans came to perceive America as a gigantic movie set inhabited by Groucho Marx, Humphrey Bogart, and Marilyn Monroe, they saw it as an almost-virgin land. Here the noble savage lived among exotic flora and fauna, in a countryside that was often more scenic than the Alps.
Farsighted observers understood that America's mercantile civilization - quite marvelous in its way - was doing its best to push aside the flora, the fauna, and the native peoples that made our country unique. To the first wave of explorers, this was no problem; they had come to stake out territory, and they did.
It was only later that a second wave of explorers and artists went out to record what was left of the virgin land. Until recently the most famous of these was George Catlin (1796-1872), who dedicated his life to painting and writing about American Indians.
After World War II, however, an archive long forgotten in a castle in the Rhineland was found to contain the original documents of a journey of exploration quite equal to Catlin's. The two principals were Prince Maximilian of Wied (1782-1867) in Prussia, a man of independent means who was also one of the earliest university-trained anthropologists, and Karl Bodmer (1809-93), a Swiss who, in his early 20s, had made a small reputation for landscape painting.
As soon as he arrived in America, in 1832, Prince Maximilian decided there was little time to lose. Of Indiana and Illinois he observed that ''we may preserve here in America neither the aborigines nor the wild beasts, because the beginning of settlement is always the destruction of everything. . . .'' Written more than a century ago, this note anticipates a way of thinking that has become widespread only in the past 20 years.
Karl Bodmer's watercolors, of Indians and trading posts and the land itself, are one of the outstanding accomplishments of 19th-century painting in America.
Although Bodmer's later career in Europe was undistinguished, the two years he spent in America with Prince Maximilian called forth his best. And because of his great attention to detail Bodmer was far superior to Catlin in accurately depicting the people and places they both saw.
Along the upper reaches of the Missouri River, in the state of Montana, the travelers found geological formations of an almost hallucinatory character. Bodmer's ''View of the Stone Walls,'' reproduced on this page, suggests a landscape of fortified hill towns. One nearby rock formation was named ''La Citadelle'' by French fur traders. Maximilian wrote of another place: ''These singular natural formations, when seen from a distance, so perfectly resembled buildings . . . that we were deceived by them until we were assured of our error. We agreed to give to these original works of nature the name of 'The White Castles.' ''
Except for a tour of West Germany and the United States during the 1950s, Karl Bodmer's watercolors have not been widely seen. That omission is splendidly made good by ''Views of a Vanishing Frontier,'' an exhibition organized by the Joslyn Art Museum, in Omaha, Neb., where the watercolors have been on loan since 1962. They are currently circulating among major museums.
Having completed its run at the M. H. de Young Memorial Museum, in San Francisco, on Nov. 18, it will be seen at the National Museum of Natural History , in Washington, from Jan. 4 to March 31, 1985, and at the Metropolitan in New York, from July 17 to Oct. 6, 1985.