Thomas Moran (1831-1926) was one of the 19th century artists. One of his noteworthy efforts came from an expedition that headed west, the Hayden Expedition (1871). The goals was for Thomas to record the wonders of the Yellowstone area. Because of Moran's efforts the U.S. Congress set this area aside to become the first national park. He made annotated drawings and watercolors.
Moran continued to produce works from his westward expedition well into the 20th century. If you get a chance take some time to search for some of his paintings online.