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The historic significance of the geologic developments leading to the formation of the Sweetwater River Valley is that they produced a break in the Rocky Mountain chain. That break became an important part of a major central east-west overland route that extended from the Missouri River to and through the Rocky Mountains. Along that route--the Oregon Trail--fur trappers, goldseekers, homeseekers, merchants and troopers rode horseback or in wagons, walked, or pulled and pushed handcarts during the century that lasted from 1812 to 1912. The former date is the year the Astorians under Robert Stuart followed the trail from west to east on their journey from the mouth of the Columbia River. The latter year is said to be the one in which the last wagon train passed over the trail. There were at least three prominent landmarks along the trail. At the eastern end of the Sweetwater Valley was Independence Rock, a large protruding granite mass. The rock was a midway point in the journeys of those bound for the West Coast. Five miles west of Independence Rock is a second Sweetwater landmark, Devils Gate. Fifteen miles upriver from Devils Gate is Split Rock, the last of the three granite landmarks along the Sweetwater. To some, such as pioneer photographer W. H. Jackson, its summit was not a split rock but was seen as ''Twin Peaks''. For a day or two following their passing of Split Rock, emigrants could look backward at the V-shaped notch as they moved up the Sweetwater Valley toward South Pass. At the base of the pass the Sweetwater country was left behind and the emigrants crossed the Continental Divide, moving into the Pacific watershed and entering the long-anticipated Oregon Country.