Or four gods, like the buda statues in asia.. God-kings? What king of ruins would be around Rushmore, anyway? It is near a city or in the wilderness?
For that mater, isnt there some sort of second Rushmore with a famous Native being built somewere on the States? Or I am misrememebring some movie?
There is a Crazy Horse monument only a few miles away, but it's not complete and has continually run into funding problems. There's also a similar monument on Stone Mountain in Georgia comemorating Stonewall Jackson, Jefferson Davis, and Robert E. Lee. The presence of similar monument sites roughly 1500 miles apart might suggest to future archaeologists that a large, contiguous civilization with the ability to carve giant faces in the sides of mountains once occupied at least the eastern half of North America.
The Rushmore and Crazy Horse monuments are in wilderness areas, so there's not a lot of ruins around them to place them in context. At least not anything that would last more than a few centuries (highway and parking areas, interpretive center, gift shop, etc.) The nearest city is only about population 300, so it's doubtful there are a lot of really impressive long-lasting structures there: maybe a bank vault might survive several centuries, but even that's doubtful.
However, there is a chamber, part of a much larger 'Hall of Records' that was originally proposed but never completed, built 70 feet into the rock in the canyon behind the monument. It contains historical information on porcelain panels (full text of Declaration of Independence and Constitution, a history of the United States, and biographies of Lincoln, Washington, Jefferson, and Roosevelt along with that of the architect). This would be an immensely important artifact for a future archaeologist to find.
It's interesting to note that the Mt. Rushmore monument was carved from a mountain that the Lakota Sioux regarded as a holy site:
Paha Sepa, or the Six Grandfathers. So at some point in the future Rushmore might come full circle.

Some geologists have suggested that the mountain may persist in a form recognizeable as a carving for as long as eight million years.