A serpentine mystery

What would it take to build an earthen mound that would last 900 years? It has to be only three feet tall (ish) but stretch 1300 feet from end to end--and not in a straight line, either. Make it serpentine, with a spiral of coils on one end and an ambiguous arrangement of shapes on the other (head and eye? mouth eating sun?), and make sure the head aligns with the summer solstice while the remaining coils indicate other important astronomical points. Oh, and build it on top of a remote hillside, where every ounce of dirt will have to be carried and put in place by hand.

I couldn't do it, but somebody did and it's still a sight to see 900 years later. I've driven past the sign pointing toward Great Serpent Mound several times but today I decided to break up my drive to Louisville with a visit to the serpent (links here). Only two cars in the parking lot, only me and a family walking around the effigy mound that overlooks rolling hills.
 
It's beautiful, but what is it for? Art? Science? Sacred space? Excavations have yielded little information, but someone must have been highly motivated to make it happen, and others have been motivated to prevent it from eroding away. 

Could the Native Americans who built this mound have imagined that it would last 900 years and provide a respite for traveling strangers? Would they be appalled at our tromping over their serpent or appreciate our wonder? What is this doing here?

I asked the snake but apparently the cat's got his tongue. (Or her tongue? Who knows?)