Not to be difficult, but what is it about Mt. Rushmore that indicates that it was designed?
First, just to make sure we're all speaking about the same thing, when folk refer to Mt. Rushmore I take it to mean the artwork and not the whole mountain.
I think there are two things that indicate design. One is we have other examples of similar faces that were produced naturally. One good example until it collapsed was the "Old Man of the Mountain" in Franconia State Park, NH. If the level of detail of the two were compared, don't you believe that the difference in detail alone would be sufficient to infer design? Secondly, all four faces on the Mt. Rushmore carving are portraits of specific, documented historical figures. Unlike generic depictions, for example portraits of Jesus, we are fortunate enough to have paintings done from life of two of the figures and photographs of the others.
What is it about Mt. Rushmore that should convince me that there are no natural processes that could have produced those faces?
I would say the tool marks left on the sculpture. The marks themselves are too regular and from processes we are intimately familar with.
Aslan is not a Tame Lion