GoForSmoke:
You know that geologic ages ago the Appalachians were big mountains? The Rushmore faces won't last forever unless it gets buried deep. Aren't they already showing signs of erosion? Notice how fossils are found in sedimentary rock? Buried, protected, impressions filled by minerals.Of course NONE of that is true if you're a Creationist. In which case you can't really think and refuse to be honest about what you do see and have.
Given enough time everything goes away, that is for sure, but I think Rushmore is going to survive 1000 years unless someone decides to deface them (there have been recent incidents of similar things happening, e.g. Buddhas of Bamiyan - Wikipedia). The pyramids have survived so far and don't seem to be going anywhere despite, as you say, some small amounts of erosion. It's going to take a lot to erode those away..