Creationists: How do you explain the Great Pyramids in Egypt?

According to Ussher Chronology, the great flood happened 2349-2348 BC. We have a problem with this, though. The Great Pyramids at Giza were built over a 14 to 20-year period concluding around 2560 BC. This was over 200 years before the great flood supposedly happened. If there was a global flood after the building of these pyramids, the pyramids would have been almost completely obliterated by turbulent flood waters, as they are made out of a lot of sand. Obviously, we see signs of erosion on the pyramids today because of wind, but there is no sign of major water erosion.