I agree with you. Was definitely an ancient quarry.

To those who put forth no research they of course would buy the story of water erosion.

To those who actually study the subject matter, have a keen eye and high IQ know that water erosion wouldn't produce 90 degree angle erosion and it would be much smoother gradual angle.

Another interesting thing to note is that each mound was found to align with a star in a constellation above. Similar to how the Giza pyramids match up to Orion belt. I forget what constellation, you can look it up.

I think some of those mounds could be defaced pyramids. They did find Egyptian artifacts in some of them in the early 1900s.