The Western pioneers were well aware of the mounds everyplace they went, but they were too busy hacking down forests and planting farms to stop and take a good look at the time.
There were mounds and pyramids in several areas of the U.S., mainly the northeast, the Mississippi Valley and along its tributaries. There weren't many at all in the West. Yes, they were disregarded by the European colonists who didn't believe that Native Americans could have built them. Many were destroyed along with much archaeological information.