The Southern Poverty Law Center has a comprehensive look at the way that pseudo/fringe history (think ancient astronauts and lost civilizations) have frequently been deployed in defense of racism and in turn fueled new generations of racists.
“In the monuments and fortifications of an unknown people, we behold the memorials of a once-powerful race,” said Jackson, “exterminated to make room for the existing savage tribes.”
This reference to a “once-powerful race” was not lost on the American public of 1830. Every schoolboy and girl knew it to be the Lost Race of the Mound Builders, believed to be the continent’s original Caucasian inhabitants. From the colonial era into the twentieth century, it was widely accepted that certain earthen structures and burial grounds proved the existence of “white” or Indo-European peoples who settled North America only to be wiped out by the arrival of Jackson’s “savage (Asiatic) tribes.”
As the country expanded west, the “Moundbuilders” myth had obvious utility: If the Indians destroyed earlier waves of (white) settlers, their own extermination was just another turn of history’s wheel.
The entire essay is well worth reading.
I also can’t recommend highly enough the blog/website of Jason Colavito, who does a meticulous job of debunking fringe historical claims, and of documenting the often strange ways such claims work themselves in and out of popular discourse and understanding of the past.