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LiDAR finds pyramids in American woods
The mound builders
Mississippian civilization begins around 900 AD, when its people got corn from Mesoamerica. Once they could grow big crops of it, they built huge communities along the major rivers, from the Mississippi out to the East Coast. Much of that land is now national park, wilderness, or a private ranch of 3,000 acres, so nobody had seen the mounds until LiDAR.
Lidar America
One archaeologist has found almost 80 pyramid sites this way. A Facebook page called Lidar America posts a new image almost every day: an aerial shot of woods that looks like nothing, then the LiDAR version with three pyramids in it.
What remains
Many pyramids were destroyed in the 1700s and 1800s as farming spread across the cropland, so the fear was that all of them were gone. The LiDAR images suggest hundreds probably remain. LiDAR cannot see underground, but half a dozen major sites already operate as archaeological parks open to visitors.
How does LiDAR see through trees?
An aircraft fires hundreds of thousands of laser pulses per second at the ground. Some pulses slip through gaps in the leaves and bounce off the soil. Software separates those ground returns from the treetop returns and builds a bare-earth map. Even a low mound or a faint ditch shows up as a shape in that map.
Why did Mississippians build mounds?
Platform mounds held temples and the houses of leaders, and some covered burials. Builders carried the earth in baskets and stacked it layer by layer. Cahokia, near modern St. Louis, was the largest Mississippian city, and its Monks Mound is the biggest earthen mound in North America. Cahokia's population peaked around 1100 CE.
What destroyed so many mounds?
Plowing leveled the mounds that stood in farm fields, and cities and railroads covered others. Early settlers also dug into mounds for soil and for objects. Surveys in the nineteenth century recorded many mounds that have since vanished. Mounds on land that stayed forest or ranchland survived because nobody plowed them.
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