soil

Posted by Ellen

Near the southern extreme of Puget Sound, around Olympia, Washington, are hundreds of acres of bumpy grassland, the Mima Mounds. The picture above–aerial imagery produced by a radar-sensitive LIDAR camera–shows what the Mima landscape looks like without all the grass and shrubs that soften the lumpy appearance. the picture below shows what it looks like to the human eye.

LIDAR is a radar technology used to determine elevation, and it works even when the actual ground level is hidden underneath vegetation or buildings; when a plane flies over an area of interest and sends radar signals straight down to earth, the signals will bounce back differently depending on what they hit. Of course you need a computer to sort it all out, but they've got computers.

What the computers haven't been able to figure out is what caused these mounds, which are pretty much all very round and low and flat; in the Mima prairie, most of the mounds are about 3 feet high and 30 feet across. They are gravelly dirt, just like the spaces between them.

Similar-looking mounds are found in dozens of other places, and no single mechanism has been identified that might account for them everywhere. Among the possible processes that have been researched in the Mina region: earthquakes shaking the soil, clay minerals shrinking and swelling in the soil, windblown dunes forming around vegetation, and–our favorite–burrowing by pocket grophers, perhaps with help from termites and ants.

Scientific consensus on this matter has yet to emerge.