by Dawn Farmer
Last Saturday we decided to explore these strange formations in southwest Washington State. These are large uniform mounds of earth covered in prairie. These formations also go by the name Hogwallows and appear in at least six other states and five other countries.
No one has been able to determine the origins of these mounds, but the leading theory about retreating glaciers got a boost when Department of Natural Resource scientists presented new laser imaging of the mounds.
These new images make a convincing argument that these mounds were formed near outburst channels of melting glaciers. The surging melt water carried the gravel and stones that form the base of these mounds. This theory goes on to say sun cups formed (a glacier surface feature) and continuing melt water deposited more sediment. When the ice was gone these mounds remained.
My simplistic explanation provided with apologies to geologists.
Some folks think these mounds were formed by the vibrations of earthquakes, or gophers, or that they are Native American burial mounds. However these mounds formed, they are currently covered in beautiful wildflowers.
The aerial and laser images are from a Seattle Times article found here.