the gas builds up in the soil, forming mounds called ‘pingoes.’
the gas builds up in the soil, forming mounds called ‘pingoes.’
effigy mounds national monument:7
effigy mounds national monument
however, mounds in the shape of mammals, birds, or reptiles, known as effigies, apparently were constructed primarily by peoples in what is now known as southern wisconsin, northeast iowa, and small parts of minnesota and illinois.