Who were the Mound Builders?

In the 19th and early 20th centuries, the term "Mound Builders" was applied to the prehistoric cultures in the Eastern United States who had left behind the earthen mounds found in many places in the Midwest and Southeast. The earthen mounds included burial mounds, platform mounds (upon which temples or other structures were built), and some effigy mounds built in the shape of an animal or bird. Today we know that these mounds were created by many different cultures spanning thousands of years. But in the mid-1800s, the mounds were seen as work of a single culture that some thought could not possibly be the ancestors of American Indians. Some were convinced that the mounds had been built by the Lost Tribe of Israel. This ridiculous notion was abandoned by the late 1800s when the discipline of archeology was established. Large numbers of Indian artifacts found within the mounds showed that these were the work of Native American cultures. Still, many simplistic notions persisted well into the 1900s including the idea that a single Mound Builder culture had existed.