[derbOac]

Many remains of the mound building civilizations in the present-day US were plowed away, as they were in prime farming geography. You're right that many of the structures were made of wood and dirt that are less lasting than stone, but there were a large number of them that were lost through field preparation. The reasons for this are complex, including a disregard for Native American structures, but in many cases people just didn't know what they were.

The mound building civilizations of North America are fascinating to me, not the least of which is because of the pyramidal-mound structures of Central America.