From the record
“Effigy Mounds National Monument preserves more than 200 prehistoric mounds built by pre-Columbian Mound Builder cultures, mostly in the first millennium CE, during the later part of the Woodland period of pre-Columbian North America. Numerous effigy mounds are shaped like animals, including bears and birds.”
Wikipedia article Effigy Mounds National Monument, CC BY-SA 4.0.
One of 21 National Register listings published here for Allamakee County · listed in 1966 · category: District · built in 1949.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service