The earliest evidence of humans in the Cedarburg area is the Hilgen Spring Mound Site, located in the eastern part of the city of Cedarburg, near Cedar Creek. The site consists of three conical burial mounds constructed by early Woodland period Mound Builders. In 1968, archaeologists from the Milwaukee Public Museum found human burials and artifacts, including stone altars, arrowheads, and pottery shards, during an excavation of one of the mounds. Radiocarbon samples from the excavation date the mounds' construction to approximately 480 BCE, making it one of the oldest mound groups in the state.