Serpent Mound, in northern Adams County, Ohio, USA, is one of the most iconic symbols of ancient America and yet there is no widely agreed upon date for the age of its original construction. Some archaeologists consider it to have been built by the Adena culture around 300 bc, while others contend it was built by the Fort Ancient culture around ad 1100. There have been three attempts to obtain radiometric ages for the effigy, but they have yielded inconclusive results. The iconography of the earthwork offers an alternative means of placing the mound in its cultural context. Serpent imagery is abundant in the Fort Ancient culture as well as in the more encompassing Mississippian Ideological Interaction Sphere. Pictographs from Picture Cave in Missouri include a serpent, a humanoid female and a vulvoid in close association. We interpret these elements, in the light of Siouan oral traditions, as First Woman and her consort the Great Serpent. The Picture Cave imagery dates to between ad 950 and 1025. We argue that these same three elements are represented in the original configuration of Serpent Mound and therefore situate its design and original construction in the Early Fort Ancient period.