The 38-km-diameter Richat Dome, in central Mauritania, has been widely regarded as a possible astrobleme. Our field examination of the structure and subsequent petrographic study of the critical rock types failed to reveal a single feature attributable to shock processes. We, therefore, dismiss an impact origin for this structure. It must, instead, be endogenic. The same conclusion applies to the much smaller, associated, Semsiyat Dome.