The Local Natural Monument of Ribeira de Anha preserves the occurrence of a geomorphological element - Giant's Box - partially eroded, with circular section and concave bottom, 140 cm deep, in the vestibular portion of Ribeira de Anha. The internal structure preserves what appear to be 3 parking levels of the dynamic flow, marked by ring protuberances on the inner wall of the pothole. The hemi-portion of the geoform associated with the riverside thalweg has been completely eroded, so the evolution of this structure may be only partially associated with that watercourse, exactly the erosion phase and eventually the abrasion of the bottom of the pothole.