A recent archaeological site survey of the island of Sanday on Orkney has identified a distinct concentration of 15 farm mounds. An immediate need is to determine the nature, age, and processes of formation of these very distinctive landform features. A geoarchaeological investigation of three mounds at Westbrough, Langskaill, and Skelbrae was carried out. For the island as a whole, the mounds vary in diameter from 50 to 205 m, with deposits up to a thickness of 4.3 m. The mounds vary in their morphology, reflecting complex processes of accumulation, often intense disturbance, and in some cases, erosion by the sea. Sections at the three sites are described with analytical data also being given for particle size, percent loss-on-ignition, total phosphate, delta¹³C, and radiocarbon dating. The Westbrough and Skelbrae mounds were formed post-7th century; more detailed dating was possible for the Langskaill site, which was initiated in the early to mid 13th century. A steady and relatively rapid sedimentation rate (ca. 1.9 cm year^-1) took place over the 200 years following the initiation of the mound. The lower part of the Westbrough mound is distinguished by the presence of peat, interpreted as discarded bedding material, but the remainder of the investigated sections are composed dominantly of mineral sediments containing organic matter of terrestrial origin. It is suggested that these materials accumulated from the ash of domestic hearths as well as from the residues of bedding material from the byres (cattle sheds). Turves (vegetation plus some underlying soil) would have been cut, dried, and then used in the byres and such turves would have retained a significant mineral content. The implication is that the inherent fertility of the island made it unnecessary for the farmers to spread the dung over the fields. This may explain why there is the distinct concentration of these mounds on the island of Sanday, an island noted in earlier times for its fertile soils developed on calcareous sands.