Mineral magnetic investigations of sediment cores and catchment materials from Llyn Geirionydd (north Wales) form the basis of a study of Holocene sediment yields and deposition patterns. Catchment soils and bedrock materials are highly variable in magnetic concentration. In contrast, the lake sediments are more homogenous in magnetic concentration. Magnetic hysteresis properties of catchment materials, bulk sediments and magnetic extracts demonstrate that the occurrence of authigenic greigite (Fe₃S₄) in early Holocene sediments is responsible for differences between the magnetic characteristics of sediments older than 5000 BP, and the more recent sediments and catchment materials. A whole-lake stratigraphy based on core correlation using magnetic measurements, pollen stratigraphy and ¹⁴C age determinations indicates sediment yields which increase almost threefold from the early Holocene to the period from c. AD 500-1850.