This paper is part of the special publication No.165, Geoarchaeology: exploration, environments, resources. (eds A.M. Pollard). Although there has been considerable speculation, there is as yet no conclusive evidence to confirm Dartmoor as a tin producing region in prehistory. This is in part due to extensive tin mining on the moor during the Medieval period destroying evidence of earlier mining activity. This paper presents the preliminary results of an integrated mineralogical and geochemical geoarchaeological approach to identifying the record of early mining activity. Floodplain sedimentary successions, that have not themselves been mined, and are downstream of known areas of tin streaming, retain a geochemical record of the mining activities because the early tin streaming released large quantities of mine waste tailings. Where these successions can be dated, they provide an indirect means of testing the record of mining activity on Dartmoor. Geochemical and mineralogical analyses of datable fluvial sediments aimed to (a) distinguish between natural and mine contaminated sediments, and (b) to date the onset of sediment aggradation caused by tin mining activity in the headwaters. Results from the Avon Valley show negligible Sn concentrations in basal silts that are dated to pre-2845±45 yrs BP. Overlying silts are enriched in Sn and were deposited after 1560±40 AD, suggesting that the Sn enrichment corresponds with tin streaming in the 16th century. In the Erme Valley, significant Sn enrichment post-dates an organic- rich silt dated at 1280±45 AD. At both sites there is sediment aggradation coupled with an enhancement in Sn concentration consistent with mine waste contamination as a result of Medieval tin streaming.