earliest means of recovery, known as streaming or streamworking, involved the collection of alluvial deposits from river and stream beds where they had accumulated after being eroded from the ore-bearing lodes. The geological processes that resulted in the deposition of the cassiterite in the stream beds often resulted in very pure tin gravel which was mixed with gravels of other, unwanted, minerals such as quartz, mica and feldspar, collectively known as "gangue". It was relatively easy to separate these minerals on the basis of their very different specific gravities – cassiterite about 7 and gangue 3 or less. The separation was performed by passing a stream of water over the gravels: the gangue would be washed away faster than the wanted tin gravel. Once a tin-bearing valley had been identified, the stream-workers would arrange a stream of water, probably carried by a leat from higher up the valley, and starting at the lowest end of the deposit they would dig a trench (known as the "tye") as deep as possible to allow the finer gangue to be washed away. From there they would start working up the valley, using the stream of water to wash over the debris they had loosened from the bed with picks. This method of working leaves characteristic evidence in the valleys - a series of ridges of the larger gangue material, sometimes roughly perpendicular to, sometimes coaxial with the line of the valley, sometimes apparently haphazard, all bounded by a scarp which marked the edge of the worked ground and whose height relates to the depth of the deposits. Eluvial deposits—those that had weathered from the lode in the usual way, but had not then been transported by flowing water—were also worked; these tended to be poorer deposits due to the lack of sorting that a stream provides, and they usually did not have such a ready supply of water available to work them. They are found on gently sloping hillsides. Where possible the water needed to work these deposits was carried by a leat from the nearest available river, or if the site was above such supplies, reservoirs were constructed to collect rainwater and runoff from hillsides. A study has shown that a phase of aggradation occurred in the Erme valley between the 4th century and 7th century, providing evidence of late Roman or early post-Roman tin mining activity in this valley. The same