without suitable controls

Washing it just before heading out on a route which involved the Roseburn path hasn't helped, though at least the freshest muck will be actual muck rather than rust and lots of tiny bits of ground-up chain and chainring and sprockets. The relatively pristine nature of the rears of some bikes seen around town makes me wonder if my regular routes just happen to involve particularly glag-covered streets, of which the Cowgate immediately suggests itself a likely candidate. At this stage in the drivetrain's life it's quite likely that the large amounts of gack accumulating between washings are little bits of carved-off tooth-metal, held in suspension in excess lubricant. Perhaps it's just not as rainy as it should be, resulting in less frequent accidental washings.