There's also the Roughstuff Fellowship;
The history of the Rough Stuff Fellowship goes way back to its foundation in 1955, long before anyone had ever heard of Marin County. It was formed by cyclists who wanted to get away from roads and cycle off-road on tracks, and byways.
@edscoble started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
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There's also the Roughstuff Fellowship;