The Marilyns:

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The Marilyns are a set of hills in Britain which are defined by their relative height: they must have 500 feet of descent on all sides (like the Corbetts) but there is no minimum height. They were defined by Alan Dawson in his excellent book The Relative Hills of Britain which is available online at that link as well as from Amazon and so on. There are 1500 Marilyns, and the attempts of people to climb them all are documented in the Marilyns Hall of Fame