From Longman Dictionary of Contemporary Englishreachesreaches[plural] a) the parts of a place that are furthest from the centrethe further/outer reaches of something the further reaches of the jungle b) the straight part of a river between two bends the upper reaches of the Nile → reach
Examples from the Corpus
the further/outer reaches of something• They explored the outer reaches of the farm and took long hikes up Talcott Mountain.• Even in the outer reaches of the capital-Bexley, say-you are looking at £137,000.• She was a thin, slender woman, somewhere in the further reaches of middle age. 