a funerary barrow having an elongate shape, sometimes constructed over a megalithic chamber tomb and usually containing one or more inhumed corpses along with artifacts: primarily Neolithic but extending into the Bronze Age.
Compare round barrow.
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Examples from the Web for long barrow
Their graves of long-barrow type are found not only in the chalk areas but on the margins of the lias formations.Ancient Man in Britain|Donald A. (Donald Alexander) Mackenzie