Menhir du camp de César Where menhirs appear in groups, often in a circular, oval, henge or horseshoe formation, they are sometimes called megalithic monuments. These are sites of ancient religious ceremonies, sometimes containing burial chambers. The exact function of menhirs has provoked more debate than practically any other issue in European pre-history. Over the centuries, they have variously been thought to have been used by Druids for human sacrifice, used as territorial markers, or elements of a complex ideological system, or functioned as early calendars. Until the nineteenth century, antiquarians did not have substantial knowledge of prehistory, and their only reference points were provided by classical literature. The developments of radiocarbon dating and dendrochronology have done much to further knowledge in this area. The word menhir was adopted from French by 19th-century archaeologists. The introduction of the word into general archaeological usage has been attributed to the 18th-century French military officer Théophile Corret de la Tour d'Auvergne. It is a combination of two words of the Breton language: maen and hir. In modern Welsh, they are described as maen hir, or "long stone". In modern Breton, the word peulvan is used, with peul meaning "stake" or "post" and van which is a soft mutation of the word maen which means "stone".