Some of the long barrows contained stone-lined chambers within them. Early 20th-century archaeologists began to call these monuments "chambered tombs". The archaeologists Roy and Lesley Adkins referred to these monuments as "megalithic long barrows". In most cases, local stone was used where it was available. The style of the chamber falls into two categories. One form, known as "grottes sepulchrales artificielles" in French archaeology, are dug into the earth. The second form, which is more widespread, are known as "cryptes dolmeniques" in French archaeology and involved the chamber being erected above ground. Many chambered long barrows contained side chambers within them, often producing a cruciform shape. Others had no such side alcoves; these are known as "undifferentiated tombs".