Most of our knowledge of Celtic religion derives from iconography and epigraphy dating to the Roman period. This is augmented by both classical and vernacular Celtic (Irish and Welsh) literary material, though much of the latter is chronologically too late in its extant form to be useful. In addition to the inevitable interpretative bias of the Graeco-Roman authors, their writings dwell largely on ritual rather than the nature of belief-systems. Thus, although the Druids and sacrificial ceremonial are discussed at length by such commentators as Strabo and Caesar, there is little mention of the gods themselves. In the pre-Roman period, the elaborate character of many Iron Age graves attests only the strong belief in an afterlife, endorsed by contemporary Mediterranean observers. Once again we are brought no closer to the deities of the Celtic world.