The lowest layer is gravel dating to the beginning of the Pleistocene 2.6 million years ago. At this time the North Sea covered much of south-east England and the site was in an inter-tidal area, but since then the Chiltern Hills have risen 170 metres. Above this gravel layer is sand, and then another layer of gravel laid down some 20,000 years ago. This was the height of the last ice age, when the site was cold tundra like western Siberia today. According to Natural England the site is controversial as the new excavation shows gravel of possible marine origin rather than glacial till as previously believed.