The theory, developed in the late 1830s and 1840s by Venetz, de Charpentier, and Agassiz, that most of Northern Europe, N. America and the north of Asia, had been covered by ice sheets during a period later termed the Pleistocene. The hypothesis was used to explain erosion, and the subsequent deposition of till or boulder clay, and the extinction of species such as the mammoth. Since that time, glacial theory has been developed to include multiple glaciation, and evidence of much older ice ages.