Page:Early Man in Britain and His Place in the Tertiary Period.djvu/170

142 of the other animals, buried in the fluviatile strata below, may be considered preglacial. If, however, it be referred to the action of the snows and the frosts of the late Pleistocene age, the strata in question, from their position below, must be older than that age. The mammalia then inhabiting the district are intermediate in character between those of the forest-bed and those of the late Pleistocene strata, and lived in that area before the cold was sufficiently severe to drive away the big-nosed rhinoceros, and cause the feeding grounds of the stags, fallow deer, and uri, to be enjoyed by the countless herds of reindeer and bisons.

For these reasons, these river-deposits in the valley of the Thames and at Clacton are taken to be older than those of the late Pleistocene so widely distributed through middle and southern England, and they may date back to the preglacial age, as Dr. Falconer inferred from the study of the mammalia. They are, on the other hand, assigned by Professor Prestwich to a late period in the Pleistocene age because they are at a low level. The use of relative levels as a test of age is, however, valid only under two conditions. The valley must be assumed to have been cut down by the stream flowing along the bottom, and the fluviatile deposits to have been formed at different levels as the river bottom became lowered. It must also be assumed that the land