We must now turn to the theories which make these terraces to be deposited by the rivers when so charged with water that the whole valleys, from their solid foundations up to a height of at least 80 feet, were filled with water. These theories postulate no gigantic denudations extending over limitless ages, but accept the valley terraces as fluviatile deposits left along their own banks by the rivers when, by means of exceptional causes, they were, at one time, so flooded, that they actually reached the upper level terraces which in fact represent their old banks when so flooded. Two causes for such abnormal floods have been assigned. Mr. Tylor postulates a Pluvial period, characterized by most exceptional rainfall; while Mr. Belt urges that these floods were caused by the pounding back of the European rivers, and the consequent formation of a European lake by a great Atlantic glacier.