The results of the great extension of ice action in that period which geologists call Pleistocene were twofold. In the first place, as the result of the presence of the ice-sheet, we have vast accumulations of débris spread over the lower grounds. These accumulations sometimes form great sheets of boulder clay; sometimes they are collected into the curious sandy and gravelly mounds called kames which in parts of, _e. g._ Scotland, have a great extension; sometimes they have formed great heaps of material at the entrances of valleys. Again, these deposits have sometimes blocked valleys and so formed lakes, and they have supplied the post-glacial rivers with a vast amount of material which has been used to scour out the river-beds, and has been often re-sorted and re-arranged by running water. Secondly, the fact that the northern region and the high grounds further south, in both Europe and North America, have been recently clad in ice is associated with many peculiarities of surface form, some of which have exercised a marked influence on human settlements and ways of communication. These peculiarities of surface moulding have been the object of singularly detailed study in late years, and from this detailed study many interesting facts have emerged. It may be well to state at once that this study has been largely stimulated by the fact that there is at present a great want of unanimity of opinion as to the exact cause of these peculiarities of form. According to one school ice is a more powerful eroding agent than water; according to another its action is largely conservative, and its power of erosion is slight as compared with that of water. The beginnings of a possible solution of the problem are perhaps to be seen in the suggestions of those who seek the causes of the peculiar features of glaciated regions in the way in which running water works when it is controlled and modified by the existence of ice; but we must admit that, on the whole, the conflict is still hot and many members of the opposing schools will have no compromise. To the geographer, however, the very fierceness of the controversy has been useful. The question as to the exact part played respectively by water and by ice in surface moulding is really a question for the geologist. It is, however, of great importance to the geographer that recently glaciated surfaces should be studied from every point of view, for from this detailed study are emerging many important generalisations. We shall, therefore, in this chapter only touch very lightly upon the actual points in dispute, but shall lay stress upon the interesting facts admitted by both parties.