From the upper Wallis, they began to spread south, west and east between the 12th and 13th centuries, in the so-called Walser migrations ("Walserwanderungen"). The causes of these further population movements, the last wave of settlement in the higher valleys of the Alps, are not entirely clear. Some think that the large "Walser" migrations took place because of conflicts with the valley's feudal lords. Other theories contend it was because of overpopulation and yet others that they were reinforced by the respective local authorities in order to settle previously unpopulated regions.