The Secondary period seems to have been one in which comparatively little folding took place, while, as already indicated, the Tertiary was one in which there was enormous folding in almost all parts of the globe, the result being the appearance at the surface of the great mountain chains of the present day. The structure of these chains makes them relatively unstable, and the forces of erosion are now acting upon them with extraordinary activity, beginning that process of wearing down which has reduced their prototypes of the Primary period to mere remnants of their former greatness. Extensive as the Tertiary folding was, however, it left great areas unaffected, or but slightly affected, and such areas form plains or basins, where the rocks are but slightly tilted, or show a very simple form of folding. In Europe such slightly modified rocks occur, _e. g._ in the Paris basin, and in the fertile plains of south-eastern England. In the United States beds of a similar character occur right over the great plains, filling what seems once to have been a great gulf between the old highlands to the east and the towering modern mountain chains of the west. It must be realised that this is only a very summary and partial account of a difficult and complicated problem; but from the standpoint of pure geography it seems desirable to distinguish between those remnants of ancient mountains which form the backbone of the continents, the recently elevated mountain chains where enormously rapid erosion is taking place, and the largely unmodified rocks which often form fertile plains. Let us next proceed to consider how the eroding agents act upon the surface of the land as soon as it is exposed. We may begin with the effect of running water upon a recently exposed surface, _e. g._ upon land slowly emerging above sea-level, or even with the effect of heavy rain upon sloping ground unprotected by a covering of vegetation. Alike in the one case and in the other the first effect is the formation of a number of shallow rills, which at first run parallel to one another. Sooner or later, however, these parallel channels tend to converge, and a torrent is formed such as may be seen in any mountain region. [Illustration: Fig. 2.--An ideal profile of a mature river (AC), showing the increase in the slope towards the source. The dotted line BC shows an earlier stage, when there are smooth reaches and rapid reaches with waterfalls, etc. Note that progressive erosion causes the source to retreat (_i. e._ from B to A).] [Illustration: Fig. 3.--An actual profile of the Loire. It will be noted that the Loire is a mature river, its profile nearly coinciding with the "ideal" condition. (After de Martonne.)]