When the conception of a just-vanished period of great glaciation was being established by the labours of many geologists, stress was naturally laid upon the obvious resemblances between parts of, _e. g._ Scotland and Wales, and those parts of the Alps which have been exposed by the retreat of the existing glaciers. Thus we find that most of the text-books emphasise the occurrence of perched blocks, of erratics, _i. e._ of blocks of rock which must have been carried from a distance, of the phenomenon of crag and tail, of giants' kettles, and so on. All these are of more geological than geographical importance; they do not in themselves greatly affect the distribution of other phenomena over the surface. We shall not, therefore, stop to consider them in detail. It is otherwise with those indications of recent glaciation which have been studied within the last few years, and they demand the geographer's most careful consideration. The most active discussion has taken place in regard to the peculiar features of the valleys in recently-glaciated districts, and we shall discuss especially this point. We have already described the general features presented by valleys which owe their origin to running water. In such valleys, as we have seen, the longer the forces work the more nearly is the valley floor reduced to an even slope, whose angle decreases in passing from the mountain to the plain track. In the ordinary river valley the shape of the valley approximates to that of a V, that is, the valley narrows downwards, the river occupying the narrowest region. Again, as a general rule there is no great difference of level between the tributary valleys--at least at their extremities--and the main valley, that is, there is no sharp discordance between the two. While, however, the "mature" river valley shows a gentle, continuous slope, we usually find that "young" rivers, at least in their mountain track, show an alternation of plain and gorge, which is very easily observed in any hilly region. In other words, we find that, owing to the inclination of the rocks, or to their varying hardness, or to other causes, particular reaches are less easily eroded than others. These form waterfalls, which ultimately, as we have seen, give place to gorges. Beyond the waterfall the diminishing slope checks the rapidity of flow, and the stream tends to widen out, and also to throw down its load of débris, so that an alluvial plain may be formed. One other character of an ordinary river valley may be noted. It heads, as we have seen, in a collecting basin, which receives the surface runnels and the outflows of the springs which form the beginning of the river.