Some river valleys are steep and narrow; others are broad, with gently sloping sides. A careful study of the different valleys in any large country such as Great Britain, shows that their forms vary according to the nature of the rocks through which rivers flow. Where hard rocks abound, the valleys are steep and narrow; where soft rocks occur, the valleys are broad and low. This is only what might be expected, for hard rocks are not easily worn away; a river must cut its way through them, leaving cliffs on either side that cannot be wasted away by rain. But in a district where clay or soft sandstone occurs, the rain, as it finds its way to the valley, will wash them away and give a smooth gentle slope to the sides of the valley. It is very instructive to notice how the scenery of any district depends on the nature of its prevailing rocks. Hard rocks give bold scenery with steep hills and rocky defiles; while soft rocks make the landscape comparatively flat and tame, though often very beautiful in its way, especially where a rich soil abounds, so that we see pleasant woods, rich pasture-land, and heavy crops in the fields. Compare, for instance, the scenery of Kent or Surrey with that of the Lake District or the west of Yorkshire. The difference is due chiefly to the fact that in Kent and Surrey we have rocks that succumb more easily to the action of rain and rivers, and consequently are worn away more rapidly than the harder rocks in the north country. Geologists have a word to express the effects of this wear and tear; namely, "denudation," which means a stripping off, or laying bare.