In Kent and Surrey the agents of denudation (rain and rivers, aided by the effects of the air, of heat and cold, and so on) wear away the whole surface of the county in a tolerably even and uniform manner, because there are no hard rocks for them to contend with. In this case rain washes away the sides of the valleys faster than the river can carve its bed, consequently the valleys are shallow compared to their width. And so the streams have broad valleys, while the hills are smooth and gently rounded. Chalk, clay, and soft sandstone abound there. The two latter rocks are washed away with comparative ease, and the chalk is dissolved; whereas in the Lake District we have very much harder and older rocks, that require to be split up and broken by the action of frost, while every stream carves out for itself a steep valley, and great masses of hard rock stand out as bold hills or mountains, that seem to defy all the agents of denudation. Here the opposite is the case, and the valleys are deepened faster than they are widened. But for all that, a vast amount of solid rock has been removed from the surface there, of which the mountains are, as it were, but fragments that have escaped the general destruction. Moreover, the rocks in this region have been greatly disturbed and crumpled since they were first formed, and thereby thrown into various shapes that give certain peculiar structures more or less capable of resisting denudation. Very effective illustrations of the power of rain by itself are afforded by the "earth pillars" of the Tyrol, and "cañons" of Colorado. The material of which they consist is called conglomerate, because it is composed of stones and large blocks of rock with stiff earth or clay between. All the taller ones have a big stone on the top which protects the softer material below from being washed away by heavy rains; and it is easily perceived that each pillar owes its existence to the stone on the top, which prevents the soft materials below it from being washed away. When, after a time, the weathering of the soft strata diminishes the support of the capping boulders, these at last topple over, and the pillar, thus left unprotected, becomes an easy prey to the rain, and is rapidly washed away. Some of the pillars are over a hundred feet in height. But it is only in places where heavy rains fall that these interesting monuments of denudation are to be seen. By way of contrast we may turn now to a district in which very little rain falls, but where the streams have a considerable slope, and so can wear away, or erode, their valleys much faster than rain and frost, etc., can bring down the rocks of which the sides are composed.