It took a long time for people to grasp the fact that running water is a great earth-shaping agent. Every valley you have ever seen was made in this way. Other things helped, but the stream was the main cause. A valley is only a great groove eaten out of the rock. It is not due to any earthquake or rending apart of the rocks; it is not an original feature of the country. There was a time when there was no valley there; but from the day the stream first began to trickle over the rocks it has gone on deepening its channel and excavating the valley, and it is still doing so. The stream not only made the valley; it shaped the hills also. We sometimes speak of “the everlasting hills.” No doubt the hills are very old, and will last a long time. Yet the little stream is older and mightier than they. It shaped them and brought them into being; in time it will remove them and level them with the plain. Let us climb the side of a hill, and see what we can learn about it by patient observation and inference. Any one of our flat-topped, round-shouldered Orkney hills will do. They were all formed in the same way, and teach the same lessons. [Illustration: _The Ward Hill, Hoy._] The ascent is gentle at first as we leave the plain or the bottom of the valley. Then it gets steeper and steeper. Often it is like a series of great steps—a sharp rise for a little, then a flat ledge; another sharp rise, followed by a gentle slope, and so on. These terraces are formed of beds of hard stone, which weather down very slowly. The softer rocks crumble fast, and form the steep slope. All our flagstone hills show these steps or terraces. They prove that the slope of the hillside is determined largely by the rate at which the different rock beds wear away. After our stiff climb we get near the top. Many of our hills are broad-backed. When we get above the steep part we find a flat top, and it is often difficult to say where the actual summit is. In many places there are great groups of hills, all of about the same height, but separated by valleys. The Orphir, Firth, and Harray hills, the Rousay, Evie, and Birsay hills, and the hills of Walls are all of this kind. Even the Hoy hills show the same feature, though less clearly. In all these cases the hilltops look like the remains of one continuous stretch of high ground, which has been cut up into pieces by the digging out of the valleys. The hills are the remnants of a plateau.