Every kind of rock has its own characteristic type of cliff scenery. When pieces are detached they separate along natural cracks which are called “joints,” and these joints have a different arrangement in sandstone, in granite, in serpentine, and in schist. Weathering then acts on the exposed surface, and, if the rock is bedded, some beds are eaten away more rapidly than others. There is much to interest us in our cliffs; there is not a detail in their form which has not a meaning. [Illustration: _A sandstone cliff._] On wild shores where storm-waves are high we find large boulders; the smaller ones are washed away and swept out to sea. Sometimes there is no beach, but the cliff plunges down into deep water, for there the waves are so powerful that they clear away all the broken rock. On sheltered beaches we find small rounded pebbles. If we look at the stones on the shore of a small fresh-water loch we find them scarcely rounded at all, for the little waves cannot toss them about and rub them against one another. The tear and wear of pebbles produces sand, and the sand is driven to and fro by currents and by storms. It rests for a time in some of the bays, but is not a fixture. A high wind drives some of it ashore to cover the grass of the sandy links. A heavy storm may drag a great deal of it out to sea. Unless it is held fast by bent or other plants, sand is always moving. Even the stones travel along the shore, driven by the beat of the waves in bad weather. There are stone beaches common in Orkney and Shetland which are often called ayres, and which have behind them a salt lagoon or oyce. The oyce opens to the sea at one end of the ayre, and a strong tidal current flows out and in through the opening. An ayre is really an army of stones on the march, constantly moving forward. In every bay there is one direction from which the biggest waves come, and the stones of the ayre have come from that direction. The opening of the oyce is at the other end of the ayre. At first there was a bay with a shallow inner end. When the big waves reach shallow water they turn over and have their speed checked. Stones carried along the shore are dropped at the edge of the shallow water, forming a bar. The bar goes on stretching across the bay as the storms fetch more stones, and in time the oyce is nearly walled in. But as the opening gets narrower and narrower the tidal flow gets stronger and stronger There is a combat between the tidal currents and the storm currents, and in time things are adjusted so that the speed of the outflow is just enough to keep the opening from being closed up. The Age of Ice.