We have seen that the valleys were eaten out by running streams. At one time the land stood higher, and the burns flowed where now the salt water covers the bottom of the bay. Thus the land was shaped. Then the ground sank a little, and the sea flooded the lower grounds. The hilltops remained above water as islands; the valleys and flat grounds were changed into bays and sounds and firths. Think what would happen if the land sank another hundred feet. Many of the present islands would become shoals, and new islands would be made where the sea flowed round the higher ground, winding out and in among them in narrow sounds and straits, just as it does among the islands of the present day. Long ago Orkney and Shetland were much larger than they are at present. Most of the North Sea was dry land, covered with trees. In several parts of Orkney we can see trunks and roots of trees uncovered after heavy storms have shifted the sand on the beach. These trees did not grow beneath the sea, of course; but the land sank, and the salt water covered the site of the old forest. Our wild animals, such as hares and rabbits, mice, voles, and shrews, were not imported in boats. They were here probably before man arrived, and they walked in on their own feet when the sea bottom was still part of the dry land of Europe. Those who have studied this question think the land is still sinking, or at any rate has not yet begun to rise. If it were rising, we should find gravels and shells and sea-beaches above the level of the present shores. Such raised beaches are found in many parts of Scotland, but not in Orkney or Shetland. The shores are always changing, and every part of them bears evidence of constant alterations. Where there are high banks or cliffs, you will often find that pieces have fallen down; this is especially the case where the bank consists of clay. Our Orkney rocks are very hard and our cliffs very lasting, but in some parts of England there are villages and churches now standing on the very edge of the cliffs which a few centuries ago were at a considerable distance from the sea. It is the sea that wears away the cliffs by hammering at the rocks; during storms the big stones on exposed beaches are rounded and worn by the billows tossing them about and driving them against the rocks. On the west coasts of our islands the great winter waves have enormous power; no breakwater could resist them, and a ship which is driven ashore soon goes to pieces. The cliffs are undermined at their base by the formation of caves; the soft parts are eaten out into geos. Frost and rain open the seams of the rocks and great masses tumble down; these are then tossed to and fro until they are converted into heaps of boulders. The boulders get less and less, and become pebbles; last of all they are ground down to fine shingle and sand.