The boulder clay, then, is clearly a glacial deposit, formed at a time when our islands were covered with moving sheets of ice. These ice sheets were travelling from the North Sea towards the Atlantic, in a west or north-west direction, for the scratches on the rock surface always have that trend. We can often prove also that the boulders found in the clay have travelled from the south-east. Thus at the Mont, near Kirkwall, the boulder clay is full of red sandstone from the Head of Holland and Inganess Bay. In Shetland stones have been carried from the east side of the mainland right over the hills to the west shores. When we piece together all the evidence about this Ice Age or Glacial Period, not only in Orkney and Shetland but in all the north-west of Europe, we learn that it lasted a very long time, and that the North Sea was filled with a great sheet of ice which must have been several thousand feet thick. This ice was pushed out of the basin of the North Sea westwards into the Atlantic by the pressure of the deep snow-cap which covered the mountains of central Europe, and on its way it passed over Orkney and Shetland. The broken stones and rubbish which gathered below it formed the boulder clay. This may seem a very strange tale, but every kind of evidence that is needed to prove its truth has been found by those who have studied the boulder clay and the scratched rocks beneath it. After the great ice sheet melted, the climate was still cold, and there were times when snow and ice gathered on our hilltops and little glaciers flowed down the valleys. These also have left traces behind by which you can know where they were. In every one of the higher glens in the Orkney hills you will find mounds of clay and stones, often forming a crescent or bow running from side to side of the valley. They are very well seen in Harray, Birsay, Orphir, and Hoy; but even in the East Mainland the hills, though low, gave rise to little glaciers. In Shetland they are almost as common as in Orkney. In many parishes there are clusters of large and small mounds, some of them grassy and others covered with heather, lying near the mouths of the main valleys. When these mounds have been cut into by streams or by roads, we see that they are not rocky hillocks but consist only of clay and stones, and that the stones are often scratched like those in the stiff boulder clay. These mounds are the “dumps” or moraines where the glaciers which filled the valleys melted and dropped their rubbish. At that time our islands must have resembled Spitzbergen, where to-day most of the hills have an ice-cap and nearly all the valleys are filled with glaciers, some of which reach the sea and give birth to icebergs, while others melt away and deposit lumpy moraines over the valley bottoms. Orkney Fossils.