However, before we consider these three parts in detail, there is one question which we might well ponder for a little while, a question which probably has never occurred to more than a few of us; and that is this: Why was there ever a River Thames at all? To answer it we must go back—far, far back into the dim past. As you know, this world of ours is millions of years old, and like most ancient things it has seen changes—tremendous changes. Its surface has altered from time to time in amazing fashion. Whole mountain ranges have disappeared from sight, and valleys have been raised to make fresh highlands. The bed of the ocean has suddenly or slowly been thrust up, yielding entirely new continents, while vast areas of land have sunk deep enough to allow the water to flow in and create new seas. All this we know by the study of the rocks and the fossil remains buried in them—that is, by the science of geology. [Illustration: HOW THE THAMES WAS MADE.] Now, among many other strange things, geology teaches us that our own islands were at one time joined on to the mainland of Europe. In those days there was no English Channel, no North Sea, and no Irish Sea. Instead, there was a great piece of land stretching from Denmark and Norway right across to spots miles out beyond the western limits of Ireland and the northern limits of Scotland. This land, which you will best understand by looking carefully at the map, p. 4, was crossed by several rivers, the largest of them one which flowed almost due north right across what is now the North Sea. This river, as you will see from the map, was chiefly produced by glaciers of the Alps, and, in its early stages, took practically the same course as the River Rhine of these days. As it flowed out across the Dogger district (where now is the famous Dogger Bank of our North Sea fishermen) it was joined by a number of tributary rivers, which flowed down eastwards from what we might call the “back-bone of England”—the range of mountains and hills which passes down through the centre of our islands. One of these tributaries was a river which in its early stages flowed along what is now our own Thames Valley. In those days everything was on a much grander scale, and this river, though only a small tributary of the great main continental river, was a far wider and deeper stream than the Thames which we know. Here and there along the present-day river valley we can still see in the contours of the land and in the various rocks evidences of the time when this bigger stream was flowing. (Of this we shall read more in Book III.) Thus things were when there came the great surface change which enabled the water to flow across wide tracts of land and so form the British Islands, standing out separately from the mainland of Europe.