The great American lakes also are probably due to differences of elevation. Round Lake Ontario, for instance, there is a raised beach which at the western end of the lake is 363 feet above the sea level, but rises towards the East and North until near Fine it reaches an elevation of 972 feet. As this terrace must have been originally horizontal we have here a lake barrier, due to a difference of elevation, amounting to over 600 feet.
In the same way we get a clue to the curious cruciform shape of the Lake of Lucerne as contrasted with the simple outline of such lakes as those of Neuchâtel or Zurich. That of Lucerne is a complex lake. Soundings have shown that the bottom of the Urner See is quite flat. It is in fact the old bed of the Reuss, which originally ran, not as now by Lucerne, but by Schwytz and through the Lake of Zug. In the same way the Alpnach See is the old bed of the Aa, which likewise ran through the Lake of Zug. The old river terraces of the Reuss can be traced in places between Brunnen and Goldau. Now these terraces must have originally sloped from the upper part downwards, from Brunnen towards Goldau. But at present the slope is the other way, i.e. from Goldau towards Brunnen. From this and other evidence we conclude that in the direction from Lucerne towards Rapperschwyl there has been an elevation of the land, which has dammed up the valleys and thus turned parts of the Aa and the Reuss into lakes—the two branches of the Lake of Lucerne known as the Alpnach See and Urner See.
During the earthquakes of 1819 while part of the Runn of Cutch, 2000 square miles in area, sunk several feet, a ridge of land, called by the natives the Ulla-Bund or "the wall of God," thirty miles long, and in parts sixteen miles wide, was raised across an ancient arm of the Indus, and turned it temporarily into a lake.
In considering the great Italian lakes, which descend far below the sea level, we must remember that the Valley of the Po is a continuation of the Adriatic, now filled up and converted into land, by the materials brought down from the Alps. Hence we are tempted to ask whether the lakes may not be remains of the ancient sea which once occupied the whole plain. Moreover just as the Seals of Lake Baikal in Siberia carry us back to the time when that great sheet of fresh water was in connection with the Arctic Ocean, so there is in the character of the Fauna of the Italian lakes, and especially the presence of a Crab in the Lake of Garda, some confirmation of such an idea. Further evidence, however, is necessary before these interesting questions can be definitely answered.
Lastly, some lakes and inland seas seem to be due to even greater cosmical causes. Thus a line inclined ten degrees to the pole beginning at Gibraltar would pass through a great chain of inland waters—the Mediterranean, Black Sea, Caspian, Aral, Baikal, and back again through the great American lakes.
But though many causes have contributed to the original formation and direction of Valleys, their present condition is mainly due to the action of water. When we contemplate such a valley, for example, as that which is called par excellence the "Valais," we can at first hardly bring ourselves to realise this; but we can trace up valleys, from the little water-course made by last night's rains up to the greatest valleys of all.
These considerations, however, do not of course apply to such depressions as those of the great oceans. These were probably formed when the surface of the globe began to solidify, and, though with many modifications, have maintained their main features ever since.
ON THE CONFIGURATION OF VALLEYS
The conditions thus briefly described repeat themselves in river after river, valley after valley, and it adds, I think, very much to the interest with which we regard them if, by studying the general causes to which they are due, we can explain their origin, and thus to some extent understand the story they have to tell us, and the history they record.
What, then, has that history been? The same valley may be of a very different character, and due to very different causes, in different parts of its course. Some valleys are due to folds (see Fig. 41) caused by subterranean changes, but by far the greater number are, in their present features, mainly the result of erosion. As soon as any tract of land rose out of the sea, the rain which fell on the surface would trickle downwards in a thousand rills, forming pools here and there (see Fig. 37), and gradually collecting into larger and larger streams. Wherever the slope was sufficient the water would begin cutting into the soil and carrying it off to the sea. This action would be the same in any case, but, of course, would differ in rapidity according to the hardness of the ground. On the other hand, the character of the valley would depend greatly on the character of the strata, being narrow where they were hard and tough; broader, on the contrary, where they were soft, so that they crumbled readily into the stream, or where they were easily split by the weather. Gradually the stream would eat into its bed until it reached a certain slope, the steepness of which would depend on the volume of water. The erosive action would then cease, but the weathering of the sides and consequent widening would continue, and the river would wander from one part of its valley to another, spreading the materials and forming a river plain. At length, as the rapidity still further diminished, it would no longer have sufficient power even to carry off the materials brought down. It would form, therefore, a cone or delta, and instead of meandering, would tend to divide into different branches. These three stages, we may call those of—