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-- 1. Deepening and widening; 2. Widening and levelling; 3. Filling up; and every place in the second stage has passed through the first; every one in the third has passed through the second. A velocity of 6 inches per second will lift fine sand, 8 inches will move sand as coarse as linseed, 12 inches will sweep along fine gravel, 24 inches will roll along rounded pebbles an inch diameter, and it requires 3 feet per second at the bottom to sweep along angular stones of the size of an egg. When a river has so adjusted its slope that it neither deepens its bed in the upper portion of its course, nor deposits materials, it is said to have acquired its "regimen," and in such a case if the character of the soil remains the same, the velocity must also be uniform. The enlargement of the bed of a river is not, however, in proportion to the increase of its waters as it approaches the sea. If, therefore, the slope did not diminish, the regimen would be destroyed, and the river would again commence to eat out its bed. Hence as rivers enlarge, the slope diminishes, and consequently every river tends to assume some such "regimen" as that shown in Fig. 46.