Permanent Records of River-Action. If, then, all the streams on the surface of the globe are engaged in the double task of digging out their channels and carrying away the loose materials that arise from the decomposition of the surface of the land, let us ask ourselves what memorials of these operations they leave behind them. In what form do the running waters of the land inscribe their annals in geological history? If these waters could suddenly be dried up all over the earth, how could we tell what changes they had once worked upon the surface of the land? Can we detect the traces of ancient rivers where there are no rivers now?

From what has been said in this lesson it will be evident that in answer to such questions as these, we may affirm that one unmistakable evidence of the former presence of rivers is to be found in the channels which they have eroded. The gorges, rocky defiles, pot-holes, and water-worn rocks which mark the pathway of a stream would long remain as striking memorials of the work of running water. In districts, now dry and barren, such as large regions in the Levant, there are abundant channels (wadies) now seldom or never occupied by a stream, but which were evidently at one time the beds of active torrents.

But more universal testimony to the work of running water is to be found in the deposits or alluvium which it has accumulated. Spreading out on either side, sometimes far beyond the limits of the ordinary or modern channels, these deposits, even when worn into fragmentary patches, retain their clear record of the operations of the river. Let us in imagination follow the course of a river from the mountains to the sea, and mark as we go the circumstances under which the accumulation of sediment takes place.

The power possessed by running water to carry forward sediment depends mainly upon the velocity of the current. The more rapidly a stream flows, the more sediment can it transport, and the larger are the blocks which it can move. The velocity is regulated chiefly by the angle of slope; the greater the declivity, the higher the velocity and the larger the capacity of the stream to carry down debris. Any cause, therefore, which lessens the velocity of a current diminishes its carrying power, and if the water is bearing along gravel, sand, or mud, some of these materials will begin to drop and remain at rest on the bottom. In the course of every stream, various conditions arise whereby the velocity of the current is reduced. One of the most obvious of these is a diminution in the slope of the channel. Another is the union of a rapid tributary with a more gently flowing stream. A third is the junction of a stream with the still waters of a lake or with the sea. In these circumstances, the flow of the water being checked, the sediment at once begins to fall to the bottom.

Tracing now the progress of a river, for illustrations of this law of deposition, we find that among the mountains where the river takes its rise, the torrents that rush down the declivities have torn out of them such vast quantities of soil and rock as to seam them with deep clefts and gullies. Where each of these rapid streamlets reaches the valley below, its rapidity of motion is at once lessened, and with this slackening of speed and consequent loss of carrying power, there is an accompanying deposit of detritus. Blocks of rock, angular rubbish, rounded shingle, sand, and earth are thrown down in the form of a cone of which the apex starts from the bottom of the gully and the base spreads out over the plain (Fig. 10). Such cones vary in dimensions according to the size of the torrent and the comparative ease with which the rocks of the mountain-side can be loosened and removed. Some of them, thrown down by the transient runnels of the last sudden rain-storm, may not be more than

a few cubic yards in bulk. But on the skirts of mountainous regions they may grow into masses hundreds of feet thick and many miles in diameter. The valleys in a range of mountains afford many striking examples of these alluvial cones or fans, as they are called. Where the tributary torrents are numerous, a succession of such cones or fans, nearly or quite touching each other, spreads over the floor of a valley. From this cause, so large an amount of detritus has within historic times been swept down into some of the valleys of the Tyrol that churches and other buildings are now half-buried in the accumulation.

Looking more closely at the materials brought down by the torrents, we find them arranged in rude irregular layers, sloping downwards into the plain, the coarsest and most angular detritus lying nearest to the mountains, while more rounded and water-worn shingle or sand extends to the outer margin of the cone. This grouping of irregular layers of angular and half-rounded detritus is most characteristic of the action of torrents. Hence, where it occurs, even though no water may run there at the present day, it may be regarded as indicating that at some former time a torrent swept down detritus over that site.

Quitting the more abrupt declivities, and augmented by numerous tributaries from either side, the stream whose course we are tracing loses the character of a torrent and assumes that of a river. It still flows with velocity enough to carry along not only mud and sand but even somewhat coarse gravel. The large angular blocks of the torrent part of its course, however, are no longer to be seen, and all the detritus becomes more and more rounded and smoothed as we follow it towards the plains. At many places, deposits of gravel or sand take place, more especially at the inner side of the curves which the stream makes as it winds down the valley. Sweeping with a more rapid flow round the outer side of the curve, the current lingers in eddies on the inner side and drops there a quantity of sediment. When the water is low, these strips of sand and shingle on the

concave side of each curve of the river form a distinctive feature in the scenery. It is interesting to walk along one of these strips and to mark how the current has left its record there. The stones are well smoothed and rounded, showing that they have been rolled against each other along the bottom of the channel for a sufficient distance to lose their original sharp edges and to pass from the state of rough angular detritus into that of thoroughly water-worn gravel. Further, they will be found not to lie entirely at random, as might at first sight be imagined. A little examination will show that, where the stones are oblong, they are generally placed with their longer axis pointing across the stream. This would naturally be the position which they would assume where the current kept rolling them forward along the channel. Those which are flat in shape will be observed usually to slope up stream. That the sloping face must look in the direction from which the current moves will be evident from Fig. 11, where a current, moving in the direction of the arrow

and gradually diminishing in force, would no longer be able to overturn the stones which it had

FIG. 11.--Flat stones in a bank of river-shingle, showing the direction of the current (indicated by the arrow) that transported and left

them.

so placed as to offer the least obstacle to its passage. Had the current flowed from the opposite quarter, it would have found the upturned edges of the stones exposed to it, and would have readily overturned them until they found a position in which they again presented least resistance to the water. In a section of gravel, it is thus often quite possible to tell from what quarter the current flowed that deposited the pebbles.

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