65. When a river enters a lake, the motion of the water is of course checked, and hence the heavier detritus, such as gravel and coarse sand, moves more slowly forward, and at last comes to rest on the bed of the lake, at no great distance from the mouth of the river. Finer sand and mud are carried out for some distance further, but eventually they also cease to move, and sink to the bottom. When the lake is sufficiently large, it catches all or nearly all the matter brought down by the river, which, as it issues from the lower end of the lake, is bright and clear. A well-known example of this phenomenon is that of the Rhone, which enters the Lake of Geneva turbid and muddy, but rushes out quite clear at the lower end of the lake. Lakes, therefore, are all being slowly or more rapidly silted up, and this, of course, is most conspicuous at the points where they are entered by rivers. Thus, at the head of the Lake of Geneva, it is manifest that the wide flat through which the river flows before it pours into the lake, has been conquered by the Rhone from the latter. In the times of the Romans, the lake, as we know, extended for more than one mile and a half further up the valley.
66. Deltas.—When there are no lakes to intercept fluviatile sediment, this latter is borne down to the sea, where it is deposited in precisely the same way as in a lake: the heavier detritus comes to rest first, the finer sediment being swept out for some distance further. So that, in passing from the river-mouth outwards, we have at first gravel, which gradually gets finer and finer until it is replaced by sand, while this in turn is succeeded by mud and silt. There is this difference, however, between lacustrine and fluvio-marine deposits, that while the former accumulate in water which is comparatively still, the latter are often brought under the influence of waves and currents, and become shifted and sifted to such a degree that fine and coarse detritus are frequently commingled; and there is, therefore, not the same orderly succession of coarse and fine materials which characterises lacustrine deposits. Often, indeed, the currents opposite the mouth of a river are so strong, that little or no sediment is permitted to gather there. Usually, however, we find that rivers have succeeded in reclaiming more or less wide tracts from the dominion of the waves, or at all events have cumbered the bed of the sea with banks and bars of detritus. The broad plains formed at the mouth of a river are called deltas, from their resemblance to the Greek letter Δ. The deltas of the Nile, Ganges, and Mississippi are among the most noted. The term delta, however, is not exclusively applied to fluvio-marine deposits; rivers also form deltas in fresh-water lakes. It is usual, however, to restrict the term to extensive alluvial plains which are intersected by many winding channels, due to the rapid bifurcation of the river, which begins to take place at the very head of the great flat—that is to say, at the point where the river originally entered the sea (or lake).
67. Frozen Water.—We have now seen what can be done by the mechanical action of running water. We have next to consider what modifications are effected by freezing and frozen water. Water, as every one knows, expands in the act of freezing, and in doing so exerts great force. Let the reader bear in mind what has been said as to the passage of water through the minute and often invisible pores of rocks, and to its presence in cracks and crevices after every shower of rain, and he will readily see how excessive must be the waste caused by the action of frost. The water, to as great a depth as the frost extends, passes into the solid state, and in doing so pushes the grains of the rocks asunder, or wedges out large masses. No sooner does thaw ensue than the water, becoming melted, allows the grains of the rock to fall asunder; the outer skin of the rock, as it were, is disintegrated, and crumbles away, while fragments and masses lose their balance in many cases, and topple down. Hence it is, that in all regions where frost acts, the hill-tops and slopes are covered with angular fragments and débris, and a soil is readily formed by the disintegration of the rocks.
River-ice is often a potent agent of geological change. Stones get frozen in along the margins of a river, and often débris falls down from cliff and scaur upon the surface of the ice; when thaw sets in, and the ice breaks up, stones and rubbish are frequently floated for long distances, and may even be carried out to sea before their support fails them, and they sink to the bottom. In some cases, when the ice is very thick, it may run aground in a river, and confuse and tumble up the deposits gathering at the bottom. Ice sometimes forms upon stones at the bottom of a river, and floats these off; and this curious action may take place even although no ice be forming at the time on the surface of the water.
68. Glaciers, Icebergs, and Ice-foot.—In certain mountainous districts, and in arctic and antarctic regions, snow accumulates to such an extent that its own weight suffices to press the lower portions into ice. Alternate thawing and freezing also aid in the formation of the ice, which soon begins to creep down the mountain-slopes into the valleys, where it constitutes what are called glaciers or ice-rivers. These great masses of ice attain often a great thickness, and frequently extend for many miles along the course of a valley. In the Alps they occasionally reach as much as five hundred or six hundred feet in depth. In Greenland, however, there are glaciers probably not less than five thousand feet thick; and the glacier ice of the antarctic continent has been estimated even to reach twelve miles in thickness. Glaciers flow slowly down their valleys, at a rate which varies with the slope of their beds and the mass of the ice. Some move only a few inches, others two or three feet, in a day. Their forward motion is arrested at a point where the ice is melted just as fast as it comes on. A glacier is always more or less seamed with yawning cracks, which are called crevasses. These owe their origin to the unequal rate at which the different parts of the ice flow; this differential motion causing strains, to which the ice yields by snapping asunder. The flanks of a glacier are usually fringed with heaps of angular blocks and débris which fall from the adjacent rocky slopes, and some of this rubbish tumbling into the gaping crevasses must occasionally reach to the bottom of the ice. The rubbish heaps (superficial moraines) travel slowly down the valley on the surface of the ice, and are eventually toppled over the end of the glacier, where they form great banks and mounds. These are called terminal moraines. The rocky bed of a glacier is invariably smoothed and polished, and streaked with coarse and fine striæ, or scratches, which run parallel to the direction of the ice-flow. These are due to the presence, at the bottom of the ice, of such angular fragments as become detached from the underlying rocks, or of boulders and rubbish which have been introduced from above. The stones are ground by the ice along the surface of its bed, causing ruts and scratches, while the finer material resulting from the grinding action forms a kind of polisher. The stones acting as gravers are themselves covered with striæ, and their sharp edges get smoothed away. In alpine districts there is always a good deal of water circulating underneath a glacier, and this washes out the sand and fine clay. Thus it is that rivers issuing from glaciers are always more or less discoloured brown, yellow, green, gray, or blue, according to the nature of the rocks which the ice has pounded down into mud. In Greenland many of the large glaciers go right out to sea, and owing to their great thickness are able to dispossess the sea sometimes for miles. But erelong the greater specific gravity of the sea-water forces off large segments from the terminal front of the ice, which float away as icebergs. Large masses are also always falling down from the ice-front. Occasionally, big blocks and débris are floated away on the icebergs, but this does not appear to be common. In Greenland there is very little rock-surface exposed, from which blocks can be showered down upon the glaciers, and the surface of the latter is therefore generally free from superficial moraines. A kind of submarine terminal moraine, however, gathers in front of some glaciers, made up chiefly of the stones and rubbish that are dragged along underneath the ice, and exposed by the breaking-off of icebergs, but partly composed also of the sand and mud washed out by sub-glacial waters. A narrow belt of ice forms along the sea-coast in arctic regions, which often attains a thickness of thirty or forty feet. This is called the ice-foot. It becomes loaded with débris and blocks, which fall upon it from the cliffs above; and, as large portions are frequently detached from the cliffs in summer-time, they sail off with their cargoes of débris, and drop these over the sea-bottom as they gradually melt away. The ice-foot is the great distributor of erratics or wandered blocks, the part taken in this action by the huge icebergs which are discharged by the glaciers being, comparatively speaking, insignificant. But when these latter run aground, they must often cause great confusion among the beds of fine material accumulating upon the floor of the sea.
69. The Sea.—Sea-water owes its saltness to the presence of various more or less soluble substances, such as common salt, gypsum, Epsom salts, chloride of magnesium, &c. Besides these, there are other ingredients held in solution, which, although they can be detected in only minute quantities in sea-water, are yet of the very utmost importance to marine creatures. This is the case with carbonate of lime, vast quantities of which are carried down by many rivers to the sea. But it must be nearly all used up in the formation of hard shells and skeletons by molluscs, crustaceans, corals, &c., for very little can be traced in the water itself. Silica is also met with sparingly, and is abstracted by some creatures to form their hard coverings.
70. Breaker-action—Currents.—The most conspicuous action of the sea, as a geological agent, takes place along its margin, where the breakers are hurled against the land. Stones and gravel are borne with more or less intense force against the rocks, and by their constant battering succeed eventually in undermining the cliffs, which by and by become top-heavy, and large masses fall down and get broken up and pounded into gravel and sand. The new wall of rock thus exposed becomes in turn assaulted, and in course of time is undermined in like manner. The waste of the cliffs is greatly aided by the action of frost, which loosens the jointed rocks, and renders them an easier prey to the force of the waves. Of course, the rapidity with which a coast-line is eaten into depends very much upon the nature of the rocks. Where these are formed of loose materials like sand, gravel, or clay, considerable inroads are effected by the sea in a comparatively short time. Thus, along some parts of the English coast, as between Flamborough Head and the mouth of the Humber, and between the Wash and the Thames, it is estimated that the land is wasted away at the rate of a yard per annum. Where hard rocks form the coast-line the rate of waste is often exceedingly slow, and centuries may elapse without any apparent change being effected. When the rocks are of unequal hardness the coast-line becomes very irregular, the sea carving out bays and gullies in the softer portions, while the more durable masses stand out as capes and bold headlands. Not unfrequently, such headlands are converted into sea-stacks and rocky islets, as one may observe along the rockier parts of our shore-lines. Close inshore, the bulkier débris derived from the waste of the land often accumulates, forming beds and banks of shingle and gravel. The finer materials are carried farther out to sea, and distributed over the sea-floor by the action of the tide and currents. Tidal and other currents may also have some denuding effect upon the sea-bottom, but this can only be in comparatively shallow water. The great bulk of the material derived from the waste of the coasts by the mechanical action of the breakers, travels for no great distance. But the fine mud brought down by rivers is frequently transported for vast distances before it settles. So fine, indeed, is some of this sedimentary material, that it may be carried in suspension by sea-currents for thousands of miles before it sinks to the bottom.
71. From this short outline it becomes evident, therefore, that the coarser-grained the deposit, the smaller will be the area it covers; while conversely, the finer the accumulation, the more widely will it be distributed. A partial exception to this rule is that of the débris scattered over the bottom of the ocean by icebergs and detached portions of ice-foot. These are often floated for vast distances by currents before they finally melt away, and hence the coarse débris transported by them must be very widely distributed over that part of the sea-bottom which is traversed by currents flowing out of the Arctic and Antarctic Oceans. Although the deeper recesses of the ocean appear to be covered only with ooze and fine mud, yet in some instances coarse sand, and even small stones, have been brought up from depths of a hundred fathoms, so that currents may occasionally carry coarser materials for great distances from the shore. The shifting action of tidal currents succeeds in giving rise to very irregular deposits in shallow seas. The soundings often shew sudden changes from gravel to sand and mud, nor can there be any doubt that, could we lay bare the sea-bottom, we should often observe gravel shading off into sand, and sand into mud, and vice versâ. But as we receded from the shore, and approached areas which were once deeply submerged, we should find that the change of material was generally from coarse to fine.