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a smaller superficial area. The deformation brought about in this way varies in extent. In some places the general subsidence of the crust has not been marked by much disturbance of the rocks; the original horizontally of the strata has been largely preserved. In other regions the reverse is the case, the strata having been everywhere folded and fractured; and between these two extremes are many gradations. The various structures assumed by disturbed rockmasses show that crustal movements are of two kinds, horizontal and vertical. Folding and its accompanying phenomena are obviously the result of tangential pressure. Sometimes the strata are so folded as to present the appearance of a series of broad, gentle undulations. At other times the folds are pressed closely together and bent over to one side in the direction of crustal movement. In certain regions so great has been the horizontal thrust, that masses of rock, thousands of feet in thickness, have sheared under the pressure and travelled forwards for miles, older rocks being pushed forward bodily over younger masses. But besides such horizontal movements there are vertical movements of the crust, typically represented by the dislocations known as normal faults. Normal faults are more or less vertical displacements, often of small amount, but not infrequently very great. Many are vast rents traversing the crust in some determinate direction, the rocks on one side of the fault having subsided for hundreds or even for thousands of feet. We may reserve for the present. however, any further discussion of the rock-structures that result from hypogene action. All that we need at present bear in mind is the general fact that the crust of the earth is subject to deformation.
We now proceed to inquire more particularly into the influence of geological structure and the character of rocks upon the development of land-forms. We shall therefore consider first the form assumed by lands built up of approximately horizontal strata. This is the simplest kind of geological structure: the tale it tells is not hard to read. We can follow it from first to last in all its details. But if we succeed in grasping what is meant by the denudation of horizontal strata, we shall have little difficulty in explaining the origin of surface-features in regions the geological structure of which is much more complicated.
As common examples of horizontally bedded strata we may take the alluvial deposits that mark the sites of vanished lakes; the terraces of gravel, sand, and silt that occur in river-valleys; deltas, and raised beaches. Fluvio-marine deposits and raised beaches of recent age generally form low plains rising but a few feet or yards above sea-level. Their inclination is seawards, usually at so low an angle that they often appear to the eye level, or approximately so. This gently sloping surface is an original configuration, for it corresponds with the structure of the various underlying deposits, the general inclination or dip of which is in the same direction as the surface. When that surface is approximately level denudation necessarily proceeds very slowly, although in time the action of rain alone will suffice to lower the general level. But however much raised beaches and deltas of recent age may have been modified superficially by subaerial denudation, we must admit that their most characteristic features are original, and due to the mode of their formation.
The same holds true to a large extent of recent lacustrine and fluviatile deposits. The wide flats that tell us where lakes formerly existed, and the broad alluvial tracts through which streams and rivers meander, are, like deltas and raised beaches, plains of accumulation. It goes without saying, however, that many of these plains are more or less eroded, and have acquired an undulating, furrowed, and irregular surface. Some alluvial tracts, indeed, have been so cut up by rain and running water that, in the rough, rolling ground over which he toils, the traveller may find it hard to recognise the characteristic features of a plain.
In a broad river-basin alluvial terraces and plains usually occur at various heights, marking successive levels at which the river and its tributaries have flowed while deepening their courses. The lowest terraces and flood-plains are, of course, the youngest, and show, therefore, least trace of subaerial erosion. As we recede from these modern alluvia and rise to higher levels, the terraces and plains become more and more denuded. The highest-lying river-accumulations, indeed, may be so much eaten into and washed down that only scattered patches may remain, and few or no traces of the original flat surface can then be recognised. Thus fiuviatile terraces and recent alluvia all tend to become modified superficially, while at the same time they are undermined and cut into by streams and rivers.
The plains of accumulation at present referred to belong to a recent geological age, and consist for the most part of incoherent deposits, such as gravel, sand, clay, silt, loam, and so forth. And it is worthy of note that the nature of the deposits has to some ex
Fig. 7. Section Across A Series Of Alluvial Terraces.
#-, solid rocks; 1, oldest terrace; 3, second terrace; 3, third and youngest terrace; 4. river and recent alluvial plain.
tent influenced the denudation of the ground. Thus terraces and plains composed mainly of gravel tend to retain their original level surface, while similar flats of clay and loam of the same age as the gravel have frequently been furrowed and channelled to such an extent that the originally level surface has largely, or even entirely, disappeared. The reason is obvious, for clay and loam are somewhat impervious, while gravel is highly porous. Consequently rain falling on the surface of the latter is rapidly absorbed, and little or no superficial flow is possible. But in the case of the more impervious deposits rain is absorbed very sparingly, and naturally tends to produce inequalities as it seeks its way over the gently inclined surface.
The origin and present aspect of such recent plains of accumulation are so obvious and so readilyaccounted for, that it is hardly necessary to do more than cite a few examples. Amongst the most notable are the great deltas of such rivers as the Mississippi, the Amazon, the Rhone, the Po, the Danube, the Rhine, the Niger, the Ganges, etc., and the broad flats and terraces which occur within the drainage-areas of the same rivers. The vast plains of the Aralo-Caspian area, and the far-extended tundras of Northern Siberia, are likewise examples of plains of accumulation, all of which belong to recent geological times. However much some of these plains may have been furrowed and trenched by running water, we yet have no difficulty in recognising that the general form of the surface is due to sedimentation. The deposits of which they are built up have been laid down in approximately horizontal or gently inclined layers, and the even or level surface is thus simply an expression of the arrangement of the bedding. In a word, the geological structure has determined the configuration of the surface.
But it is needless to say that horizontal strata are not confined to low levels, nor do they always consist of unconsolidated materials, like gravel, sand, and clay. Horizontal strata of such rocks as sandstone, shale, limestone, basalt, etc., enter largely into the composition of certain lofty plateaux and mountain