Page images

antiquity. All alike, large and small, are of recent age. As regards their geographical distribution, it is singular and suggestive that they appear most abundantly in glaciated lands, in mountains, plateaux, and lowlands alike. None of these can be shown to have existed before the Glacial Period, and, with few exceptions, all must be attributed to the direct and indirect action of flowing ice. The preglacial hydrographic systems have been disturbed mainly by glacial erosion and accumulation. Many of the larger basins, however, such as those of Lakes Ladoga and Onega in Europe, and Lakes Superior, Michigan, and others in North America, are probably to a large extent tectonic, and due to warping or deformation of the crust. Not a few of the smaller lakes, again, occupy hollows caused by the irregular accumulation of fluviatile sediments, or by the blocking of streams, etc., by rock-falls and landslips, while here and there they rest in depressions produced by the dissolution and removal of soluble materials. Outside of the glaciated areas comparatively few lakes of any kind exist, and the most important of these occupy tectonic and volcanic basins.

6. Coast-Lines. Two types of coast may be distinguished, namely, regular or smooth, and irregular or indented. The former may be high and steep or gently shelving, and when expressed upon a map show a softly undulating or sinuous course. The shape assumed by the coasts themselves is naturally determined by the nature of the rock-masses and th-tir gee \ >gical structure, and the manner in which they succumb before the action of waves and breakers. The co<a5iaI conr.guration is likewise influenced in mar.y places by accumulation, for the coast-line is not r.xei but continually oscillates, retreating in some places, advancing elsewhere. Irregular or indented coast-lines are typically represented by such regions as Norway. Here the continuity of the coast-line is repeatedly interrupted by long inlets, while a multitude of islands fringe the land. Obviously, the trend of such a coast-line is determined by the configuration of the land ; the long inlets and fiords are merely the submerged lower reaches of mountain-valleys. All highly indented coasts, indeed, are evidence that the land is either sinking now or has recently sunk.

In general, it may be said that the average trend and configuration of the coast-lines of the globe are determined by the position of the continents in relation to the great oceanic depression. The former are nowhere co-extensive with what is known as the continental plateau, considerable areas of which are below the sea-level. When the coast-lines approach the margin of that plateau, they generally continue for long distances in one particular direction, are rarely much indented, and show few or no fringing islands. Conversely, when they recede from the edge of the plateau, their trend becomes irregular, following now one direction, now another, numerous inlets appear, and marginal islands usually abound. Indented or irregular coasts are not the result of marine erosion. Fiords, rias, and other indentations are simply submerged valleys. The intricate coastlines of North-west Europe, of Greece and other parts of the Mediterranean lands, of Alaska, and many other regions have been determined by antecedent subaerial erosion.



IN the preceding chapters we have been inquiring into the origin of surface-features, and have come to the general conclusion that these cannot be accounted for without some knowledge of geological structure. We have learned that the crust of the earth has experienced many changes—rocks have been tilted, compressed, folded, fractured, and displaced. In some places elevation, in other places depression, has taken place, or both kinds of movement have affected the same area at different times. The crust has further been disturbed in many regions by vast intrusions of molten matter; while frequently volcanic action has cumbered the surface with lava and fragmental ejecta. It might seem, therefore, as if the varied configuration of our lands—mountain and valley, height and hollow—might be largely if not exclusively due to subterranean action. But the study of geological structure has shown us that enorm

ous masses of material have been removed from the land-surface, and that however much that surface may have been influenced by crustal disturbance, yet its varied features, as a rule, owe their origin directly to denudation. Great mountain-chains have, indeed, been upheaved from time to time, fractures and displacements have again and again taken place; but even the youngest mountains have been so modified by the various epigene agents of change that frequently their original configuration has been almost completely destroyed. Earth sculpture, in a word, is everywhere conspicuous, and in regions which have remained for long ages undisturbed by subterranean action the latter has had only an indirect influence in determining the form of the surface. All the great ranges of tectonic mountains are of relatively recent age. Time has not yet sufficed for their complete reduction. On the other hand, the mountains that were upheaved in the earlier stages of the world■s history have been either completely remodelled or entirely demolished. If elevations still often mark the sites of the chains and ranges of Palaeozoic times, their internal geological structure yet shows that they are no longer tectonic but relict mountains. In short, we see that epigene agents are constantly endeavouring to remove the irregularities which result from crustal disturbance. Elevations are gradually lowered, and sunken areas filled up. But the process of levelling the land is not infrequently interrupted by renewed crustal movements.

« PreviousContinue »