« PreviousContinue »
valleys of erosion, until they suddenly plunge below. Sometimes they emerge from their subterranean courses again and again; at other times they never reappear at the surface, but discharge their waters on the sea-floor. Owing to the frequent collapse of tunnels and caverns, the surface of a calcareous region is apt to show many irregular depressions, and the superficial hydrographic system is necessarily very imperfectly developed.
5. Basins. The large and small depressions of the surface, like other superficial features, have been formed in various ways. Some are the result of hypogene action, others owe their origin to epigene action, while yet others are due to both. Not a few, for example, are hollows caused by deformation of the crust, and may be termed tectonic basins. Some, again, occupy the site of extinct volcanoes, or are due in one way or another to volcanic action; they are our volcanic basins. Depressions caused by the removal of soluble materials from below, as in limestone countries, may be called dissolution basins; while the terms alluvial and ceolian may well be applied to all basins which are the result of fluviatile and seolian action respectively. Landslips, etc., by obstructing drainage form a series of rock-fall basins, while glacial action is responsible for a large class of basins, some of which are rock-basins, others barrierbasins, and yet others partake of the character of both. All these are termed glacial basins.
Unless they are very capacious and extensive, basins soon become obliterated. Erosion and sedimentation are too active to permit of their prolonged duration. Exceptionally, however, tectonic basins may long outlive the land-surface upon which they first appeared. If the deformation of the crust to which they owe their origin be continued, erosion and sedimentation may be unable to obliterate them. Should the bed of a great lake subside at approximately the same rate as alluvial matter accumulates upon it, while at the same time the effluent river cannot succeed in draining the lake dry, it is obvious that the latter may endure for a very long time. Sediments reaching a thickness of many thousands of feet might come to be deposited in such a lake, although the water itself had never been more than a few hundred feet in depth. The lake would form the base-level for all the surrounding region, the surface of which, perhaps mountainous to begin with, would be gradually lowered, and might pass through a complete cycle of erosion before the lake ceased to exist. In a word, a great lake or inland sea may become the burial-place of the high grounds that surround it, for it bears the same relation to these as an ocean to a continent.
The great majority of lakes, however, do not occupy tectonic basins, and must sooner or later disappear. Even tectonic basins, the beds of which have ceased to subside, must eventually be obliterated. As a matter of fact, none of the existing lakes of the world can be shown to be of great geological 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.