Page images
PDF
EPUB

complete, but it will serve our purpose to group basins as follows :—

1. Tectonic basins.

2. Volcanic"

3. Dissolution"

4. Alluvial

5. /Eolian

6. Rock-fall

7. Glacial

1. Tectonic Basins. These owe their origin directly to deformation of the earth■s crust, whether the result of warping or of fracture, or both. In this class are included many inland seas, and most of the larger lakes of the globe. The Aralo-Caspian depression, with its numerous sheets of water and desiccated basins, the Dead Sea, Issyk-Kul, the lakes of Equatorial Africa, the Great Salt Lake of Utah, and very many others are true tectonic basins. A large number of such basins occur in relatively dry and rainless regions. On the other hand, many are met with in temperate regions. The great fresh-water lakes of North America and Europe (Superior, Huron, Michigan, Ladoga, Onega, etc.) occupy tectonic basins. These lakes, it will be noted, are confined to the glaciated areas of the two continents, and their character as tectonic basins has been modified and obscured by glacial erosion and accumulation. There seems no reason to doubt, however, that the depressions are the result of crustal deformation. Tectonic basins are usually somewhat flat-bottomed or gently undulating. Occasionally they are traversed by narrow winding hollows, which have been traced for longer or shorter distances. These have frequently the character of river-ravines and valleys, and are suggestive, therefore, of a former land-surface which has become depressed. Similar indications of depression are afforded by the highly indented coastlines of some of the larger lakes of this class, the long inlets and projecting headlands recalling the appearances presented by the fiord-coasts of Norway and Scotland.

The crustal deformation may consist of simple subsidence—a wide area of relatively flat or gently undulating land sinking below the level of adjacent tracts; or the subsidence may be the effect of dislocation and displacement. Again, basins have come into existence between contiguous high grounds undergoing elevation. Once more, the formation of an anticline across the drainage-area of a lowland region might bring extensive lakes into existence. Similarly it is conceivable that lakes might be formed in mountain-valleys by the swelling up of the crust at the base of the mountains, or by the formation of new flexures in the mountains themselves, having a direction transverse to the valleys. We cannot, however, point to any particular valley-basin formed in this way. Earth-movements of this kind would seem to take place very slowly, so slowly, as* a rule, that rivers are able to saw across the obstructions as fast as they rise.

2. Volcanic Basins. The lakes of this class form a ■well marked group, many of them occupying the sites of extinct volcanoes. Not a few, therefore, occur in the cup-shaped depressions of volcanic cones. Others, again, may not be walled round by volcanic ejecta, but occupy explosion-craters—the more or less deep concavities produced by paroxysmal outbursts. No hard-and-fast line, however, can be drawn between these two varieties of crater-lake. Some explosioncraters are encircled by ridges of ejecta, while the cup-shaped depressions of certain volcanic cones are of such a depth that, were the cones themselves to be removed, a considerable concavity would still remain. Amongst well known crater-lakes are the Maars of the Eifel, some of which are 70 feet or less in depth, while others are not much below 200 feet. Of the same character are the crater-lakes of Auvergne, which vary in depth from less than 100 to 350 feet; and the similar lakes of Central Italy, one of which, Lake Bracciano, is said to be 950 feet deep. In all the great volcanic regions of the globe, indeed, lakes of this character are recognised. Other volcanic lakes have had a different origin. Sometimes lava, at other times fragmental ejecta, or streams of tufaceous mud and ddbris have entered valleys and obstructed the drainage. The Lac d■Aydat of Auvergne, for example, is confined by a barrier of lava, and the same is the case with the large Yellowstone Lake. So, again, the enormous torrents of mud and dibris which poured down to the low grounds during the great eruption of Bandaisan in iS&S gave rise to four volcanic barrier-lakes. After volcanoes have erupted for a prolonged time the ground often becomes depressed, and large and small subsidences ot the surface are not infrequently the result of the earthquakes that accompany volcanic action.

[graphic]

3. Dissolution Basins. In regions of soluble rocks. as we have seen, many inequalities of the surface are brought about by the chemical and mechanical action of underground water. Most frequentlv the depressions produced by the collapse of subterranean galleries and caves contain no water. Sometimes, however, as Professor Penck has pointed out. warping of the crust has brought the corroded and tunnelled limestone rocks under the influence of the subterranean water-level, so that sink-holes and other superficial depressions have become more or less deeply filled. Again, should tectonic movements carry down a honeycombed calcareous region so that its basal portions sink below the sea-level, the meteoric water descending from the surface will be dammed back in sinks and other hollows. The water-surface of wells in such districts is known to rise and fall with the tide. From various causes, also, the underground outlets of dolinas, etc., become closed with accumulations of insoluble earthy materials, and the bottoms of other depressions are rendered impermeable by similar deposits washed into them by rain-or snowwater. Similar changes have been brought about by glacial action, the outlets for the escape of under

ground water having been closed by morainic ddbris. For these and other reasons lakes are by no means always wanting in regions of highly honeycombed and tunnelled calcareous rocks.

Soluble rocks deeply covered with strata of more

durable character do not escape corrosion, but are

gradually removed by underground water, and thus

bring about slow subsidence or sudden collapse of the

surface, and the shallow basins formed in this way

may become filled with water.

4. Alluvial Basins. The broad alluvial flats of rivers often show slight depressions caused by irregular accumulation. These during floods may become lakes, and endure for a longer or shorter time. Again, rivers tend to change their courses, and their deserted loops often persist as lakes. In rainless regions the rivers flow with a gradually lessening volume, until eventually they may dry up. It is obvious that the sediment transported by such rivers must gradually raise the level of their lower courses, and in time produce shallow basins. In the dried-up courses themselves pools and "creeks" not infrequently occupy the deeper hollows, and are probably maintained by water coming from underground sources. Once more, in well watered regions rivers now and again form lakes. A main stream, for example, by carrying down large quantities of detritus, tends to raise the surface of its bed above that of its tributaries, in the lower reaches of which lakes thus come into existence. In like manner tributary streams occasionally

« PreviousContinue »