« PreviousContinue »
that extend outwards from the base of the mountainarea. Eventually, however, we pass into the region of dominant accumulation—the region of groundmoraines and eskers, of terminal moraines, lakes, and fluvio-glacial plains. ,
LAND-FORMS MODIFIED BY sEOLIAN ACTIO AT
INSOLATION AND DEFLATION IN THE SAHARA—FORMS ASSUMED BY GRANITOID ROCKS AND HORIZONTAL AND INCLINED STRATA—REDUCTION OF LAND-SURFACE TO A PLAIN—FORMATION OF BASINS—DUNES OF THE DESERT—SAND-HILLS OF
OTHER REGIONS TRANSPORT AND ACCUMULATION OF DUST
— LOESS, A DUST DEPOSIT—LAKES AND MARSHES OF THE STEPPES.
AT the outset of our inquiry into the origin of surface features, we briefly considered the general nature of the work .done by the principal epigene agents. We saw that these agents are often so closely associated in their operations that their individual share in the final result can hardly be determined. In our country, for example, erosion is effected by the combined action of the atmosphere, of frost, and of rain and running water. There are many regions, however, in which one or other of these agents is by far the more conspicuous worker. Thus, at lofty elevations in temperate regions, and throughout the higher latitudes, the most potent causes of rock-disintegration and removal are frost, snow, and ice. In warm-temperate, subtropical, and tropical lands, on the other hand, it is usually the chemical and mechanical action of the rain and running water which impresses the observer, while in rainless and desiccated regions insolation and deflation play the most important role. It is in the latter, therefore, that the erosive action of wind is best displayed. Not that this action is confined to such areas, for it may be observed almost everywhere, and more particularly in mountain-regions. Outside of deserts, however, the wind acts chiefly as a transporter of rock-material. In all latitudes incoherent deposits of sand, exposed and dried, come under its power, and tend to be piled up in heaps and ridges or spread out in sheets. In this way certain more or less prominent land-features owe their origin directly to wind; and as we have devoted some space to the consideration of the action of ice as a special agent of erosion and accumulation, we shall now take a rapid glance at the more notable surface-features that result from the destructive and reproductive action of the atmosphere.
In desiccated regions rock-disintegration and the transport and accumulation of superficial materials are mainly the work of insolation and deflation—rain and running water necessarily play a very subordinate part. This is certainly the case in the Sahara—the most extensive tract of desert in the Old World. This vast region stretches across Africa from the Atlantic coast to the valley of the Nile, and from the northern borders of the Soudan to the Atlas Mount
ains and the Mediterranean, an area equal in size to two-thirds of Europe. The surface of the Sahara is sufficiently diversified, and is not, as popularly supposed, entirely covered with blowing sand. Dunes, no doubt, spread over enormous territories, but wide tracts and broad basins of loam and clay, with saline lakes and marshes, likewise present themselves, whilst elsewhere rocky and stony plateaux, and even lofty mountains, occupy extensive areas. Ever and anon, moreover, verdant oases appear, and these are so numerous that they must altogether form no inconsiderable portion of the whole Sahara. The entire area might be described as an old plateau of accumulation, built up, as it appears to be for the most part, of horizontally or gently inclined strata. Probably its mean altitude is not less than 2000 feet, only a small portion lying to the south of Algeria being below the level of the Mediterranean. The rocky areas of the region are broken up into a succession of narrower and broader terraces or plateaux—now in many places traversed by dry, winding gullies, ravines, valleys, and other abandoned watercourses, or largely replaced by groups of bare pyramidal hills, buttes, mesas, and irregular rock-masses. Over wide areas blowing sands are absent, while elsewhere they are heaped up and spread out to such an extent that the rocky framework of the country becomes entirely concealed.
Wind erosion is naturally best studied in the bare portions of the desert. Under the influence of insolation the rocks crumble down, and the disintegrated material is swept onward by the wind. Hard, compact stones acquire a polish like that given by a lapidary■s wheel, while rocks of unequal consistency yield irregularly, the softer portions being removed and the harder parts left standing in relief. Where the surface of the land is very uneven the air-currents streaming between opposing heights have ground out deep hollows and gullies. In like manner curious niches, cirques, and amphitheatres have been excavated in the walls of the dry wadies. Everywhere, indeed, the rocks are abraded, fretted, honeycombed, and undermined. Undermining is, in truth, one of the most notable stages in the general reduction of the surface. The bulk of the sand driven forward by the wind rises only a few feet above the surface, hence cliffs and stacks wear away rapidly below until the overhanging mass collapses and topples down, whereupon the same action is repeated upon the fallen ddbris. Hence isolated rock-masses often take peculiar mushroom-shapes.
Among the most fantastic forms assumed under the action of the wind are those met with among granites and granitoid rocks. Often rising boldly above the general level, they show no trace of talus or ddbris, but are swept bare to the base, and to the fanciful Arab they often simulate the appearance of elephants, apes, camels, panthers, and the like. In Europe granite hills and mountains frequently show rounded summits, and are usually well mantled with talus. In the desert, on the other hand, they are