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area of dominant accumulation, has very considerably modified the aspect of the land. Could we remove all glacial deposits from our own broad lowland valleys, it is certain that the sea would in many places penetrate far inland. On the continent the Baltic would overflow wide tracts in the plains of Northern Germany, for the bottom of the deposits of that region descends frequently below the level of the sea. And similar changes would be brought about were the glacial accumulations of North America to disappear —the sea would encroach upon the land. Very considerable .modifications were likewise effected in the drainage-systems of extensive regions. In Europe and North America alike, the irregular deposition and distribution of glacial and fluvio-glacial accumulations have often led to remarkable changes in the directions followed by the streams and rivers, which reappeared as the great mers deglace melted away. Throughout the peripheral areas of dominant deposition preglacial courses and channels were largely filled up with detritus, and not infrequently had become in this way obliterated, so that the streams and rivers of postglacial times were often deflected and compelled to erode new channels.
It is not with such changes, however, that we are at present concerned, but rather with the various forms assumed by glacial accumulations. Groundmoraines, as we have already seen, present certain typical configurations. And the same is true of lateral and terminal moraines, and of fluvio-glacial deposits. In areas of dominant glacial accumulation, as in Schleswig-Holstein and North Germany, the ground-moraines often occupy the surface over extensive regions, and form wide plains with a softly undulating surface. The ground rises and falls gently in long, broad swellings and depressions, which do not seem to follow any particular direction. In other regions, as in the Lothians and elsewhere in our own lowlands, the undulations of the boulder-clay not infrequently show a rudely parallel arrangement. Ever and anon, however, all traces of definite orientation disappear, and the ground then simply rises and falls irregularly as in the plains of North Germany. But in some of the broader dales of Scotland the configuration of the boulder-clay becomes strongly defined, the .accumulation being arranged in a well marked series of long parallel banks known as "drums" or "sowbacks." Elsewhere, again, as in Galloway and in many parts of Ireland, the ground-moraines often assume the form of short or more or less abrupt lenticular hills, or "drumlins," as they are termed.
Another set of characteristic glacial land-forms are the eskers, or osar. These are somewhat abrupt banks and ridges of gravel and sand, which are believed to have been formed in tunnels underneath the great mers de glace. They are well seen in certain tracts of our own islands, but reach their greatest development in Sweden, where they traverse the land as great embankments, rising to a height of 50 or 100 feet above the general level, and following a sinuous or river-like course for distances of sometimes 150 miles or more.
Other hillocks and hills of glacial origin are lateral and terminal moraines. The former are practically confined to mountain-valleys, while the latter are met with, not only in mountain-valleys, but in lowlands often far removed from any elevated region. In mountain-valleys such moraines consist chiefly of angular rock-dSris, but in low grounds opposite the mouths of mountain-valleys they are usually composed more largely of ground-moraine, together with gravel and sand and a certain admixture of angular ddbris and blocks, sometimes the one and sometimes the other kind of material predominating. In Europe, the most remarkable terminal moraines are those which denote the limits reached by the glaciers and ice-sheets of the Glacial Period. They are strongly developed in the Vorldndcr of the Alps, in Southern Scandinavia, Schleswig-Holstein, North Germany, and Finland; and on a smaller scale they abound in our own islands. Looked at broadly, such moraines occur as more or less abrupt mounds and crescentlike or undulating ridges. Opposite the mouths of important mountain-valleys they are often disposed in concentric series, one or more dominant lines of banks and ridges with many subordinate hummocks, heaps, and irregular low mounds lying behind and between them. Not infrequently they present a most tumultuous appearance—cones, mounds, banks, and ridges confusedly heaped together, and thus enclosing
multitudinous hollows and depressions of all shapes and sizes, many of which contain lakes or pools, while others are occupied by bogs or simply clothed with grass and herbage. The hillocks and ridges vary much in height and size, among the most conspicuous being those of Piedmont and Lombardy, where they occasionally attain the exceptional elevation of more than a thousand feet above the adjacent low grounds. More usually in the Alpine Vorldnder they do not exceed two or three hundred feet. The terminal moraines of the great Baltic Glacier in Finland, North Germany, Denmark, and Southern Sweden present much the same appearance as those of the Alpine Vorldnder. The most conspicuous are those which mark the extreme limits reached by that great icestream. These rise more or less abruptly above the level of the broad plains of gravel, sand, and boulderclay which sweep outwards from their base into the low ground of North Germany and Poland. The land lying between those external ridges and the shores of the Baltic forms a typical paysage morainique—wide plains traversed now and again by winding irregular ridges of gravel and sand, and more or less abundantly sprinkled with mounds and banks of similar materials. Here and there these hillocks crowd more closely together, giving rise to a tumultuously undulating surface; while in other places they are drawn out in curving lines and belts, or bands. Throughout the whole area shallow lakes and lakelets, bogs, and morasses are abundantly developed. The surface of the flat lands lying within this great morainic tract is usually formed superficially of fluvio-glacial deposits, and the same is the case generally with the low grounds immediately outside of the pay sage morainique.
To sum up the general results of glacial action, we may say that this action is entirely mechanical. Under the influence of ordinary weathering each particular kind of rock tends to assume a more or less characteristic outline. With glaciation, however, this is not the case. All rocks subjected to glacial action become abraded after one and the same fashion. The tendency of that action is to reduce asperities, to smooth and flatten the surface. But glacial action has usually been arrested long before its work has been completed. It is only here and there that projecting rocks have been ground away and reduced to a plain surface. In most cases they are simply rounded off, and so rocky hill-slopes tend to assume mammiform outlines. Some rocks are, of course, more readily reduced than others; but whether the rocks be hard or soft, they all acquire the same undulating configuration. In regions of dominant glacial erosion the rounded and undulating surface is often in part due to glacial accumulation, the abrupt depressions of the ground being not infrequently filled up and replaced by smoothly outlined hollows.
Where the region of glacial erosion merges into that of glacial deposition, it is often hard to say whether morainic matter or solid rock enters more largely into the formation of the banks and hillocks