ICE AND SNOW AS GEOLOGICAL AGENTS It has already been stated that ice is an important agent of transport. It is now necessary to consider its efficacy from a more general point of view in moulding the surface of the earth into the existing forms. For there is no doubt that in the colder regions of the earth land-ice has a great power of erosion.
The existence of permanent snow in any given region is a mat ter of climate. When the temperature is below freezing-point, precipitation falls as snow. If the average temperature of any region is below this point it is obvious that the snow will remain, and will continue to accumulate. In the arctic 'and antarctic zones this condition prevails down to sea-level: even on the equator the snow-line is reached at about I 6,000 feet. Consequently in many temperate regions we find permanently snow-clad mountains, as in the Alps, the Caucasus, the Himalayas, the Rockies and the Andes. The snow-line is the level at which the snowfall of winter and the melting of summer exactly balance. But in many valleys glaciers extend far below this level.
In addition to the mountain glaciers of the type just sketched, it is necessary to take into account the ice-caps and ice-sheets such as are now found only in the highest latitudes of both hemispheres, but formerly existed in many other parts of the globe (see GLA CIAL PERIOD) . These have of ten covered enormous areas, and have transported vast masses of material. Thus the great Scan dinavian ice-sheet of the latest northern glaciation brought innum erable boulders of Norwegian rocks to the east of England, and effected great modifications of the topography of Yorkshire and East Anglia by deposition and erosion.
The material transported by mountain glaciers of the Alpine type is derived from two sources. Part of it falls on to the surface of the ice from the steep slopes above, where it is mainly loosened by frost action, and part of it is torn from the bed of the glacier by the movement of the ice. Much of the surface material eventu ally falls down cracks (crevasses), and reaches the floor where it becomes mixed up with the material torn from the bed, the two forming the ground-moraine. The rock waste that remains on the surface also forms moraines, ridges of gravel and sand, whose form and distribution depend on the number of valleys that join to form the glacier. Where two valleys join, the trains of moraine from the right bank of one and the left bank of the other combine to form a medial moraine down the middle of the compound glacier. When a glacier ends in a wide open valley the moraine material is dumped as transverse ridges, often crescent-shaped, called terminal moraine. Many of the large Alpine glaciers however end in deep gorges, like the Aletsch, the biggest glacier in Europe. In such a case the moraine material is usually carried away by the stream that always issues from the end of a glacier, and forms torrential river deposits below.
In a good many cases where glaciers have retreated owing to a change in climate, they have left moraine barriers across valleys ; these barriers often hold up lakes, as described in a later section.
Ice-Erosion and Hanging Valleys.—For many years there has been discussion and difference of opinion as to the power of glaciers to scoop out the valleys in which they lie. Some authori ties maintain that they have little effect, being able only to bring about slight modifications in the form of pre-existing valleys due to rivers. Glacier ice is a powerful agent of erosion and has been the main factor in the formation of many great mountain valleys. At present probably the balance of opinion inclines to the latter view. One of the strongest arguments in favour of ex tensive ice-erosion is the existence of "hanging valleys," a term denoting (a) a sudden steepening of slope in the floor of a main valley for a short distance—a kind of step, and (b) the sudden fall of a tributary valley of fairly flat grade in its upper parts down a steep slope into the main valley, sometimes forming a waterfall. The last-named is probably the more significant for the present purpose. The argument is that the valley system was once occu pied by ice, which was thickest in the main valley and therefore more powerful in erosion. Hence the main valley was deepened more rapidly than the tributaries, whose mouths were left hanging in the air, as it were. Or it might have so happened that there was no ice at all in the tributaries. Further it is noticed in many cases that the main valley shows a U-shaped section, with a flat floor, steep sides, and no projecting spurs, whereas the tributaries have the V-shaped section and alternating spurs characteristic of valleys in regions that have never been glaciated. Borrowdale in the Lake District shows some good examples of hanging tributaries, e.g., the Falls of Lodore, and Sour Milk Ghyll, above Seatoller.
Glaciers also produce certain characteristic features in the rocks over which they travel, features often only properly observable when the ice has disappeared. The surface of the rocks forming the sides and floor of the valley is often remarkably smoothed and polished, with sometimes scratches and grooves produced by stones embedded in the ice, showing clearly the direction of motion. In the same way stones embedded in the sole of the ice are scratched by the rock below, and are often found in terminal moraines thus marked. The presence of such scratched stones is one of the best criteria for the glacial origin of a deposit.
Roches Moutonnees.—Again, the rocks over which a glacier has passed often show a peculiarly hummocky outline, known as roches moutonnees, a term derived from a supposed resemblance to the curly fleece-like wigs of the i 8th century. These rocks often show a smooth gentle slope up-stream, due to the grinding action of the ice, and a rough abrupt scarp down-stream, where blocks of stone have been torn off bodily along joints. Below a large roche moutonnee or any other projecting boss, there is often a long line of debris, this formation being known as crag and tail. Another notable feature of glaciated valleys is that the smooth and polished rock-surfaces often end abruptly upwards, at the original line of the top of the ice, the slopes above showing the sharp pinnacles of f rost=riven rocks, which have not been ground down by the ice. All these phenomena are clearly apparent, along with mo raines, in regions where there are now no glaciers, giving clear evi dence of their former presence.
Erratic blocks.—Further evidence of the same fact is afforded by what are called "erratic blocks," great boulders which have often been transported far from their original home and lef t stranded when the ice melted (see GLACIAL PERIOD). Such are abundant in all formerly glaciated areas, and are often of the ut most value in tracing the courses of former ice-streams.
As a matter of fact wind is much more important in geology as an agent of transport and deposition than of erosion. More will have to be said about this when we come to consider the charac teristic deposits of arid regions (see also DUNES).
Many limestones are very pure, such as the Chalk and a good deal of the Carboniferous limestone of England and North Amer ica. Hence the whole rock can be dissolved away, leaving very little residue, which is mostly a fine sand or more commonly a red dish clay. Soils on such rocks are therefore often very thin, or even non-existent. A large bare expanse of limestone rock is by no means rare ; rain falling on such a bare surface with open joints will of course flow down into the joints and will widen them by solution, thus giving rise to the peculiar forms known as "grikes" in the north-west of England (Ingleborough, etc.) where there is in some places nearly as much open joint as rock; the joints usu ally run parallel for long distances and the surface consists of knife-edges of rock with fissures many feet deep between them. If there are two equally well developed sets of joints more or less at right angles the surface will consist of isolated four-sided blocks.
Caves.—It is easy to understand that if solution is so active at the surface it will also go on underground. Rain and small streams falling down joints and running underground often widen the joints very greatly far below the surface, to such a degree as to form great caverns, which are very numerous in many limestone regions, as in west Yorkshire, Derbyshire and the Mendips in England ; south-eastern Belgium, the country east of the Adriatic; and Kentucky among many others (see CAVE). Besides actual caves in many places, indeed in most limestone areas, the main part of the river system is underground ; surface streams are rare, and often disappear at some part of their course. Dry valleys some times occupied by streams in very wet weather are numerous in the Chalk Downs of southern England.
Another remarkable phenomenon is afforded by the swallow holes or pot-holes often seen in such regions, where a stream sud denly disappears down a vertical shaft, often running into a cave below. One of the most remarkable is Gaping Ghyll on Ingle borough, Yorks., which is 365 ft. in vertical depth; the stream runs for a long way underground and eventually comes out in Clapham Cave. Most of its course has been explored.
The above-ground topography of limestone regions is often very remarkable. Rock-pillars, cliffs and gorges are common in many places, such as Dovedale and Matlock in Derbyshire. The Ched dar Gorge in Somerset is supposed to be due to the collapse of a cave. Some of the most remarkable limestone hills are found in Kedah and Perak, in the Malay peninsula; some show vertical or nearly vertical faces from 1,500 to 2,000 ft. high and many of these. cliffs overhang considerably for a height of hundreds of feet.
Limestone pinnacles.—Another remarkable feature seen in the same area, especially in the Kinta Valley, is the formation of pin nacles of limestone, below a covering of newer deposits. These pinnacles, where uncovered in the alluvial tin-mines of the Kinta Valley, show very extraordinary forms. From the disposition of the overlying deposits it is quite clear that the upper surface of the limestone was once smooth and that the solution went on underground and is even now in progress.
All the features of limestone denudation, which may be equally well developed in dolomite rock, are specially well seen in the Karst plateau east of Trieste; hence, geologically, this kind of scenery and formation is known as the Karst type.
In order to save space and avoid repetition a general account will here be given of the geology of coast—lines; this involves a consideration of the joint effects of causes belonging to several different categories, namely denudation, deposition and earth movements.
The geology of coast-lines is a complicated subject, because it is only rarely that one simple process or set of related processes is in operation. Still it is possible to state that on broad lines some coasts are being destroyed while others are being built up. Thus in England it has been shown by a Royal Commission on Coast Erosion that in a given period the gain of land in England, mainly on the west, was five times as great as the loss, mainly on the east. Thus the country is not being washed away, but is actually getting larger, though the destruction of existing land is more striking and spectacular than the building up of new areas and so receives more notice.
Loss of land may be due to two causes, marine denudation, or actual sinking of the land. Gain of land may be due to deposition of new material along a fixed coast line, or to an uplift of the whole area. In either case it is usually easy to decide which causes have been at work.
On coasts composed of soft material undergoing destruction, conditions are rather different. There is generally not much cliff but a gradual slope, the outline of the land is smoother and the wave-cut platform is not conspicuous, being replaced by a uniform slope seaward. Beach material is also more uniformly dis tributed and consists of smaller fragments; also, under the in fluence of the tidal currents, it tends to travel more along the coast, owing to the absence of sharp promontories to check it.
The two foregoing paragraphs describe simple, ideal cases, but in reality there are usually complications, and every shore-line has to be studied on its own merits. In a general way, if the effects of uplift or depression are excluded, a deeply indented coast-line indicates a country composed of rocks of very varying hardness, exposed to the fury of a prevailing on-shore wind and powerful tides. A smooth outline indicates a country of uniformly soft rock, sheltered from violent winds and with tides running parallel to the shore. Such in its broadest outlines is the reason of the contrast between the coasts of the west and east of the British Isles.
Drowned Valleys and Raised Beaches.—But a smooth coast line can be broken up, or a rugged coast-line made simple in plan, by relative changes in level of land and sea. When the land sinks relatively to the sea the lower parts become submerged ; the sea runs up the valleys and forms those long narrow winding estu aries which are so common in Cornwall and Brittany, and in north-west Spain, where they are called Rias. Within them sub merged forests are often found. When, on the contrary, the land rises, its outline is frequently smoothed off and simplified in plan. The shallow sea floor is raised to form a low plain, often bounded by a row of inland cliffs, and if the rise is spasmodic, raised beaches may be formed at various levels. Thus, in many places round Scotland can be traced raised beaches at 25, 5o and ioo ft. respectively, above present sea-level; they are not uncommon in S. Wales, Devon and Cornwall; and in Norway they are well developed at great heights.
Fiords.—Striking effects are produced when an area of highly developed glacial topography is submerged. The sea runs far up the long, straight and steep-sided (U-shaped) valleys already described as characteristic of ice-erosion ; waterfalls drop straight from the mouths of the hanging valleys to the sea and there is little or no beach at the foot of the cliffs. In some of the fiords of Norway, British Columbia and New Zealand great walls of rock ascend for 4,000 or 5,000 f t. almost sheer from the water's edge, giving rise to some of the finest scenery of the world, similar to that of the western Highlands of Scotland, which also originated in this way.
The Materials of Marine Deposits.—These are in part brought down by rivers, in part derived from the waste of the coast, and in part provided by the activity of animals and plants. When the material forming any ordinary beach is examined it is usually found that there is a distinct gradation in the size of the frag ments composing it. Near the base of the cliff, or about high water-mark if there is no cliff, are found large blocks of rock, more or less angular or rounded. Below this comes shingle or gravel, usually well rounded, which gradually passes downwards into sand, extending commonly below low-water-mark. The mate rial usually gets finer and finer out to sea, and soundings show that sand eventually passes into mud. Very often too, in the estuaries of large rivers and along certain low coast-lines, mud banks can be seen between tide-marks. Besides all this there are often shells and other organic matter, either living or dead, and in many of the warmer parts of the world, solid organic deposits, coral reefs, etc., are found. We thus arrive at a natural classification of modern marine deposits—boulders, gravel, sand, mud and organic material. The peculiar deposits found in the very deepest parts of the oceans will be disregarded for the present.
Distribution of Deposits.—One of the fundamental principles of geology is that the stratified rocks of past ages have been built up of exactly similar material, and that the modern deposits of the sea-beach will some day form rocks just like the older ones (see SEDIMENTARY ROCKS for mechanism of process). Of course the distribution of beach material is never uniform, for it is con trolled by many agencies such as waves, tides and currents, and is sometimes transported for long distances ; it is well known that along the south coast of England, for example, the shingle mostly travels from west to east, movement easily accounted for by the prevailing winds and tides. At one point, near Portland, there is a local eddy due to the shape of the land, and the shingle turns back west, thus forming the well-known Chesil Beach on the Dorset coast. In the east of England the set of the tides is mainly from north to south, and long gravel spits are formed, such as Spurn Head and the great beach at Aldeburgh, where the river is turned south and runs for 9 or Io m. parallel to the shore before it can get through the shingle banks.
An arid region may be defined as one in which the average an nual rainfall is less than io inches. The limit thus fixed is of course quite arbitrary, but it represents approximately a boundary between moist and dry regions possessing different geological and biological characters. Within this category, however, there is room for a considerable amount of variation, and a great deal depends on the seasonal distribution of such rain as there is. Some areas with a quite appreciable amount of rain may be absolute deserts, if the rain all falls at once, or within a very limited range of time, whereas on the other hand a surprisingly small total, well dis tributed, may give comparative fertility. Obviously, also, tem perature is an important factor. It is well to remember that in reality the arctic regions are rather dry, if the snow that falls is calculated as rain, but the effect of the low temperature is to create here a special type of geology, which is dealt with else where.
Deserts.—What we are here concerned with is the hot, dry regions of the world (see CLIMATE and CLIMATOLOGY) ; the idea conveyed to most people by the mention of a desert is sand, and this is for the most part a true conception, but there are also rock-deserts, as in parts of Egypt and the Sahara.
The true arid regions of the world are defined also as those from which there is no drainage to the sea, and it is stated by Walther that no less than one-fifth of the whole land surface of the globe belongs to this type. Such arid regions naturally include all types of topography, mountains as well as plains or basins, and some parts of them are below sea-level (e.g., the Dead Sea region, the lowest part of the earth's surface and one of the hottest) .
Salt Lakes.—Now when a low plain or basin is surrounded by high mountains or plateaux there may be considerable rainfall or snow on the high ground, giving rise to rivers ; these often dry up or become very small when they reach the low ground, owing to evaporation, or they may reach a lake, which is generally salt, be cause the dissolved matter carried down by the streams is con centrated as the water evaporates from the surface of the lake. The nature and origin of these salt deposits is described under PETROLOGY (q.v.) and will not be pursued further here, but it should be emphasized that salt deposits and salt lakes form some of the most characteristic features of deserts. If a dry region lies fairly near the sea a prevailing wind may carry a lot of salt inland for a long distance; the saltness of part of the desert region of N. W. India is attributed to this cause.
Desquamation.—In a dry region the chief weathering agent is sudden changes of temperature, aided by the chemical action of dew and strong salt solutions. The changes of temperature shatter the rocks and often cause thin slabs to scale off and accumulate in piles at the foot of slopes (the desquarnation of Richthofen) : chemical action and wind carve the rocks into fantastic shapes, and the rocks are often covered by a peculiar dark enamel, due to evaporation of solutions containing iron or manganese.
Wind Transport.—The chief and practically the only trans porting agent is wind. This differs from other geological agents in that it can, and often does, carry material uphill, against gravity. It can also carry fine particles across rivers and lakes and across even quite broad stretches of sea. During gales sand derived from the Sahara is often noticed in southern Spain and Sicily. The deposits of salt formed in lakes are often discoloured by fine dust blown in by the wind.
But it is the behaviour of sand under wind action (see DUNES) that is the most notable characteristic of deserts. The grains of wind-borne sand-deposits show special features (see SAND) which enable a specimen to be distinguished readily from sands of other origins, for such sand grains are usually very well rounded ("millet seed sands"), and are thus readily distinguished when occurring among the older formations, cemented to solid rocks. Such de posits are sometimes of a red colour, since the iron present exists in the anhydrous ferric condition, but they are more commonly yellowish grey.
In deserts the direction of the wind is often very constant (trade wind or monsoon type) hence the sand drift often makes very well-marked grooves in definite directions on rock surfaces, and pebbles fixed in the ground show a peculiar triangular cross section of the exposed part (Dreikanter), often with an exceed ingly sharp ridge at the top. One of the most remarkable types of deposit found in a dry region is the Loess (q.v.) of central Asia and eastern Europe.
Oases.—As a rule the barrenness of an arid region is simply due to want of water ; the soil contains plant-food in abundance if only water can be got into it. Hence arises the great fertility of the oases, places where natural springs occur, and the success of artificial irrigation schemes in the western United States, South Africa and Australia among other places. The soil is sometimes poisoned by soluble salts, but when irrigation is carried far enough to wash these out it may become very fertile, as in the case of the alkali soils of the western U.S.A.
The British Isles.—Light is often thrown on the geological his tory of an island by a study of its living fauna and flora, as well as by its fossils. Thus Great Britain has fewer species of mammals and reptiles than the continent of Europe, and Ireland has fewer species than Great Britain. The natural inference is that Ireland was cut off from Europe first, before many species had arrived there in their westward migration, and that Great Britain was cut off at a later date, when more animals had arrived. Further, the study of island faunas and floras has been of enormous impor tance in the development of the doctrine of organic evolution, as set forth by Darwin, Wallace and many later biologists.
Continental Islands.—Taking first the group of continental islands, it is obvious that a tract of land may be isolated in two ways, or by a combination of them. The connection with the Con tinent may be severed either by denudation, terrestrial or marine, or by submergence, and in most cases, probably, the finishing touch is given by submergence. This is what seems to have hap pened in the opening up of the English Channel, which was once a great river valley with its watershed between Dover and Calais. A river cannot well completely erode its own watershed to sea level, but either submergence or the waves of the North Sea might finish the work and break down the land bridge by which the ani mals migrated from the Continent.
Complex Formations.—In other cases it is clear that earth movements played the most important part. Thus, Madagascar is separated from Africa by a very deep strait undoubtedly due to the sinking of a long narrow strip of land. It is in fact a rif t valley, a term to be explained later. The formation of the islands of the Malayan region was evidently very complex, due partly to crumpling of the earth's crust into folds and partly to changes in the relative levels of land and sea due to some more general cause. The Malay peninsula only just escaped being another island very like Sumatra, as there is quite a low isthmus in Siamese territory to the north, where it was once proposed by Germany to cut a canal and intercept the trade of Singapore.
Coral Islands.—Apart from volcanic islands like St. Helena and Ascension (see VOLCANO), one of the most interesting types com prises those islands partly or entirely composed of coral and other calcareous organisms. These are only formed in the warmer parts of the world, like the Pacific and Indian oceans, and the West Indies, because corals cannot live where the temperature of the water falls more than a degree or two below 70° F ; there are none on the west coast of America, owing to the cold currents. Coral structures (see CORAL REEFS; JURASSIC, TRIASSIC), are not always islands: they may also be reefs along the coasts of large land-masses either close to, or at some distance from the shore. Both forms existed in many parts of the world, even in high northern latitudes, in earlier geological periods : their f os silized remains are found in several geological formations in Europe, indicating very different climatic conditions from those now prevailing.
Lakes definitely formed by earth-movement are usually large, and some of them are salt. Thus it appears that the Caspian and the Sea of Aral were once connected with the open ocean, but were separated by uprise of the land. They have now no outlet, and are salt. The great lakes of North America were probably formed by the cutting off of an extension of Hudson Bay; at one time they drained to the Mississippi, but the drainage was much interfered with by land-ice, and the St. Lawrence became the outlet. Recent movements in the area are indicated by the bending or warping of old beaches. All the great lakes of East Africa, except the Victoria Nyanza, are in "rift-valleys," which were formed by the letting down of long narrow strips of ground between two fractures. Hence their shape, and hence, too, they are very deep and have extremely steep sides, both above and below water. Tanganyika is the most typical example. Many of the lakes of mountain regions like the Alps, etc., are determined by the uprise of chains (see EARTH MOVEMENTS, below), but they are often modified by glaciation or other causes.
Solution Lakes.—Lakes due to erosion of various kinds are numerous. One of the simplest types is where a hollow has been formed by solution of the rock below, such as limestone, or beds of gypsum and rock salt. The soluble rock need not lie at the surface : the solution may take place underground, below an im pervious cover, which sinks down in the form of a basin and thus holds water. Lakes of this kind are not uncommon in various salt-producing districts, e.g., some of the meres of Cheshire. Arti ficial ones are often formed by subsidence after salt-mining or brine-pumping. Very similar are the large flooded areas often seen in coal-mining regions, also of artificial origin, but good illustra tions of the natural process. Some of the Swiss lakes in the St. Gothard region are believed to be due to solution of beds of dolomite-rock, etc.
Rock-basins.—Of more importance are the lakes due to glacial erosion. There has been in the past much controversy as to whether a glacier can scoop out a rock-basin, but the question is now considered to be settled in the affirmative. Such lakes are commonly associated with U-shaped valleys and all the other signs of glacial action previously described. Many of these rock basins extend far below sea-level and yet have a continuous rocky margin all round, even at the outlet. A precisely similar state of affairs exists in many of the fiords and sea-lochs of Norway, Scotland, British Columbia and New Zealand, except that the barrier is below sea-level. Only a glacier could scoop out a basin of this kind, deep inside and shallow at the mouth. Some of the world's finest scenery is around such lakes and fiords—and it is of importance to notice that there is no real difference between a freshwater lake of this kind and a fiord ; one happens to have its lip above sea-level, the other below.
Barrier Lakes.—This category includes a good many varieties. Thus the barriers and therefore the lakes may be of a very tem porary nature. Examples of these are afforded by the lakes some times held up by landslips or avalanches, such as are not uncom mon in Switzerland. Behind such a dam water may accumulate to a great depth, and sometimes the dam bursts suddenly. Several disastrous floods produced in this way are on record in the Rhone valley and also in the Himalayas. It makes very little difference whether the barrier is ice and snow or rock. Sometimes it hap pens that a glacier in its advance creeps slowly down and holds up a lake, as is seen in several places in the Alps. A famous example is the Marjelen See in the Bernese Oberland, where the great Aletsch glacier holds up a lake in a tributary valley, which overflows over a low watershed into the next main valley; the lake shows a distinct beach line at the level of this outlet, and such appears to have been also the origin of the famous Parallel Roads of Glenroy, in Scotland. On the other hand a glacier coming down from a side valley may build a dam across the main valley, as at the Mattmark See, above Stalden in the Saas valley, near Visp, Switzerland.
Moraine Lakes.—One of the most important groups of lakes comprises those held up behind the heaps of moraine left by re treating and melting glaciers. As described in an earlier section the terminal moraines of glaciers take the form of ridges lying across the valleys, which obviously form admirable dams for lakes, which may be of any size, from the small tarns and lochans, so common in the higher parts of the Lake District mountains and of the Highlands of Scotland, to big sheets of water like Windermere and Loch Lomond. As a matter of fact both the lakes just named appear to be rock-basins as well as moraine lakes; that is, the deepest parts lie in a rock basin, but the level of the water has been raised by a moraine dam. This state of affairs appears to be exceedingly common, and naturally, because the glacier at its maximum scoops the rock-basin and on its retreat deposits moraines.