Alluvial plain

“ Other things being equal, the greater their age the more meagre their remnants.
Terraces developed from river plains formed chiefly by alluviation stand a better chance of long life than most other alluvial terraces. This is because of the configuration of the original valley, the aggradation of which gave origin to the plain. The principle involved is illustrated by Fig. 197. In developing the second flood-plain the river encounters the rock wall of the valley. This greatly retards lateral erosion, and the terrace above, defended [92] by the rock, is likely to be long-lived. ”

“ Every one is familiar with the fact that many rivers overflow their banks at certain seasons, and spread themselves out over a wide area, sometimes reaching to the foot of the hills that bound the valleys through which they flow. This is the origin of those Alluvial Plains so remarkable for their surpassing richness and fertility. In each successive year a thin film of sediment is deposited on the surface of the land; and thus in the course of ages a soil is formed capable of producing, season after season, the most luxuriant crops without manifesting any symptoms of exhaustion. ”

“ The action of rain, ice and rivers conspires with the movement of land waste to strip the layer of soil from steep slopes as rapidly as it forms, and to cause it to accumulate on the flat valley bottoms, on the graceful flattened cones of alluvial fans at the outlet of the gorges of tributaries, or in the smoothly-spread surface of alluvial plains. ”

“ For, the alluvial deposit having been brought down by the rivers, they must needs be older than the plain it forms, as navvies must needs antecede the embankment painfully built up by the contents of their wheelbarrows. For thousands of years, heat and cold, rain, snow, and frost, the scrubbing of glaciers, and the scouring of torrents laden with sand and gravel, have been wearing down the rocks of the upper basins of the rivers, over an area of many thousand square miles ”

“ It is evident from the foregoing remarks, that while even the smallest stream may make deposits of alluvial character it is in the flood-plains and deltas of large rivers that the great alluvial deposits are to be found. The finer material constituting alluvium, often described as ``silt,'' is sand and mud. Although it may be exceedingly fine-grained, there is usually very little clay in alluvium. The larger materials include gravel of all degrees of coarseness; carbonaceous matter is often an important element. The amount of solid matter borne by large streams is enormous ”

“ When the engineer finally reaches the mouth of the stream at one of the lakes, we should expect to find his difficulties at an end, as the lakes are usually so deep that the alluvion makes little impression on them, and their areas are such that floods are not much to be feared. But he is confronted here with a new difficulty, that of anchoring or securing the foundation of his artificial river bank. The soil is generally alluvial over a large area, and is very damp. ”

“ At present, this species of inundated land offers no inconvenience to the people, except in the production of miasm, and even that, perhaps, becomes too much diluted with the atmosphere to produce mischief before it reaches the settlements on the borders of the prairie. Hence the inference is correct, that our inundated lands present fewer obstacles to the settlement and growth of the country, and can be reclaimed at much less expense, than the swamps and salt marshes of the Atlantic States.
2. River Bottoms or Alluvion. The surface of our alluvial bottoms is not entirely level. ”

“ The land-surface is carried beneath the water, the estuary then silts up, becomes fresh water, marsh-plants grow, and even trees may flourish on this marsh before it subsides again. But there is no sign that the strata were ever raised above the level to which ordinary floods could build up an alluvial flat. The land-surfaces seem always to have been swampy, and bed succeeds bed in fairly regular sequence, without the deep channelling we might expect to find when an alluvial flat was raised to a noticeable extent above the level of high water. ”

“ On these little rises, for they are not hills, trees grow, and grass also; but grass and weeds are the only occupants of the soil where there is no rise of ground. The soil is alluvial to greater or less depth in these barrens, though on some of the highest rises there is little or none; the lower the ground the deeper the alluvion. On these gentle rises, where there is no alluvion, we find stiff, blue clay, and no pebbles. Under the alluvial black soil, in the lower grounds, we find pebbles similar to those in the prairies, owing to similar causes. ”

“ The line where a land-born river meets the sea tends to become a sandbar or a delta, created by the river-borne silt and the wash of the waves, a form intermediate between land and sea, bearing the stamp of each, fluid in its outlines, ever growing by the persistent accumulation of mud, though ever subject to inundation and destruction by the waters which made it. The alluvial coastal hems that edge all shallow seas are such border zones, reflecting in their flat, low surfaces the dead level of the ocean, in their composition the solid substance of the land ”

“ To what extent these agents actually operate in changing the present surface of the globe, is an enquiry beyond the objects of this brief notice, but we have, in this valley as in numerous other parts of this country, abundant proof, that many of the most conspicuous and extensive alluvial deposits have had their rise in causes of a much more general and extensive nature. ”

“ So far, wherever we have a carefully noted section of the lowest deposits in these valleys, the tin ground or the gravels are directly succeeded by a growth of oak trees. It looks as though the climate ameliorated, the more violent floods ceased, and an oak forest grew across the alluvial flats, without there being any, or much, change of sea-level. ”

“ In consequence of the gradual rise of the river's bed, the annual flood is constantly spreading over a wider area, and the alluvial soil encroaches on the desert, covering, to the depth of six or seven feet, the base of statues and temples which the waters never reached 3000 years ago. ”

“ It is an old experience that certain infectious diseases have their favorite seats in the so-called alluvial soils, in lands subject to overflow. Alluvial soil consists chemically and geognostically of substantially the same mineral matters as the compact mountain-masses, from the disintegration of which it has originated—except that its physical aggregation is essentially different; and it is distinguished from rock soils by the great permeability for air and moisture arising from its great porosity, that is, from the spaces in which air and water, as well as organic matters, can find place. ”

“ Still, in all contemplated new enclosures, the lands should be raised by warping, that is, accumulating the alluvial matter, which can always be done if properly managed up to the level of high water of neap tides. ”

“ The streams have dug their channels deeper into the flanks of the hills and have spread their alluvial soil further and wider over the valleys and lake-floors. The frosts of winter have been splintering the crags, the springs have been sapping the cliffs, and from time to time landslips have been launched into the stream-channels below. The sea has cut away large slices of land from some parts of the coast-line, while to others it has added strips of alluvial ground and mounds of shingle. ”

“ Where the trees of the submerged forest can be seen rooted into hard rock, or into firm undisturbed strata of ancient date, there can, however, be no question that their position below sea-level is due to subsidence of the land or to a rise of the sea, and not to compression. But in certain cases it is found that our submerged land-surface rests on a considerable thickness of soft alluvial strata, consisting of alternate beds of silt and vegetable matter. ”

“ This new alluvial land, formed by the rivers and the sea, is of astonishing fertility, and is so much the more valuable, as the ancient soil of these countries, being covered with heaths and peat-mosses, is almost everywhere unfit for cultivation. The alluvial lands alone produce subsistence for the many populous cities that have been built along these coasts, since the middle age, and which perhaps would not have attained their present flourishing condition, without the aid of the rich deposits [131] which the rivers had prepared for them, and which they are continually augmenting. ”

“ If a fast stream meets a slower one deposition of material will take place; and the same thing will occur when the rivers meet a lake or a sea. Whatever checks the swiftness of a current weakens its carrying power and causes it to drop some of its sediment to the bottom. Therefore accumulations of sediment occur at the foot of torrent slopes along the lower and more level ground. These deposits we call alluvium, and sometimes when the mountain torrent ends abruptly in the plain they may stand up in cones of silt. ”

“ The former as far S. as the Roanoke river is sandy and not naturally fertile, though capable of being made highly productive; from the Roanoke to the Mississippi it is generally swampy, with sandy tracts here and there, and a considerable proportion of rich alluvial soil. The inland slope is one of the finest districts in the United States, the soil consisting for the most part of alluvium from the mountains and the decomposed primitive rocks which underlie the surface. ”

“ Lowland Tree Growth.—The variations occurring in the valleys or alluvial bottoms are less obvious to superficial observation, yet equally important and cogent to the close observer. In the properly alluvial lands, one dominant condition, that of adequate moisture supply, is almost always fulfilled, irrespective of soil quality. In addition to this, as stated in chapter 2 (see page 24) , practically all the alluvial lands of the humid region may be considered as being of a more or less calcareous character, as compared with the adjacent uplands. ”

“ At the present day the tidal part of each of these rivers extends right through the Downs into the lower Wealden area, and it is obvious that their valleys tend to silt up, not to deepen, and scarcely anywhere to become wider. When we examine further we find that the true valley-bottom lies far below the present alluvial flat; though the scarcity of borings and the uncertainty of many of the records make it difficult to say exactly how deep it lies. ”

“ This may account for many immense beds of alluvial materials, as gravel, rounded sand granulated limestone, and chalk, covering such extensive plains as Lincoln-heath, having become dry without the supposition of their having been again elevated from the ocean. At the same time we acquire the knowledge of one of the uses or final causes of the organized world, not indeed very flattering to our vanity, that it converts water into earth, forming islands and continents by its recrements or exuviae. ”

“ Else why do we find no alluvion, no grass, but a thick growth of ancient forest-trees on the higher lands? Else why do we find beneath the alluvion of the prairies, pebbles and shells similar to those at the bottom of lakes and ponds? ”

“ Perhaps Nature thinks fit to give a long preparation to these regions, in order to elevate the alluvial deposits for the habitation of man, and to form the stream territory, and thus to realise the principles of humidity ”

“ The alluvial deposits of which the river bottoms are formed, consist of particles of mud and sand, more or less minute. The coarser and more ponderous particles are of course soonest deposited, while the finer are transported by the current to a greater distance, and deposited near the mouths of the rivers. Thus it happens, that the bottoms situated nearest to the sources of the western rivers, are sandy, and contain but a small proportion of vegetable mould, while those nearer their mouths are generally furnished with a rich and fertile loam. ”

“ Here and there a valley has become filled with alluvial detritus owing to some local impediment in the drainage, and when this occurs there is usually to be found a fertile and productive field for agriculture. ”

“ The drainage of an alluvial cone or of a delta is independent of the structure of the bed-rock beneath; and if in the course of time erosion takes the place of deposition and the alluvial formation is cut through, the drainage system which is acquired by the rocks beneath is not consequent upon their structure but is superimposed by alluviation. ”

“ The waters of the Po in floods are dense with rich alluvial matter, of the fertilising properties of which evident proofs may be observed throughout the course of this river. After great floods, as if by magic, bare shoals of gravel become covered with a deep strata of alluvial soil, on which the seeds of trees and shrubs carried down by the waters soon take root, and in a very short time they are covered with a luxuriant vegetation. The waters of the Po on this account are highly valued for irrigation, as also from the fact of its temperature being higher than that of its tributaries. ”

“ Wherever the hill is formed of a soft alluvium they possess the greatest breadth, while, on the contrary, wherever the ground is rocky, they are scarcely to be traced. It is plain that they should, if they had been roads, have exhibited superior permanence in the most durable materials. ”