A plain is an expanse of low and nearly level land. Some plains, such as the Fens of England, large parts of Holland, and extensive areas of Russia, are almost perfectly flat; but generally an extensive plain consists of wide, gently sloping valleys separated by low hills. Such a plain is termed ' rolling' or 'undulating'. Many plains look extremely flat when viewed from the top of the neighbouring hills, but on descending one finds numerous instances of steep gradients, the valley slopes of the streams which cross the plain.
Plains are not all of the same type. Some plains may have been formed by the wearing down of lands that were once much higher. Such areas are more correctly called 'peneplains', e.g. Finland, a lowland area of very old rock, the HudsonBaylowlands, and Anglesey. Secondly, where layers of rock have not been folded but remain almost horizontal, extensive plains also occur. The Central Plains of the United States and the great plains of European Russia are in this group. Thirdly, plains may have been formed by the gradual accumulation of silt brought down by rivers. These are usually called alluvial plains. Good examples are the plain of North China, the Indo-Gangetic plain, the plains of Iraq, and much of the Amazonlowland. Some plains are the beds of old lakes. Rivers entering a lake deposit silt which is spread by the movement of the water over the lake floor. Such plains, though not large in size, are usually very fertile. Much of the great wheat land of Southern Manitoba is the bed of an old lake -lake Agassiz. The fertile plains of Hungary are of similar origin. Finally, some plains, such as the coastal plains of the United States from ChesapeakeBay to Florida, have been formed by the uplift of part of the sea floor bordering a continent.
The plains of the world tend to be areas of most advanced development and densest settlement. They are easier to cultivate than highland areas as the soil is usually deeper and more fertile. Hence the great plains, except where covered with large tracts of uncleared forest or occasional deposits of infertile soil, are important agricultural lands. Some plains, such as portions of Central Asia or of the Murray-Darling Basin, are too dry for successful agriculture. Unless irrigation is a practical possibility such plains are occupied by pastoral farmers engaged in rearing animals, and even the pastoral farmers sometimes have to bore wells for water, as in Hungary and in Australia. Where coal is found in or near plains, densely populated industrial centres usually develop, as in the North-Central United States. Movement is easy in all directions over lowlands, and rivers are generally slow and easily navigable, so that they are used as commercial highways. This is well illustrated by the United States, where the Mississippi and its tributaries provided the main lines of communication before the period when railways were developed.
A valley is a long narrow depression in the earth's crust, flanked by well defined ridges and usually due to the erosive action of rivers or glaciers but sometimes due to trough-faulting.
Longitudinal valleys are the hollows between the up-folded mountain ranges, parallel to the mountains, and they usually contain a largee river. Similar valleys occur between upfoldod mountains and the crustal plateau which has resisted upheaval. The Indo-Gangetic valley between the upfolded Himalayas and the Deccanplateau is the largest example of this type.
The valley cut by vertical erosion is usually V-shaped in cross-section and irregular in its course, its gradient being punctuated by sudden drops and long shelves. These irregularities represent local base levels which are gradually removed by denudation, so that as the falls are worn back and lakes infilled the breaks in the profile are reduced. In southern England the valleys of the Severn and the Thames show the results of denudation, which has carved away the softer rocks, and left the more resistant ridges of the Cotswolds, Downs, and Chilterns, which confine the drainage system.
With lateral erosion and mass movement, the valley broadens. Deposition occurs as the gradient slackens, and floodplains fill the valley floor. Rejuvenation leaves remnants of old floodplains above the new ones in the form of terraces, the highest of which are the oldest. A lowering of the water-table may leave dry valleys, and sudden uplift may leave hanging valleys, while the flooding of valleys by the sea gives rias or 'drowned valleys' which are existing estuaries where the sea has encroached upon the lower courses of rivers, such as the Gulf of St Lawrence. Research Valley
In geology an uplift is a raising or upheaval of strata so as to disturb their regularity and uniformity, and to occasion folds, dislocations, and the like. Research Uplift
An uplift bra is a brassiere designed to enhance and amplify the perceived size and shape of the breasts. The most famous uplift bra is the 'Wonderbra'. Research Uplift Bra
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Valley
