Lakes are accumulations of water in hollows on the earth's surface. When they are drained by rivers their waters are fresh, but when they have no outlet they are salty, e.g. the Dead Sea, Sea of Aral, etc.
Lakes may owe their origin to:
Barriers across a river valley hold back the water, which forms a lake. Such barriers may be of various types. (a) Sometimes artificial barriers of concrete and masonry are built across a valley so as to make a lake which can act as a reservoir for the water-supply of a large city, e.g. LakeVyrnwy for Liverpool. (b) A glacier may deposit a mass of morainic material across a valley. In this way the lakes of the Lake District and many of the Scottish lakes were formed. (c) A landslip may occur. A lake was formed thus in the Upper GangesValley in 1892. Two years later the landslip dam gave way, and disastrous floods occurred downstream. (d) Oxbow lakes are formed from the meanders of rivers. The deposition of silt at the two ends of the 'oxbow' closes the channel between the main river and its old loop. Many oxbow lakes border the River Murray in Australia, and the lower Mississippi. (e) Sometimes a lavastream may flow across a valley and cause the formation of a lake, e.g. LakeTaupo in New Zealand. (f) Sometimes large estuaries are partially filled with silt. In the portions not so filled are large shallow lagoons. Such lagoons are found in deltaic areas. The NorfolkBroads are portions of an old river estuary. (g) When a silt-laden stream enters a lake its speed is checked and a barrier or delta is built across the lake splitting it into two portions. This has happened in the Lake District, where Keswick stands in the alluvial flats between Lakes Bassenthwaite and Derwentwater, and in Switzerland, where Interlaken is situated in the flats between Lakes Thun and Brienz. (h) The action of the sea often causes an accumulation of sand and pebbles which cuts off a lagoon of sea water. The Fleet in Dorset is such a lagoon, cut off from the sea by Chesil Bank, a long pebble beach which joins Portland Island to the mainland.
The nehrungs of East Prussia are sand-spits which enclose the shallow salt-water lagoons or halls, such as Kurische Haff. Earth movements cause lakeformation when subsidence occurs. This is most easily seen in rift valleys. Examples of riftvalley lakes are the Dead Sea, Lakes Nyasa and Tanganyika in Africa, and LakeTorrens in Australia. These are all long, narrow, and very deep lakes.
In Cheshire, the removal of underground beds of salt has caused subsidence resulting in the 'meres' of the Weaver Valley. The 'folding' of the earth across the line of a river valley may partially block a river and help to form a lake. The study of a good physical map will reveal the connection between mountain building and the formation of LakeGeneva and LakeConstance in Switzerland. Where there are large areas of depressed lowland wide and shallow lakes are formed in the lowest part of the depression, for example the Sea of Aral in Asiatic Russia, LakeBalaton in Hungary, and LakeEyre in Australia. Ice sheets and valley glaciers may scoop out hollows to form 'rock basins'. Mountain tarns and corrie lakes in North Wales and Scotland have been formed in this way. Water also accumulates in the hollows of unevenly- distributed glacial drift. Such are the lakes of East Prussia, and also those of the Cheshire-Shropshire borders near Ellesmere. Subsidence of the land surface and consequent lakeformation may be directly related to volcanic action. Lough Neagh in Northern Ireland is a shallow lake formed by subsidence of this type. Lakes are often formed by the accumulation of water in the craters of extinct volcanoes, for example the Laachersee in the Eifel region of Germany. Research Lakes
A nebula is a cloud of gas and dust in space. Before the invention of the telescope, the term nebula was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were originally called nebulas.
Nebulas exist within other galaxies as well as in our own Milky Waygalaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas called Herbig-Haro objects are found in dense interstellar clouds and are probably the products of gas jets expelled by new stars in the process of formation. Planetary nebulas, or planetaries, are so called because many of them superficially resemble planets through telescopes. They are actually shells of material that an old average star sheds during a late, red giant stage in its evolution, before becoming a white dwarf. The Ring nebula of the constellationLyra, a typical planetary, has a rotational period of 132, 900 years and a mass calculated to be about 14 times that of the earth's sun. Several thousand planetaries have been discovered in the Milky Way. More spectacular but fewer in number are nebulas that are the fragments of supernova explosions, perhaps the most famous of which is the Crab nebula in Taurus, now fading at the rate of about 0.4 percent per year. Nebulas of this kind are strong emitters of radio waves, as a result of the explosions that formed them and the probable pulsar remnants of the original star. Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighbouring stars. They include some of the most striking objects in the sky, such as the Great nebula in Orion. The tremendous streams of matter in the diffuse nebulas are intermingled in violent, chaotic currents. Many thousands of luminous nebulas are known. Spectral studies show that light emanating from them consists of reflected light from stars and also, in so-called emission nebulas, of stimulated radiation of ionised gases and dust from the nebulas themselves.
Dark, diffuse nebulas are observed as nonluminous clouds or faintly luminous, obscuring portions of the Milky Way and too distant from the stimulation of neighbouring stars to reflect or emit much light of their own. One of the most famous dark nebulas is the Horsehead nebula in Orion, so named for the silhouette of the dark mass in front of a more luminous nebular region. The longest dark rift observed on photographic plates of the star clouds of the Milky Way is a succession of dark nebulas. Both dark nebulas and luminous nebulas are considered likely sites for the processes of dust-cloud condensation and the formation of new stars. Research Nebula
The federal Democratic Republic of Ethiopia (formerly known as Abyssinia until the 20th century) is a country in north-east Africa. It has a total area of 1,221,900 km2. The climate is tropical monsoon with wide topographic-induced variations, the country is also prone to extended droughts. The terrain is high plateau with a central mountain range divided by Great RiftValley. Natural resources are small reserves of gold, platinum, copper, potash. The religion is 40-45% Muslim, 35-40% Ethiopian Orthodox, 15-20% animist, 5% other. The official language spoken is Amharic with Afaan, Tigrigna, Oromo, Arabic and English also spoken.
Ethiopia was a powerful Christian kingdom from the 1st century AD until by 1889 it had become fragmented. King Menelik II subsequently reunified the country and repulsed an Italian invasion. Research Ethiopia
The State of Israel is a Jewish country in south west Asia, founded in 1947 in Palestine by a United Nations resolution. It has a total area of 20,770 km2. The climate is temperate and hot and dry in desert areas. The terrain is comprised of the Negev desert in the south; a low coastal plain; central mountains and the JordanRiftValley. Natural resources are copper, phosphates, bromide, potash, clay, sand, sulphur, asphalt, manganese, small amounts of natural gas and crude oil. The religion is 83% Judaism, 13.1% Islam (mostly SunniMuslim), 2.3% Christian, 1.6% Druze. The official language is Hebrew with Arabic used officially for the Arab minority; English is the most commonly used foreign language.
Israel is a township in Preble County, Ohio, USA. Research Israel
The Republic of Kenya is a country in east Africa. It has a total area of 582,650 km2. The climate varies from being tropical along the coast to arid in the interior. The terrain is comprised of low plains that rise to central highlands bisected by the Great Rift Valley with a fertileplateau in the west Natural resources are gold, limestone, diatomite, salt barytes, magnesite, feldspar, sapphires, fluorspar, garnets, wildlife The religion is 38% Protestant, 28% Roman Catholic, 26% indigenous beliefs, 6% Muslim The official language is Swahili with English and numerous indigenous languages also spoken. The area of Kenya was originally inhabited by African tribes before being colonised by Arabs during the 8th century and later conquered by the Portuguese in the 15th century and taken by the British in 1895, becoming part of British East Africa before becoming a British colony in 1920 and independent in 1963 following an uprising and a republic in 1964. Research Kenya
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Lakes
