EROSION The A River is a large stream of water flowing through the land. Creeks, Brooks, and Rills are smaller than rivers. Parts of Streams. - Streams have their origin in springs, swamps, and melting snow and ice. The place of origin is called the Source. The Mouth is the place where a stream flows into another body of water. bed in which a stream flows is called its Channel. Stream Systems and Basins. — Nearly every stream is joined by smaller streams, which flow into it from either side, and are called its Branches, Tributaries, or Affluents. A stream System is a river with all its tributaries. A stream Basin is all the land drained by a river and its tributaries. Lakes. - A Lake is a body of water situated entirely within the land. Ponds are small lakes. Lake basins are depressions in the land. They have been formed (1) by the wrinkling of the earth's crust, (2) by the damming of valleys, (3) by moving masses of snow and ice wearing away basins in the land. Water collects in the depressions until it runs over the surrounding rim, or until evaporation balances the inflow of water. Hence lakes are of two classes, those with outlets and those with no outlets. Lakes with no outlets are usually salt, for only pure water is lost by evaporation, and the impurities brought in by the inlets accumulate in the lake. Lakes with outlets are usually fresh, because the salts can not accumulate. Text Questions. How is the land supplied with water? In what three ways does this water leave the land where it falls? Describe what becomes of that part which sinks into the ground. How are hot and mineral springs caused? How and where are caverns formed? What is a geyser? How are streams formed? What is a river? In what do rivers originate? What are the parts of a river? What is a tributary? What is a river system? a river basin? What is a lake? How are lake basins formed? Name and define the two classes of lakes. Weathering. - Rain water dissolves some part of the surface rocks, and what is left then slowly crumbles to pieces. Water expands in freezing, hence when the ground water freezes in the fine crevices of rocks they are broken into finer and finer pieces, until at last they are reduced to gravel, sand, and mud. This slow breaking up of rocks is called Weathering. Transportation. -The rock waste produced by weathering is slowly moved down the slopes of the land by its weight, by winds, and by the wash of the rain. Finally it reaches some creek or river, and by it is carried away as sand and mud toward the sea. portation. This is called Trans Corrasion. Every stream carries with it some sand or mud. When the flow is rapid this material acts as a file or sandpaper on the bed of the stream, and wears it deeper. This is called Corrasion. A stream with a rapid current can carry more rock particles and much larger ones than a stream with a more gentle current. Hence rapid streams generally deepen their beds more than do streams with gentle currents. The cutting may go on until the stream bed is corraded or cut down nearly to the level of the sea. Sediment. When the flow of a muddy stream becomes slower, some of its load of sand and mud sinks to the bottom, forming a layer of Sediment on the bed of the stream. Each time the stream is muddy and becomes clear a new layer of sediment is laid down. But if the current later becomes more rapid, it picks up the sediment again from its bed, and carries it farther on. In this way the sediment may at last be laid down on the sea bottom at the mouth of the stream. Formation of Valleys. Nearly all the valleys on the land have been formed by erosion. Streams flowing in the depressions of the land corrade their beds, and thus make the depressions deeper, while weathering on the slopes. ROCK WEATHERING (MONTANA). of depressions broadens them. Thus valleys grow deeper by corrasion and wider by weathering. In highland regions, where the slopes are usually steep, streams flow swiftly, and may deepen the valleys faster than weathering can widen them. For this reason highland valleys are usually in the form of deep, narrow gorges or canyons. In lowland regions, where the slopes are gentle, the streams flow slowly, and often can not deepen the valleys so fast as they are widened by weathering. Hence in lowland regions the valleys are usually broad and shallow. The widening of valleys slowly wears away the hills between them. Thus, after many ages, the hills may be worn entirely away by erosion, and a hilly region may become an almost level lowland plain. Such a plain made by erosion is called a Peneplain. Rapids and Ripples. -A stream with a gentle current is often unable to carry away quickly all the sand and gravel brought into it by its swift tributaries. A deposit, or Bar, of sand, gravel, and bowlders, therefore, often forms in the main stream just below the mouth of a swift tributary. Such deposits act as dams, over which the stream rushes swiftly, forming Rapids or Ripples, while above each dam the stream, at low water, is a long Reach, or pool, having a very gentle current. Dams and rapids may also be formed where a stream flows over inclined layers of hard rock, which it can not wear away so quickly as it does the softer rock above and below. Cataracts are formed when the rock layers are nearly level and the softer rocks beneath are eroded in such a way as to cause a perpendicular plunge of the water. Small cataracts are called Cascades. Deltas. When a stream flows into a body of quiet water, nearly all its load of mud and sand sinks to the bottom. This deposit may accumulate until it reaches the surface of the water. It then becomes a tract of low, marshy land, which divides the stream into several channels, and causes it to have several mouths instead of one. Such a tract of land at the mouth of a stream is called a Delta, because some deposits of this kind have roughly the shape of the Greek letter delta, A. A CASCADE. THE ATMOSPHERE One The Air. The earth is surrounded by an atmosphere of air, which is at least fifty miles thick and probably much thicker. Air is a mixture of several gases. of these gases in the lower part of the atmosphere is the vapor of water. It is the source of clouds, rain, and snow. The air presses on the earth at sea level with a weight of nearly fifteen pounds on each square inch. This pressure is determined by an instrument called the Barometer. As we ascend into the air there is less of the atmosphere above us, and its pressure therefore decreases. Weather and Climate. The condition of the atmosphere at any time, with respect to the heat, moisture, and movement of the air, constitutes Weather. The RAPIDS (NIAGARA RIVER). Deltas are very apt to be formed where streams flow into lakes or seas having no tidal currents. The delta deposit constantly extends outward, and may eventually entirely fill up a lake, and convert it first into a swamp and then into dry land. Flood Plains. During a freshet a stream swells to greater width and deposits sediment along its margins, where the current is less than near the center. When the freshet subsides, this deposit is left as a coating of mud on the land that was flooded. Each freshet leaves a fresh coating of mud on top of those left before. Thus a strip of smooth, fertile "bottom land" is at last built up nearly to high water level on one or both sides of a stream. This strip of bottom land is called a Flood Plain. The river flows in a winding channel, which gradually shifts about in the flood plain. Copyright, 1900, by Detroit Photographic Co. A FLOOD PLAIN (CONNECTICUT RIVER, weather and the weather changes that are usual at any place throughout the year, taken one year with another, constitute the Climate of that place. Heating of the Atmosphere. The temperature of the air is ascertained by an in Movements of the Atmosphere. Air, like other substances, expands when heated, and contracts when cooled. Thus heat makes air lighter, and cold makes it heavier. This variation in the weight of air causes currents in the atmosphere from regions where air is heavy toward regions where it is light. We call these currents of air Winds. COLD BELT The boundaries of the heat belts are isothermal lines that is, lines drawn through places having the same temperature. On this map all parts of the earth whose mean temperature for the year is warmer than 70° are included in the hot belt, and all parts whose mean annual temperature is colder than 30° are included in the cold belts. The map shows the heat belts as they are in the spring and autumn, when the sun is vertical over the equator. But these belts lie much farther north in July, when the sun is nearly vertical over the Tropic of Cancer, and farther south in January, when the sun is nearly vertical over the Tropic of Capricorn. Heating of Land and Water. The boundaries of the heat belts are quite irregular. Both the hot belt and the northern cold belt are wider over the continents than over the oceans. This is caused partly by ocean currents, but chiefly by the fact that the sea heats and cools more slowly than the land. (1) The sun's rays penetrate and warm the water to a considerable depth. Thus the heat is distributed through a large depth of water, while it is concentrated at the surface of the land. (2) Part of the heat falling on water is used up in causing evaporation, and does not affect the temperature of the water. (3) The ocean currents cause an interchange of the warm equatorial and cold polar waters. Water cools slowly, because when it cools at the surface it becomes heavier and sinks, forcing the warmer, lighter water below to rise. Thus the sea Since it is always hotter near the equator than at places nearer the poles, the cool, heavy air pushes in toward the equator, forcing the lighter warm air there to rise. This warm air ascends for some distance, and then flows off toward the poles as upper currents. Thus are formed two great systems of surface winds, one on each side of the equator, moving toward the equator, near which they must rise and move away from the equator as upper currents. Winds, however, do not blow in straight lines. The earth's rotation causes them to turn to the right in the northern hemisphere, and to the left in the southern, so that the winds which blow toward the equator come from the northeast and the southeast. The Equatorial Calm Belt. Near the equator, where the winds meet, light breezes or calms prevail, for the air is rising and is not felt as winds. This narrow region is called the Equatorial Calm Belt. The Trade Winds. The cool winds which push in along the surface from the northeast on one side of the equatorial calm belt, and from the southeast on the other, are known as Trade Winds. On the sea they are more noticeable and regular than on land. The Tropical Calm Belts. Near each tropic some of the air that rises near the equator descends again, thus are affected so differently by the sun's rays, climates are sometimes producing belts of calms called the Tropical Calms. can not become very much warmer or cooler at one time than at another, while the land may be very cold in winter and very warm in summer. Classes of Climate. Because the atmosphere gets most of its heat from the surface on which it rests, and because land and water surfaces classified as continental and oceanic. A Continental climate is very changeable, with great extremes of heat and cold. It prevails over the interior of the continents. An Oceanic climate is much milder and more equable. It prevails over the ocean, and often near its borders. Temperature of Highlands. -The higher we ascend on highlands, the thinner and drier the air becomes. The heat received by the ground, passing out through the thin atmosphere, imparts but little heat to it. Hence the higher the altitude the colder the air becomes. Map Exercise. - In what heat belts does North America lie? South America? Eurasia? Africa? Australia? Compare northern North America with southern South America. Compare the north Pacific Ocean with America north of the equator. — Prevailing Westerlies. Some of the air that descends at the tropical calms moves along the surface toward the poles, and forms the Prevailing Westerlies. These winds blow from the southwest in the North Temperate Zone, and from the northwest in the South Temperate. Polar Winds. In the temperate regions, cold Polar Winds sometimes occur blowing from the poles. They blow from the northeast in the northern hemisphere, and from the southeast in the southern. All the wind belts shift north and south during the year with the movements of the heat belts. Monsoons. Because land heats and cools more rap Text Questions. What is the atmosphere? What is said about its idly than water, the continents in the Temperate zones are warmer in summer, but colder in winter, than the composition? its pressure? What is the difference between weather changes. terly colder than 32°, the moisture is deposited in little ice crystals, and Frost is produced. Clouds, Mist, and Fog are countless little drops of water or ice crystals in the air. Rain and Snow. - Continued condensation may cause the formation in the atmosphere of larger drops of water or ice crystals which may fall to the earth as Rain or Snow. When the atmospheric moisture falls to the earth as little pellets of ice, it is called Sleet or Hail. Rainfall. — All atmospheric moisture which reaches the earth's surface in liquid or solid form, that is, as dew, rain, snow, hail, etc., is usually spoken of as Rainfall. Little or no rain Moderately heavy rains Heavy rains quatorial Equatorial Calm's Southeas Frequent Calms Westerly m s ولولا WINDS AND RAINFALL. 4.4.... ad Calms Wi ̧n ̈d ́s, Zones of Rainfall. The trade winds blowing over the ocean bring warm, moist air to the equatorial calm belt. Here, as we have seen, this air rises, and its vapor is chilled and condensed as it ascends. Hence rain is likely to occur every Small and very severe local cyclones in the United | day, and the equatorial calm belt is a zone of daily rains. -States are called Tornadoes. Evaporation and Condensation. The atmosphere always contains water in the form of invisible vapor, but most of it is within a mile of the earth's surface. Vapor is nearly always rising into the atmosphere from every exposed surface of water. When the water is warm vapor usually forms rapidly, but when it is cold vapor forms slowly. The process of forming vapor is called Evaporation. When vapor is chilled it changes back to water or ice. This process, called Condensation, produces dew and frost, clouds, fogs, mists, rain, hail, sleet, and snow. Dew is formed when the vapor in the air is chilled by contact with the earth, and deposited in little drops of water, usually on cold vegetation. If the ground is This narrow rain zone follows the vertical rays of the sun northward or southward over the Torrid Zone every six months, giving abundant rain while it passes to places that receive no rain at all at other times. Thus, in many countries in the Torrid Zone which are always hot, the year is divided into wet seasons and dry seasons, instead of into winter and summer as with us. The trade winds on either side of the equatorial calm belt grow warmer as they advance, and are full of moisture, but they yield little or no rain to the ocean and level lands, because there is nothing to chill and condense the vapor. When, however, these winds encounter a mountain chain, and are forced up its sides, their vapor is chilled in the ascent. This causes abundant rains on the windward slopes of the Torrid Zone. But there is little or no rain on the opposite or leeward slopes, for the air has lost most of its vapor in ascending the mountains. The tropical calms are regions of no rainfall, since the air descends, and is therefore warmed. The region of prevailing westerlies may be rainy, since these winds. move from warmer to colder latitudes, and are therefore chilled. These winds bring ample rain to parts of the western coasts of the continents. The polar winds move toward warmer regions. They therefore produce little or no rain. The monsoons are likely to cause a rainy season on the land when the winds blow from the ocean, and a dry season when they blow from the land. The winds on the front or eastern side of a cyclone almost always bring cloudy, rainy, or snowy weather, for the winds always whirl around. the cyclone center in such a direction that on the eastern side they blow from warmer to colder latitudes, and hence their vapor is chilled. Glaciers. The air at high altitudes and near the poles is very cold. Hence on high mountains and in the polar regions most of the rainfall is in the form of snow. If the amount of snow falling during the winter be greater than can be melted during the summer, the snow accumulates until its weight converts the lower part into plain each. Why do the monsoons usually divide the year into a rainy ice, and causes it to move slowly down the valleys of the mountain sides as a river of ice called a Glacier. Some glaciers extend downward to the sea, where great masses of ice break off and float away as Icebergs. In polar regions, glaciers cover not only mountains, but most of the lowlands as well. Thus the island of Greenland is almost entirely cov ered with a mass of snow and ice thousands of feet thick, which is constantly moving slowly downward and outward to the sea. Sometimes large masses of snow, carrying rocks and bowlders with them, roll down mountain sides into the valley below as Avalanches. Work of Glaciers. As glaciers move down their slopes they pick up bowlders and rock waste, and carry them along. This rocky material is deposited at the end of the glacier, forming what is known as a Terminal Moraine, or as Glacial Drift. Moraines frequently dam up valleys and cause the formation of lakes. The bowlders that glaciers carry with them are often frozen into the ice at the bottom of the glacier. As the glaciers move along, these bowlders scratch and plane the surface of the land below. Soft rock layers are worn more than the hard ones, and thus depressions are formed. When the glaciers melt, waters may collect in depressions and form lakes. Glaciers, in moving from mountains down into the sea, cut and furrow the coasts, and make them irregular. Many of the deep steepsided valleys which occur along the coasts of lands in high latitudes have been scoured out by glaciers and afterwards submerged to form fiords. Text Questions. - What is meant by evaporation? by condensation? Explain how each is caused. Name the different forms that water takes when vapor is condensed, and explain each. Is it rainy or dry in the zone of equatorial calms? in the trade wind belts? in the tropi cal calms? in the belts of prevailing westerlies? in the polar winds? Exand a dry season? What effect do cyclones have upon weather? How are glaciers formed? What is an avalanche? What is a terminal moraine? Tell all you can about the work of glaciers. What are icebergs? How are they formed? DISTRIBUTION OF PLANTS AND ANIMALS In the Torrid Zone. Plants and animals are found in all parts of the world, but in the Torrid Zone the great heat and moisture cause plants to flourish in luxuriance, and, since plants furnish animals with much of their food, all forms of animal life are abundant. In the Temperate Zone the greater variety of climate gives greater variety to the animal and plant life, though there may be fewer animals and plants. In the Frigid Zone, where it is cold throughout most of the year, there are comparatively few life forms. Barriers. Most animals and plants can not spread over the whole world because they can not pass over the sea. The sea is thus a barrier. Mountains and deserts are also barriers to the spread of life forms. Adaptation. - Plants and animals living in any region. become fitted to the temperature, rainfall, food, and elevation of that particular region, and in time they do not thrive so well anywhere else. AN ICEBERG. Thus camels live best in the desert, llamas and chamois at high altitudes, the tapir and moose in damp forests, the reindeer on Arctic swamps or tundras, and the antelope on dry, grassy plains. Animals also adjust themselves to a locality in such a way as to be protected from their enemies or better fitted to secure their prey. Thus, the grouse of the Arctic region is snow-white in winter and mottled like partridges in summer. Its enemy, the Arctic owl, also changes color in the same way. This change of plumage fits them to their surroundings. |