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Weathering – Section 1

Weathering and erosion generally go hand-in-hand but the two terms have subtly different scientific meanings. Weathering is the name given to processes that change the surface material of the Earth as a result of exposure to the effects of water and the atmosphere. Erosion means the removal or transportation of material from one position on the surface of the Earth to a different position, again by the effects of water and the atmosphere. There are circumstances where weathering and erosion occur simultaneously. For example, storm waves along our coasts hurl rocks at the cliffs, causing abrasion. Pieces are broken off and carried away. Weathering (by breaking the solid rock) and erosion (removal of the broken material) occur together. We will begin by discussing the two types of weathering: physical weathering and chemical weathering.

PHYSICAL WEATHERING Physical weathering (also known as mechanical weathering) is the process by which rocks are broken down into smaller fragments without undergoing any change in chemical composition. Physical weathering is mainly caused by the freezing of water, the expansion of rock, and the activities of plants and animals. Frost Wedging. In areas with temperate and cold climates, as well as in high mountainous areas, rocks are weathered by the action of freezing water. During the daytime, when the temperature is above the freezing point of water (O°C), rainwater, melted snow, or ice trickles into cracks in the rocks. At night, when the temperature falls below the freezing point of water, the water trapped inside the rock changes into ice. When water freezes, it increases in volume by about 9 percent. This

expansion is responsible for the bursting of water pipes that are allowed to get too cold in winter. The expansion of water into ice pushes against the sides of the cracks with great force, wedging the rocks apart. This process, which is characterized by a cycle of daytime thawing and refreezing at night, is called frost wedging (also known as ice wedging). Frost wedging causes large rock masses, especially the rocks exposed on mountain tops, to be broken into smaller pieces. Frost wedging can have the same effect on pavement. During the winter, water trapped in cracks in the pavement freezes into ice. The ice often expands enough to crack the pavement and form potholes.

Frost wedging is most prevalent in cool, temperate climates where freezing and thawing occurs many times in the year. In the arctic, frost wedging actually occurs less frequently because the temperature tends to stay below freezing for long periods of time. Exfoliation. Weathering continually breaks down exposed bedrock into smaller fragments, which are then carried away by wind and water. Consequently, rock formed deep underground (under great pressure) becomes exposed at the Earth's surface. The release of overlying pressure causes the newly exposed bedrock to expand, forming cracks parallel to the rock's surface. Then, frost wedging causes large, curved slabs of rock to peel away from the main body of the rock.

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The peeling away of the outer layers from a rock is called exfoliation. Rounded mountaintops called exfoliation domes are formed in this way. Miners and quarry workers have been injured and killed by exfoliation. As a mine shaft or quarry pit is dug into the bedrock, the removal of rock causes a rapid decrease in pressure on the surrounding bed rock. The reduced pressure can result in sudden exfoliation, sending dangerous missiles of rock through the air. Animals and Plants. Insects, earthworms, rabbits, woodchucks, and many other animals burrow through the soil. Perhaps the most effective of all are humans. The tunnel the organisms make often expose parts of the bedrock to the weathering action of the air and water. Frost wedging or chemical action can then break down the bedrock. In addition, the act of burrowing through soil helps to break down rock particles into smaller and smaller pieces. Mining and quarrying, highway construction projects and excavations for building foundations and basements all lead to significant physical weathering of the Earth's crust. Root Wedging. You may have seen trees that appear to be growing out of solid rock. Trees occasional grow in soil that has collected in small cracks in the bedrock. In other cases trees grow in soil in the cracks of a boulder. In either case, as the tree continues to grow it exerts great pressure. The pressure causes the rock to split apart even wider. CHEMICAL WEATHERING Exposure to air, water, and organisms can change the minerals in a rock into new substances that have different chemical compositions. These new substances are generally softer or weaker than the original

materials, so they tend to cause the rock to crumble and fall apart. The breaking down of rocks through changes in their chemical composition is called chemical weathering. An example of chemical weathering is the change of feldspar in granite to clay. When acted upon by water, the feldspar changes into powdery clay minerals. Feldspar and clay have different chemical compositions. Water, oxygen and carbon dioxide are the main agents of chemical weathering. When water and carbon dioxide combine chemically, they produce a weak acid that can break down rocks. Rocks also are weathered chemically by the action of acids produced by plants and animals. These acids are formed by living plants and by the decomposition of plant and animal materials. Action of Acids. You may have noticed the exposed surfaces of bedrock sometimes appear to be covered by a grayish or bluish-green crust. A closer look reveals that this colored crust actually consists of tiny plants called lichens. Lichens "eat" rocks by releasing acids that slowly dissolve the minerals in the rocks. In addition, the action of bacteria and other microscopic organisms on dead plants and animals changes the composition of their remains. As these changes take place, acids are produced. The acids then dissolve the minerals in the rocks, and the rocks crumble apart. Water. When water comes into contact with certain minerals the water combines chemically with these minerals and changes them. This kind of chemical weathering is called hydrolysis. You learned that when water reacts with the feldspar in granite, the feldspar changes into clay. Clay is much softer than the original feldspar. Thus, the process of hydrolysis weakens granite, making it more susceptible to physical weathering.

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Oxygen. When an iron nail is exposed to the atmosphere rusting takes place. Iron, in the presence of moisture, combines chemically with oxygen in the atmosphere. The combining of oxygen with another substance is called oxidation. The oxidation of an iron nail produces a powdery reddish brown substance called rust. Rust is composed mainly of a chemical compound called iron oxide (Fe2O3). Oxidation also takes place in rocks that contain iron-bearing minerals. When these rocks are exposed to oxygen in the atmosphere, the minerals slowly change into softer, more crumbly substances. Like an iron nail that rusts when exposed to oxygen and moisture, the iron-bearing minerals in these rocks also rust when exposed to the atmosphere. The chemical weathering of rocks that contain iron often produces reddish or brownish soils. Many of the world's largest iron ore deposits were formed by chemical weathering. The great deposits of iron ore in Minnesota and upper Michigan were formed over a vast period of time by the chemical weathering of basalt, an igneous rock that contains iron. Carbon Dioxide. When carbon dioxide and water combine chemically, they form a weak acid called carbonic acid (H2CO3). A common name for carbonic acid is soda water. Soda water is made by dissolving carbon dioxide gas in water. When you place a drop of hydrochloric acid on a rock containing calcite, bubbles of carbon dioxide gas appear on the rock. At the same time that the hydrochloric acid is dissolving the calcite, carbon dioxide gas is being released. Like hydrochloric acid, carbonic acid dissolves calcite from rocks, but much more

slowly. The world's largest underground caverns were formed by this type of chemical weathering. The original limestone bedrock was gradually dissolved by water containing carbonic acid. The water then carried the dissolved limestone away, leaving the hollow spaces called caverns. RESISTANCE TO WEATHERING The ability of a rock to resist weathering depends mainly on the mineral composition of the rock and on the number of cracks in the rock. Rocks that consist mostly of quartz are more resistant to either physical or chemical weathering than most other kinds of rocks. The chemical weathering of quartz is very slow because quartz does not combine readily with other substances. Unlike rocks that contain abundant quartz, rocks that consist mainly of feldspar weather rapidly, especially in warm moist parts of the world. In these areas, chemical weathering rapidly changes feldspar into clay. Granite consists mainly of quartz and feldspar. Although granite is a relatively hard rock, the chemical weathering of feldspar eventually causes granite to crumble. Quartz weathers also, but at a much slower rate than feldspar. Eventually, physical weathering breaks down the quartz into small grains of sand. In general, sedimentary rocks weather more rapidly than other rock types because sedimentary rocks consist of many small grains that are cemented together. Poorly cemented sedimentary rocks, such as some shales and sandstones, contain small air spaces between the grains. Consequently, water easily penetrates the rocks. If the water freezes within these rocks, the rocks are broken apart by frost wedging. If water dissolves the cement holding the grains of a sedimentary rock together, the grains will separate. Cements that consist of

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iron compounds or calcite dissolve more rapidly than do cements composed of silica (quartz) compounds. In moist climates, therefore, sedimentary rocks cemented by calcite and iron compounds weather rapidly, whereas those cemented by silica are more resistant to weathering. Cracks in a rock also cause the rock to weather more rapidly. The more cracks in a rock, the faster that weathering will split the rock into fragments. Once a large rock has been split into smaller fragments, the smaller fragments can then be broken down more rapidly by the action of air and water. SOIL The soil consists of particles formed by physical and chemical weathering. Soil usually contains particles of sand, clay, various minerals, tiny living organisms, and humus. Humus is the decayed remains of plants and animals. In addition, some types of soil have large numbers of air spaces between their particles. Formation of Soil How often have you been told, "Take off those dirty shoes before you come into this house"? Ever since you were a young child, you've had many experiences with dirt. Dirt is actually soil. You can find soil in lots of places. An empty lot may have exposed soil in it. A garden or a flowerbed has soil.

But what is soil and where does it come from? As you learned, weathering gradually breaks rocks into smaller and smaller fragments. When plants and animals live in these fragments, organic matter, such as leaves, twigs, and dead worms and insects, is added. When organic matter is gradually added to the weathered rock, soil evolves. Soil is a mixture of weathered rock and

organic matter. Most soil is made up of about 50 percent, rock and mineral fragments and 50 percent air, water, and organic matter. Soil can take hundreds of years to form and can range in thickness from 60 meters in some areas to just a few centimeters in others. As rock weathers into smaller and smaller fragments, plants begin to grow in the weathered rock. Then worms, insects, bacteria, and fungi begin living among the plant roots. These organisms don't just live in the weathered rock they help it evolve into soil by adding organic matter. When the plants and animals that live in the soil eventually die, they break down in a process called decay. The dark-colored organic matter made of pieces of decaying plants and animals is called humus. As worms and insects burrow throughout the soil, they mix the humus with the fragments of rock. As you can see, weathered rock is constantly evolving into soil. SOIL TYPES Soils are divided into three main classes according to texture. These classes are sandy soils, clay soils, and loamy soils. You can determine the texture of each soil type by squeezing and rubbing a small amount of moist soil between your fingers. Sandy soils feel gritty, and their particles do not bind together firmly. Sandy soils are porous, which means that water passes through them rapidly. Consequently, sandy soils do not hold much water. Adding a large amount of humus to a sandy soil permits the soil to hold more water. Clay soils feel smooth and greasy, and their particles bind together firmly. Clay soils are usually moist, but they do not permit water to pass through easily.

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Loamy soils feel somewhat like velvet and their particles clump together. Loamy soils consist of a mixture of sand, clay, and silt. A loamy soil holds water well and permits some water to pass through. Soil Profiles You may have seen layers of soil if you've ever been near a steep slope such as a road cut where the soil and rock are exposed. You might have noticed that plants grow in the top layer of soil, which is a darker color than the other soil layers below it. These different layers of soil make up what is called a soil profile. Each layer in the soil profile is called a horizon. There are generally three horizons, and they are labeled A, B, and C.

The A horizon is the top layer of soil. It's also known as topsoil. If you could dig up a scoop of topsoil from the top of that steep slope and look at it very closely, what would you see? The soil would be dark in color and would contain sediments, decayed leaves, the roots of plants, and even insects and worms. The A horizon is the most fully evolved soil layer in a soil profile. This means that the A horizon has changed the most since it was just weathered rock. It generally has more humus and smaller sediments in it than the other, less evolved layers in a soil profile. The next layer, below the A horizon, is the B horizon (subsoil). This layer is less evolved and lighter in color than the A horizon because it has little or no humus. Some plant roots reach into this layer. This horizon

usually contains minerals that were washed down from the A horizon. The process in which minerals are dissolved in water and carried down in a soil profile is called leaching. The process of leaching is similar to how coffee is made in an automatic drip coffee maker. Like water seeping into the A horizon, hot water drips into coffee grounds in a filter. There, like water dissolving minerals in the B horizon, the water absorbs flavor and color from the coffee grounds. Then, like water carrying the dissolved minerals to the C horizon, water in a coffee maker flows through the filter and into the pot. Below the B horizon you will find the C horizon. This is the bottom layer in a soil profile. Some of the materials in this layer were leached from the B horizon. The C horizon also contains partly weathered rock, but no humus. This rock is just beginning the long, slow process of evolving into soil. What do you suppose you will find if you dig all the way to the bottom of the C horizon? As you might have guessed, there will be solid rock. Although the bottom two layers of the soil are relatively thick, the topsoil is always a thin layer. Even fertile farmlands have a relatively thin layer of topsoil. Because topsoil is a loose material, it is easily carried away by wind and water. In the 1930s, winds blew away billions of tons of topsoil from the farmlands of the Plains states. The dust was carried in great clouds more than 3000 kilometers (1860 miles) east to the Atlantic Ocean. The United States still loses large amounts of topsoil each year because of the destructive action of wind and water.

Erosion – Section 2 Have you ever ridden your bike by a river just after a heavy rain? The water was so muddy

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that it looked like chocolate milk. Where do you suppose all of the mud came from? As you might guess, some of it came from dirt along the river's bank, but the rest of it was carried to the river from much more distant sources. Mud is a product of erosion. Erosion is the process that moves or transports weathered sediments from one location to another. As you investigate the processes of erosion, you will see that eroded sediments are eventually deposited. But what moves the sediments in the first place? The four major agents of erosion are gravity, wind, running water and glaciers. As you investigate these four agents of erosion, you will notice that they have several things in common. For one thing, they all carry sediments only when they have enough energy of motion. For example, air doesn't have the ability to erode sediments as long as it's standing still. But once air begins moving and develops into wind, it can carry dust, soil, and even large rocks along with it. Another thing that the agents of erosion have in common is that they all drop their load of sediments when their energy of motion decreases. This dropping of sediments is called deposition. Now let's take a look at how gravity, wind, running-water and glaciers erode and deposit sediments. Gravity. Gravity causes loose materials to move down slope. When gravity alone causes materials to move down slope, it's called mass movement. Some mass movements are very slow; you hardly notice that they are happening. Other types, however, happen very quickly. Let's examine some different types of mass movements. Slump. A slump is a type of mass movement that takes place on steep slopes. It occurs when loose materials or rock layers slip

downward as one large mass. The material does not travel very fast or very far, but when it happens, a curved scar is left where the slumped material originally rested. Slumps occur because the material underlying the slumped material weakened. It could no longer support the overlying material, so it slipped down slope.

Creep. The next time you travel by car or bus, look along the roadway for slopes where trees, utility poles, and fence posts lean downhill. These indicate that another type of mass movement is happening. It's called creep. Creep gets its name from the way sediments slowly creep down a hill. It is especially common in areas where freezing and thawing occur. Small sediments are pushed up by the expanding water in the soil as the ground freezes. When the soil thaws the sediments fall down slope, often less than a millimeter at a time. Several years of soil creeping down slope can cause objects such as utility poles and fence posts to lean.

Rockslides. You may have seen signs along the road warning you, "Beware of Falling Rocks”. This movement is called a rockslide. A rockslide happens when large blocks of rock break loose from steep slopes and tumble quickly to the bottom. As they fall, these rocks crash into other rocks and they too break loose. Rockslides commonly occur in mountainous areas. They happen most often after heavy rains or during earthquakes. But

Slump

Creep

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they can happen on any rocky slope at any time without warning. Piles of broken rock at the bottom of a cliff tell you that rockslides have occurred there in the past and are likely to occur there again. Mudflows. Can you imagine being in a car traveling along a mountain road during a storm, when suddenly a wall of chocolate pudding slides clown a slope and covers your car? This is similar to what might happen if you were caught in a mudflow. Instead of chocolate pudding, a thick mixture of sediments and water would flow down the slope in a type of mass movement called a mudflow. Mudflows usually occur in relatively dry areas where weathering forms thick layers of dry sediments. When heavy rains fall in these areas, the water mixes with the sediments and forms a thick, pasty substance. Gravity causes this mass to slide downhill. A mudflow has enough energy to move almost anything in its path, including houses, cars, and large rocks. When a mudflow finally reaches the bottom of a slope, it loses its energy of motion and deposits all the sediments and debris it has been carrying. These deposits are usually a cone-shaped mass. Now that you've thought about mudflows, rock slides, creep and slump, think about how all these mass movements are similar. They're all more likely to happen where there are steep slopes. They all depend on gravity to make them happen. And, regardless of the type of mass movement, it will occur more often after a heavy rain because the water adds mass and makes the sediments slippery. Wind In dry environments, such as deserts, very few plants cover the ground. For example, in parts of the southwestern United States, deposits of loose sand, silt, and clay lie exposed at the

surface. Wherever these fine materials lie uncovered at the surface, the wind is an important agent of erosion.

The wind changes the landscape by moving sand, silt, and clay from one place to another. Lightweight silt and clay particles can be lifted and carried by light breezes. This type of erosion is deflation. However, only strong winds can carry and move the heavier sand grains. Strong winds lift silt and clay particles high into the atmosphere and carry them long distances. Heavier sand grains, however, are rarely lifted more than one meter off the ground and travel relatively short distances. On windy days at the beach, you may have notice while standing that most sand grains strike you below knee level. Even during violent desert windstorms, most of the flying sand grains do not rise more than one or two meters above the ground. Flying sand grains erode pebbles, boulders, and even large rock outcrops by wind abrasion. During wind abrasion, flying sand grains sandblast, or chip away, any solid surface that they are blown against. In some windy areas of the United States, the bases of wooden telephone poles have been completely sandblasted away by wind abrasion. On a longer time scale, isolated rock outcrops have been carved into strange, mushroom like shapes by windblown sand. Loss of Topsoil. A long drought in the 1930s led to wind erosion of the fertile topsoil of the Plains states. Crops withered and died, leaving bare fields unprotected from the wind. Moist topsoil turned into powdery dust and farmers were helpless as the wind stripped the precious topsoil from their fields. On several occasions, strong winds carried thick clouds of dust-like topsoil from the Great Plains to the Atlantic Ocean—over 3000 km away. Because of the disastrous loss of topsoil during such dust storms, the region from

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Texas to South Dakota was called the "Dust Bowl." DEPOSITS OF WINDBLOWN MATERIAL Sand, silt, and clay particles are deposited when the wind slows down or is blocked by an object. Two deposits often formed by wind blown sediments are sand dunes and loess. Sand Dunes. Boulders, rock outcrops, and isolated shrubs are often found in regions that have a great deal of loose sand. Such objects obstruct the wind, allowing windblown sand to be deposited. For example, a boulder blocks the wind, slowing the wind velocity in the immediate area. Consequently, sand grains suddenly fall to the ground and pile up around the boulder. In time, the sand grains form a sand hill, or sand dune. The side of a sand dune facing into the wind is called the windward side. The windward side of a sand dune has a gentler slope than the side facing away from the wind, or the leeward side. For example, winds blowing from the north will form dunes with gentle north slopes and steep south slopes. In places where the wind blows steadily from one direction, crescent-shaped sand dunes form. Sand dunes range in height from one meter to more than 100 m. In the Sahara Desert there are sand dunes more than 200 m tall. Dunes also are found in the Great Sand Dune National Monument in Colorado.

The wind not only forms sand dunes but also moves dunes from place to place. As the wind blows against the windward side of a sand

dune, some sand is blown over the top, falling on the leeward side. Gradually, the entire dune is moved in the leeward direction. Entire farms, towns, and forests have been buried by advancing sand dunes. However, in regions where grasses and shrubs grow on the dunes, the movement of sand dunes is slowed or even stopped.

Most dunes are made of quartz sand. A notable exception is the White Sands National Monument in New Mexico, which has beautiful white dunes made of gypsum sand. Another exception would be islands made of coral, such as Bermuda, which have dunes made of calcite sand. The calcite sand is produced by the weathering of coral reefs that make up the islands. Loess. Unlayered deposits of windblown silt are called loess (pronounced less). Loess particles are angular and may be derived from many types of rocks. Loess weathers to form very fertile topsoil. The topsoil of several Mid-western states, including Kansas, Iowa, Missouri, and Illinois, consists largely of loess deposits. On some Illinois farmlands, fertile loess deposits are more than 90 m thick. Loess often consists of glacial rock flour. When glaciers melt and the melt-water drains away, rock fragments of various sizes are left exposed on the ground. The wind picks up the lightweight particles of rock flour, leaving behind the heavier materials. The windblown rock flour eventually settles to the ground, forming deposits of loess. Loess is an unusual kind of sediment. Most deposits of loose sediment collapse to form slopes when they are cut into by streams or by

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road builders. Loess deposits, however, form steep cliffs when they are cut into, because the angular loess particles stick together. Loess cliffs are often found along the banks of the upper Mississippi and Missouri rivers. RUNNING WATER The most important agent of erosion is running water. Brooks, creeks, streams, and rivers are constantly at work removing weathered fragments of bedrock and carrying them away from their original location. Almost all of the water that erodes the land originates in the ocean. Water evaporates from the ocean and becomes water vapor. The water vapor cools and condenses into huge masses of tiny water droplets called clouds. In time, precipitation falls from clouds in the form of rain or snow. Small amounts of the precipitation that falls on Earth's surface soaks into the ground. Most of the water that falls as rain or snow quickly evaporates back into the atmosphere or returns to the ocean by way of streams and rivers. Water that seeps into the ground moves slowly downward and becomes groundwater. When groundwater meets an impermeable layer of rock, the groundwater flows sideways along the impermeable layer until it meets openings in the surface. At these surface openings, lakes, ponds, and springs appear. Lakes, ponds, and springs provide streams with a steady supply of water. RIVER SYSTEMS When rain falls on sloping land, the runoff water flowing downhill carries away loose rock fragments and soil. When water erodes without being in a stream channel, sheet erosion takes place. Soon, small channels called rills, might be created by this flowing water. If water continues to flow along the same path a gulley may form. Gulleys are deeper channels carved into Earth's surface by

running water. Runoff water flowing downhill from repeated rainfalls deepens and widens a gully. Eventually, the bed of the gully may reach the water table. If there is sufficient groundwater throughout the year, a permanent stream forms in the gully. Otherwise, the gully contains flowing water only when it rains a lot or when snow melts. For example, many gullies in the deserts of the southwestern United States are dry for much of the year. They fill with water after heavy rains and then dry up because water does not flow continuously in such a gully. This stream is called an intermittent stream. Small streams often join to form larger streams. In turn, these streams combine and form a still larger stream called a mainstream. All of the streams that supply a mainstream are called tributaries of that mainstream and all of its tributaries form a river system. The Missouri River and Ohio River are examples of river systems; both rivers are themselves tributaries of the Mississippi River. The Mississippi River and its thousands of tributaries form a huge river system that extends from the Rocky Mountains to the Appalachian Mountains and from Canada to the Gulf of Mexico. The area of land drained by a river system is called its watershed. River systems usually empty into a large body of water such as the ocean. The place where a river empties into a larger body of water is called the mouth of the river. EROSION BY STREAMS AND RIVERS You have seen that running water carries away loose rock materials. Streams and rivers can also wear down solid bedrock, gradually eroding high mountains into low hills and flat plains. Two ways that streams and rivers erode bedrock are by abrasion and by solution of minerals in the bedrock.

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Abrasion. A stream erodes bedrock by the grinding action of the sand, pebbles, and other rock fragments it carries. Like sandpaper against wood, these materials scrape against the streambed and banks. Particle by particle the bedrock is worn away. The grinding action of rock fragments scraping against the streambed and against each other is called abrasion. When large rock fragments first enter a stream, the fragments are usually angular with sharp edges. These sharp edges act as cutting tools. As the sharp-edged, angular rocks bounce and scrape along the riverbed, they chip off pieces of the bedrock. In time, the angular rocks lose their sharp edges and become rounded. The roundness of a stone is an indication of how long it has been carried along by a stream. Streams may carry a large volume of fine-grained silt and clay. The silt and clay are produced when coarse-grained rock particles collide with each other or with the streambed, or when loose materials are eroded from the banks of a stream. The motion of the stream prevents the silt and clay particles from settling to the streambed. Rivers that carry abundant silt and clay have a muddy appearance. The Mississippi River and the Missouri River are two examples. In fact, the Missouri River has been nicknamed the "Big Muddy." It has been estimated that the rivers of the United States carry about 700 million tons of sediments into the oceans every year. Solution. Besides abrasion, streams also erode rocks by the dissolving, or solution, of their minerals. Limestone and marble are

easily dissolved by streams. Running water also causes sedimentary rocks cemented together by calcite or iron compounds to crumble, because these cementing agents are easily dissolved. The dissolved mineral substances in water are invisible. However, they can be detected by chemical analysis. Geologists estimate that the rivers of the United States carry about 250 million tons of dissolved minerals into the oceans each year. HOW A STREAM ERODES ITS BED How quickly a stream erodes its bed depends on several factors. These factors include the velocity of the stream, the volume of water carried by the stream, the size and shape of sediments in the stream, and the type of bedrock beneath the stream. The lowest level to which a river can erode its bed is called base level. Sea level is the ultimate base level, but the floor of a lake or basin into which a river flows may become a local and temporary base level. Cliffs or escarpments and differences in the resistance of rocks create irregularities in the bed of a river and can thus cause rapids and waterfalls. A river tends to eliminate irregularities and to form a smooth gradient from its source to its base level. As it approaches base level, downward cutting is replaced by sideways cutting and the river widens its bed and valley and develops a winding course that forms exaggerated loops and bends called meanders. Velocity. Rapidly flowing streams erode their beds faster than slow-moving streams. A rapidly flowing stream has great cutting power because of its speed and because it can carry large, coarse-grained sediments such as gravel, pebbles, and even small boulders. These coarse-grained materials scrape with great force against the streambed. In contrast, a slow-moving stream has little cutting power because it can carry only fine-grained

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sediments such as silt and clay. Such fine-grained sediments have almost no effect on the streambed. A stream moving about 1 kilometer (km) per hour can carry silt, clay, and small sand grains. A stream moving at about 8 km per hour can move small boulders. Volume. A deep, wide stream can transport a greater volume of water than a narrow, shallow stream can. Suppose two streams move with the same velocity but carry different volumes of water. The two streams also flow over the same type of bedrock. In this case, the stream with the greater volume of water will erode its bed faster. This stream erodes its bed faster because it carries more sediment. Size and Shape of Sediments. Most stream erosion is produced by the abrasion of coarse grained, angular sediments. Large rock fragments with sharp edges make much better cutting tools than do small, rounded particles. Consequently, large coarse-grained sediments such as gravel and pebbles enable a stream to erode its bed most effectively. Type of Bedrock. Streams readily erode bedrock that is soft or soluble in water. For example, most streams will easily erode bedrock made of limestone and shale, which are relatively soft rocks. Streams also readily erode sedimentary rocks cemented together by calcite or iron compounds, which are soluble in water. In contrast, bedrock made of granite, slate, or quartzite is much harder and more resistant to chemical weathering than are limestone and shale. Consequently,

streams erode bedrock made of granite, slate, and quartzite very slowly. RUNNING WATER AND DEPOSITION You already know that the ability of a stream to carry sediments depends partly on its velocity. A stream's velocity decreases when

the stream (a) enters a larger body of water, (b) overflows its banks, (c) changes direction, or (d) flows from a steep slope to a gentle slope. As the velocity of a stream decreases, its ability to carry rock particles also decreases. Thus, when a stream slows down, the coarse-grained sediments settle out first, followed by the fine-grained sediments. An accumulation of sediments left by a stream or river is called a deposit. As a deposit of sediment grows, it may produce landscape features such as deltas, alluvial fans, levees and sandbars. Deltas. When a river enters a large body of water, (such as a lake or bay), the river immediately slows down. This sudden decrease in speed causes sediments to be deposited on the bottom of the larger body of water. In time, these deposits extend far into a lake or bay, producing a landform called a delta. The name delta comes from the fact

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that many of these deposits have a triangular shape that resembles the Greek letter delta. For example, the Nile River in Egypt has built a triangular delta where it empties into the Mediterranean Sea. Not all deltas have this shape. Another type of delta looks like long toes extending into a larger body of water. This type is called a bird-foot delta. In the United States, the Mississippi River has built a bird-foot delta out into the Gulf of Mexico.

Alluvial Fans. A stream that flows from steep mountains out onto a broad, flat plain abruptly slows down and deposits sediments on the plain. The resulting fan-shaped deposit is called an alluvial fan. Alluvial fans are most common in dry regions, such as the southwestern United States, where they are often seen at the base of mountain passes. Levees. When a river overflows its banks the water that leaves the channel slows down when it enters the floodplain. As the water loses speed, it deposits sediments on the banks of the river. Each time the river overflows its banks more sediment is deposited on top of the previous deposit. Layer by layer, the river builds up long, low walls of sediments along its banks. These built-up riverbanks are called levees. Towns located near a flood-prone river often build artificial levees along the riverbanks. These artificial levees usually prevent the river from overflowing and flooding the nearby towns. Many levees, both natural and artificial, are found along the banks of the Mississippi River.

Sandbars. The velocity of the river changes when it flows around a curve or bend. The water in the river slows down on the inside of the curve and speeds up on the outside of the curve. The loss of speed on the inside of the curve causes sediments to settle out of the water and to be deposited along the inside bank of the river. This type of deposit is called a sandbar. A sandbar may also form when a boulder or some other obstruction, such as a fallen tree, partly blocks a river channel. Water Diversion. The changing of the natural flow of water by people is called water diversion. Dams and pipelines are often successfully used to take water from one location to another. This is usually done if the demand for water is greater then the local

supply. However, this leaves less water for the people at the original location. Water diversion can significantly affect the velocity and the volume of a stream.

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LIFE CYCLE OF A RIVER As a river flows and erodes the land, the river undergoes changes, passing through different stages of development. These stages make up the river's life cycle. There are three main stages in the life cycle of a river: youth, maturity, and old age. Each stage has its own characteristic features. Youth. During youth, a river flows rapidly down mountains or hills. A young river tumbles noisily over falls and races through rapids, enclosed within a narrow, rock-walled valley. Because a young river moves swiftly, it carries large quantities of coarse-grained sediments. Consequently, the river cuts rapidly downward into the underlying bedrock, eroding a deep, narrow, V-shaped channel. The Colorado River, which has produced the Grand Canyon, is an example of a young river. A waterfall is perhaps the most spectacular feature of a young river. A waterfall occurs wherever the elevation of a riverbed drops suddenly. For example, as the Niagara River flows from Lake Erie to Lake Ontario, the elevation of the riverbed drops about 50 meters at one spot. The sudden drop of the river's bed in this location produced the famous Niagara Falls. A waterfall also may form where a river travels over different types of bedrock. For example, a river flowing over a layer of basalt followed by shale erodes the softer shale faster than the harder basalt. In time, the uneven erosion produces a basalt cliff in the river channel. A waterfall is formed as the

river flows over this cliff. Yellowstone Falls in Yellowstone National Park is an example of a waterfall produced in this way. Maturity. As a young river continues to erode and smooth out its channel, the river loses its waterfalls and rapids. Erosion also decreases the slope of its channel. As a consequence, the velocity of the river decreases as well. The river is then said to be in its mature stage. The Missouri River is an example of a mature river. A mature river flows more slowly and has less cutting power than a young river. Consequently, when a mature river encounters an obstruction, the river flows around the obstruction. In this way, a mature river begins to form a curved, or S-shaped, channel. In time, the curves become larger, and the river channel becomes more winding. These looping, S-shaped curves in the river's course are called meanders. During maturity, a river's once-steep valley walls are worn down by the agents of physical and chemical weathering. Instead of being V-shaped, the valley walls of a mature river are somewhat rounded and slope gently toward the river. As a mature river flows back and forth across its valley, the river gradually widens the valley floor. When flooding causes the river to overflow its channel, coarse-grained sediments, such as gravel and pebbles, are deposited along the banks to form levees. Finer grained sediments, such as silt and clay, are carried beyond the riverbanks and deposited on the valley floor. These sediments level out the valley floor to form a relatively flat area called a floodplain. Swamps often form on a floodplain because the deposits of fine silt and clay do not readily allow the

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floodwaters to soak into the ground. Old Age. When the slope of a river channel becomes almost flat, the water flows very slowly. Then, a river is said to be in old age. The lower part of the Mississippi River is in this stage of development. Instead of causing erosion, an old river mainly affects the land by depositing sediments. Consequently, the natural levees found along an old river are higher than the levees along a mature river. An old river also has a wider floodplain than a mature river because the old river flows back and forth over a much wider valley. Sometimes, an old river cuts through the narrow part of one of its meanders. This causes a sharp decrease in the velocity of water flowing through the meander. As a result, sediments accumulate where the ends of the meander meet the new river channel. In time, sediments completely cut off the meander from the river. The cutoff meander is called an oxbow lake. When sediments fill in an oxbow lake, a swampy area, or bayou, is formed.

You should be aware that a river may not be in the same stage of its life cycle along its entire course. In fact, many rivers show several different stages of development along their length, depending on the steepness of the land. For example, a single river may exhibit signs of youth in its upper reaches, near its source; characteristics of a mature river in its middle stretch; and features of old age near its mouth, where it empties into the ocean. Sometimes an old or mature river becomes youthful again. For example, forces inside the Earth may raise the crust, causing the slope of an old, meandering river channel to become

steeper. The steeper slope increases the water velocity, enabling the river to cut through the underlying bedrock again. With its renewed cutting power, the river begins to deepen its old meanders. A deepened meander looks like a curved trench with steep walls. These deeply eroded curves of a river are called entrenched meanders. You can see these along the Susquehanna River in New York and Pennsylvania, and along the San Juan River in Utah and New Mexico.

Glaciers Section 3 In some areas of the world, temperatures are low enough that it snows year-round. If the snow doesn't melt, it begins piling up. When it accumulates, the weight of the snow is great enough to compress the bottom layers into ice. When the snow piles up to 50 to 60 meters high, the ice on bottom partially melts and becomes putty-like. The whole mass begins to slide on this putty-like layer and it moves downhill. This moving mass of ice and snow is a glacier.

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Movement of Glaciers The movement of glaciers is caused in part by the great pressure exerted by the surface ice on the ice located deep within the glacier. This pressure causes the deep ice to become plastic-like. This process is called Plastic Deformation. In other words, the deep ice behaves like dough or taffy. Materials that look solid but that can flow slowly and change shape. You may have noticed how the asphalt along the edge of a busy highway seems to have been pushed up against the curb. That is because the weight of passing cars and trucks causes the solid asphalt to become plastic-like and flow slowly toward the curb. In a similar way, deep ice in a glacier becomes plastic-like and flows outward because of the enormous weight of the overlying snow and ice. Glaciers can also move due to a process called Basal Slip. Basal Slip is a process where the underlying ice crystals rearrange themselves so they are parallel to each other. When the ice crystals are parallel to each other, the massive weight of the glacier causes the ice crystals to slide across one another causing the glacier to advance. Glacial movement also involves the melting of ice at the base of a glacier. Deep ice in a glacier melts under great pressure, forming a thin layer of water beneath the glacier. The melt water acts as a lubricant, causing the entire glacier to slip a small distance downhill. This movement decreases the pressure at the base of the glacier, allowing the melt-water to refreeze. In time, the weight of the ice causes the deep ice to melt again, and the process is repeated. A row of stakes driven into the surface of a glacier will reveal the center of the glacier

moves faster than its sides. The sides of a glacier move more slowly than the center because of friction between the glacier and the valley walls. Some glaciers move as far as 20 meters in a day. Other glaciers move very slowly, advancing only a few centimeters in a day. Because of additional weight, glaciers move faster after winters with heavy snowfall than after winters with light snowfall. Also, the sleeper the slope, the faster a glacier moves.

Glaciers, along with gravity, running water, and wind, are agents of erosion. There are two types of glaciers: continental glaciers and valley glaciers. Continental glaciers are huge masses of ice and snow found near the Earth’s polar regions. Today, they are found only in Greenland and Antarctica. But during past ice ages, continental glaciers covered large portions of the world including where the city of Baraboo is today.

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An ice age is a period of time when ice and snow cover much of Earth's surface. The last ice age in Wisconsin occurred from 35,000 to 10,000 years ago. During this time glaciers dramatically changed the landscape of Wisconsin. Glaciers were the most important agent of erosion during this time period. Wisconsin was divided into two regions: glaciated and non-glaciated (driftless) areas. The glaciated area was the northern and eastern 2/3 of the state. The southwestern 1/3 of the state was the driftless area.

The glaciated area was covered by four main glacial lobes (from east to west) the Lake Michigan lobe, Green Bay lobe, Chippewa lobe and Superior lobe. The driftless area was not covered by glacial ice during the previous ice ages. The driftless area was not covered by ice for three reasons. The highlands of hard igneous rocks in north- central Wisconsin slowed the Chippewa lobe. It was going uphill and trying to carve

through hard igneous rock making advancement very difficult. The Green Bay and Lake Michigan lobes to the east and the Superior lobe to the west were moving faster and pinching in on the sides of the Chippewa lobe. This pinching created massive friction and further slowed the Chippewa lobe. Finally, the glacier began to retreat before it could reach the driftless area. Glaciers are considered to be retreating when more ice and snow are melting than are being added to the glacier. Results of Wisconsin glaciation include changes to soil quality, landscape and land features. Northern Wisconsin soil is poor and shallow, central and southeastern Wisconsin soil is very good and thick because of the rich soil deposited there by the glacier. South-western soil is also very good. Northern Wisconsin is generally flat, southeastern Wisconsin has gentle rolling hills while southwestern Wisconsin has many steep hills. The glaciers also left many land features such as drumlins, moraines, eskers, kettle lakes and outwash. These will be discussed later. The last ice age even changed the route of the Wisconsin River.

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When the glaciers retreated from Wisconsin they left huge quantities of water forming Glacial Lake Wisconsin. This glacial lake is believed to have drained in a matter of weeks creating most of the stunning sandstone sculpture along the Wisconsin River in the Wisconsin Dells region. Causes of the Ice Ages Scientists have learned that the Earth’s orbit changes every 100,000 years from relatively circular to a more elliptical orbital pattern. The change in orbit is due to the gravitational pull of the sun and the moon. The Earth’s average temperatures are warmer when the Earth’s orbit is more circular. When the orbit is more elliptical the Earth’s average temperatures are cooler thus creating favorable conditions for an ice age. The elliptical orbits create cooler summers allowing more snow and ice to remain for longer periods of time. Scientists have also discovered the Earth’s inclination or tilt of axis changes every 40,000 years. Today the Earth’s axis is tilted at approximately 23 degrees. This axis tilt allows the northern hemisphere more direct sunlight creating warmer average

temperatures. During times of ice ages the axis is tilted at approximately 13 degrees. The low axis tilt means less direct sunlight for the northern hemisphere resulting in cooler average temperatures allowing the snow and ice to accumulate year-round. During the last ice age the average temperature on Earth was only about 40C lower than it is today. The final condition needed for an ice age is called the Albedo Effect. The Albedo Effect is the measure of reflectivity of light and heat from the surface of the Earth. The oceans remained dark in color compared to the landmasses that were covered in snow. The dark colored oceans continued to collect solar heat creating warm air over them because they reflected less of the sunlight back into space. The snow-covered landmasses reflected much of the solar energy back into space creating very cold air over the landmasses. When the two air masses of differing temperatures met, the moisture-laden air from over the oceans condensed forming large amounts of snow in the frigid air over the landmasses. This continued year after year to provide the snow and ice for the glaciers. This snow and ice created the continental glaciers. Valley glaciers are fairly common even in today's warmer climate. They are located in mountainous areas where the average temperature is low enough that snow doesn't melt over the summer season. The glaciers of Glacier National Park in Montana are valley glaciers. How is it possible that something as fragile as snow or ice can erode something as hard as rock? It may not seem likely, but much of Earth's landscape has been shaped by glacial ice. Let's explore how this is possible. Glacial Erosion As they move over land, glaciers act like bulldozers, pushing any loose materials out of their path. These eroded sediments are added to

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the mass of the glacier or piled up along its sides. But glaciers do more than just move loose sediments. They also weather and erode rock and soil that isn’t loose. Glacial snow and ice melt creating runoff that flows into cracks in rocks. Later, the water refreezes in these cracks, expands, and breaks the rock into pieces. The rock fragments are moved along with the glacial ice. This process, called plucking results in boulders, gravel, and sand being added to the bottom and sides of a glacier. These materials at the base of a glacier act like the blade of a plow. They scrape the soil and bedrock the glacier moves over. They cause the glacier to erode even more than the ice and snow alone could. Striations are left behind when bedrock is gouged. These striations are usually long, parallel scars in rocks. Very large striations are called glacial grooves. If you live in the mountains and want to know if there were ever valley glaciers in your area, how could you find out? You might begin by looking for striations. But what other evidence of glacial erosion could you find? Glacial plucking often occurs near the top of a mountain. Where a glacier is in contact with a wall of rock a bowl-shaped basin, called a cirque, is created in the side of the mountain. If two or more glaciers erode a mountain summit from several directions, a ridge or sharpened peak forms. Valley glaciers flow down mountain slopes and valleys, eroding as they go. Valleys that have been eroded by glaciers are a different shape from those that have been eroded by streams. Stream-eroded valleys are normally V-shaped; glacially eroded valleys are usually U-shaped. This is because glaciers pluck and scrape soil and rock along their sides as well as on their bottoms. Streams tend to erode downward into underlying rock more than glaciers do.

Deposition by Glaciers

When glaciers begin to melt, they no longer have enough energy of motion to continue carrying sediments. Therefore, these materials are deposited. Glacial deposits are classified into two major types. One is a jumble of different-sized sediments that is deposited from the glacial ice and snow. This mixture of boulders, sand, clay, and silt is called till. When a glacier stops moving, till begins dropping from its base. These sediments cover huge areas of land. Continental glaciers moving across the northern United States dropped so much till that it completely filled valleys and covered hills during the last ice age. Today, these areas appear quite flat. Till is also deposited in front of a glacier when it stops moving forward. Because it's made of the rocks and soil that the glacier has been pushing along, it looks like a big ridge of material left behind by a bulldozer. Similar ridges are deposited along the sides of glaciers. These mounds of material are called moraines. Moraines can easily be found in the Baraboo area today. If you have ever driven on highway 159 between highway 12 and Devil’s Lake you have driven over a moraine. You also have driven over it if you have driven on highway 136 past K-mart. The other major type of glacial deposit is outwash. Outwash is deposited from the glacier's melted ice. This melt-water carries sediments and deposits them much like a river does. For example, one type of outwash deposit is an alluvial fan made of sediments eroded by the glacier. It forms when a stream of melt-water drops sand and gravel in front of the glacier. Another type of outwash deposit looks like a long winding ridge. This deposit forms beneath a melting glacier. Melt-water forms rivers within the ice. These rivers carry sand and gravel and deposit them within their channels. When the glacier melts, a winding ridge of sand and gravel, called an esker, is left behind.