Which formations appear to be the most resistant to weathering

This is because the Moon has no atmosphere and, as a result, has no weathering. Weathering is one of the forces on Earth that destroy rocks and landforms. Without weathering, geologic features would build up but would be less likely to break down. Weathering is the process that changes solid rock into sediments. Sediments were described in the Rocks chapter.

With weathering, rock is disintegrated. It breaks into pieces. Once these sediments are separated from the rocks, erosion is the process that moves the sediments. The four forces of erosion are water, wind, glaciers, and gravity. Figure 1. A once smooth road surface has cracks and fractures, plus a large pothole.

While plate tectonics forces work to build huge mountains and other landscapes, the forces of weathering gradually wear those rocks and landscapes away.

Together with erosion, tall mountains turn into hills and even plains. No human being can watch for millions of years as mountains are built, nor can anyone watch as those same mountains gradually are worn away. But imagine a new sidewalk or road. The new road is smooth and even. Over hundreds of years, it will completely disappear, but what happens over one year?

What changes would you see figure 1? What forces of weathering wear down that road, or rocks or mountains over time? Follow this link to view some animations of different types of weathering processes. Mechanical weathering also called physical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock, just smaller. That means the rock has changed physically without changing its composition.

The smaller pieces have the same minerals, in just the same proportions as the original rock. There are many ways that rocks can be broken apart into smaller pieces. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point figure 2. Ice wedging works quickly, breaking apart rocks in areas with temperatures that cycle above and below freezing in the day and night, and also that cycle above and below freezing with the seasons.

Ice wedging breaks apart so much rock that large piles of broken rock are seen at the base of a hillside, as rock fragments separate and tumble down. Abrasion is another form of mechanical weathering. In abrasion, one rock bumps against another rock. Figure 3. Rocks on a beach are worn down by abrasion as passing waves cause them to strike each other.

Abrasion makes rocks with sharp or jagged edges smooth and round. If you have ever collected beach glass or cobbles from a stream, you have witnessed the work of abrasion figure 3. Now that you know what mechanical weathering is, can you think of other ways it could happen?

Plants and animals can do the work of mechanical weathering figure 4. Burrowing animals can also break apart rock as they dig for food or to make living spaces for themselves.

Figure 4. Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases figure 5. With more surfaces exposed, there are more surfaces on which chemical weathering can occur. Figure 5. Another phenomena that occurs when salt water evaporates can leave behind a square imprint preserved in a soft sediment , called a h opper crystal.

Chemical weathering is the dominate weathering process in warm, humid environments. It happens when water, oxygen, and other reactants chemically degrade the mineral components of bedrock and turn them into water-soluble ions which can then be transported by water. Higher temperatures accelerate chemical weathering rates. Chemical and mechanical weathering work hand-in-hand via a fundamental concept called surface-area-to-volume ratio. Chemical weathering only occurs on rock surfaces because water and reactants cannot penetrate solid rock.

Mechanical weathering penetrates bedrock , breaking large rocks into smaller pieces and creating new rock surfaces. This exposes more surface area to chemical weathering , enhancing its effects. In other words, higher surface-area-to-volume ratios produce higher rates of overall weathering. Carbonic acid H 2 CO 3 forms when carbon dioxide, the fifth-most abundant gas in the atmosphere , dissolves in water. This happens naturally in clouds, which is why precipitation is normally slightly acidic.

Carbonic acid is an important agent in two chemical weathering reactions, hydrolysis and dissolution. Hydrolysis occurs via two types of reactions. In another type of hydrolysis , carbonic acid molecules react directly with minerals , especially those containing silicon and aluminum i. Feldspars , to form molecules of clay minerals. Hydrolysis is the main process that breaks down silicate rock and creates clay minerals. The following is a hydrolysis reaction that occurs when silica-rich feldspar encounters carbonic acid to produce water-soluble clay and other ions:.

Clay minerals are platy silicates or phyllosilicates see Chapter 3, Minerals similar to micas, and are the main components of very fine-grained sediment. The dissolved substances may later precipitate into chemical sedimentary rocks like evaporite and limestone , as well as amorphous silica or chert nodules. Dissolution is a hydrolysis reaction that dissolves minerals in bedrock and leaves the ions in solution , usually in water. Some evaporites and carbonates , like salt and calcite , are more prone to this reaction; however, all minerals can be dissolved.

Non-acidic water, having a neutral pH of 7, will dissolve any mineral , although it may happen very slowly. Water with higher levels of acid, naturally or man-made, dissolves rocks at a higher rate. Natural rainwater can be highly acidic, with pH levels as low as 2. Dissolution can be enhanced by a biological agent, such as when organisms like lichen and bacteria release organic acids onto the rocks they are attached to.

Regions with high humidity airborne moisture and precipitation experience more dissolution due to greater contact time between rocks and water. Minerals at the top of the Bowen series crystallize under high temperatures and pressures, and chemically weather at a faster rate than minerals ranked at the bottom. Olivine and pyroxene are rarely found as end products of weathering because they tend to break down into elemental ions. Dissolution is also noteworthy for the special geological features it creates.

In places with abundant carbonate bedrock , dissolution weathering can produce a karst topography characterized by sinkholes or caves see Chapter 10, Mass Wasting. The figure shows a cave formation created from dissolution followed by precipitation — groundwater saturated with calcite seeped into the cavern, where evaporation caused the dissolved minerals to precipitate out.

Oxidation , the chemical reaction that causes rust in metallic iron, occurs geologically when iron atoms in a mineral bond Two or more atoms or ions that are connected chemically. Any minerals containing iron can be oxidized. The resultant iron oxides may permeate a rock if it is rich in iron minerals.

Oxides may also form a coating that covers rocks and grains of sediment , or lines rock cavities and fractures. If the oxides are more susceptible to weathering than the original bedrock , they may create void spaces inside the rock mass or hollows on exposed surfaces. These iron oxides coat and bind mineral grains together into sedimentary rocks in a process called cementation , and often give these rocks a dominant color. These oxides can permeate a rock that is rich in iron-bearing minerals or can be a coating that forms in cavities or fractures.

When the minerals replacing existing minerals in bedrock are resistant to weathering , iron concretions may occur in the rock. When bedrock is replaced by weaker oxides , this process commonly results in void spaces and weakness throughout the rock mass and often leaves hollows on exposed rock surfaces.

Erosion is a mechanical process, usually driven by water, gravity, see Chapter 10 , wind, or ice see Chapter 14 that removes sediment from the place of weathering. This is well demonstrated in the cliffs of the Grand Canyon. Rocks with different levels erosion resistant also create the unique-looking features called hoodoos in Bryce Canyon National Park and Goblin Valley State Park in Utah.

If erosion does not remove the sediment significantly, organisms can access the mineral content of the sediments. These organisms turn minerals , water, and atmospheric gases into organic substances that contribute to the soil A type of non-eroded sediment mixed with organic matter, used by plants.

The organic component of soil A type of non-eroded sediment mixed with organic matter, used by plants. Nitrogen is the most common element in the atmosphere , but it exists in a form most life forms are unable to use.

Special bacteria found only in soil A type of non-eroded sediment mixed with organic matter, used by plants. These nitrogen-fixing bacteria absorb nitrogen from the atmosphere and convert it into nitrogen compounds.

These compounds are absorbed by plants and used to make DNA, amino acids, and enzymes. Animals obtain bioavailable nitrogen by eating plants, and this is the source of most of the nitrogen used by life. That nitrogen is an essential component of proteins and DNA. Freshly created volcanic soil A type of non-eroded sediment mixed with organic matter, used by plants. The nature of the soil A type of non-eroded sediment mixed with organic matter, used by plants.

For example, soil A type of non-eroded sediment mixed with organic matter, used by plants. The quantity and chemistry of organic matter of soil A type of non-eroded sediment mixed with organic matter, used by plants. Temperature and precipitation , two major weathering agents, are dependent on climate.

Fungi and bacteria contribute organic matter and the ability of soil A type of non-eroded sediment mixed with organic matter, used by plants. In well-formed soil A type of non-eroded sediment mixed with organic matter, used by plants. These soil A type of non-eroded sediment mixed with organic matter, used by plants. Each soil horizon reflects climate , topography, and other soil A type of non-eroded sediment mixed with organic matter, used by plants.

The horizons are assigned names and letters. Differences in naming schemes depend on the area, soil A type of non-eroded sediment mixed with organic matter, used by plants.

The figure shows a simplified soil profile that uses commonly designated names and letters. O Horizon : The top horizon is a thin layer of predominantly organic material, such as leaves, twigs, and other plant parts that are actively decaying into humus. A Horizon : The next layer, called topsoil , consists of humus mixed with mineral sediment.

As precipitation soaks down through this layer, it leaches out soluble chemicals. In wet climates with heavy precipitation this leaching out produces a separate layer called horizon E, the leaching or eluviation zone. B Horizon : Also called subsoil , this layer consists of sediment mixed with humus removed from the upper layers. The subsoil is where mineral sediment is chemically weathered. The amount of organic material and degree of weathering decrease with depth.

The upper subsoil zone, called regolith , is a porous mixture of humus and highly weathered sediment. In the lower zone, saprolite , scant organic material is mixed with largely unaltered parent rock. C Horizon : This is substratum and is a zone of mechanical weathering. Here, bedrock fragments are physically broken but not chemically altered.

This layer contains no organic material. R Horizon : The final layer consists of unweathered, parent bedrock and fragments. The United States governing body for agriculture, the USDA, uses a taxonomic classification to identify soil A type of non-eroded sediment mixed with organic matter, used by plants.

Xoxisols or laterite soil A type of non-eroded sediment mixed with organic matter, used by plants. Ardisol forms in dry climates and can develop layers of hardened calcite , called caliche. Andisols originate from volcanic ash Volcanic tephra that is less than 2 mm in diameter. Alfisols contain silicate clay minerals. These two soil A type of non-eroded sediment mixed with organic matter, used by plants. In general, color can be an important factor in understanding soil A type of non-eroded sediment mixed with organic matter, used by plants.

Black soil A type of non-eroded sediment mixed with organic matter, used by plants. This is true for many sedimentary rocks as well. Not only is soil A type of non-eroded sediment mixed with organic matter, used by plants. Careless or uninformed human activity can seriously damage soil A type of non-eroded sediment mixed with organic matter, used by plants.

A prime example is the famous Dust Bowl disaster of the s, which affected the midwestern United States. The damage occurred because of large-scale attempts develop prairieland in southern Kansas, Colorado, western Texas, and Oklahoma into farmland. The prairie soil A type of non-eroded sediment mixed with organic matter, used by plants. With government encouragement, settlers moved in to homestead the region. They plowed vast areas of prairie into long, straight rows and planted grain.

The plowing broke up the stable soil profile and destroyed the natural grasses and plants, which had long roots that anchored the soil A type of non-eroded sediment mixed with organic matter, used by plants. The grains they planted had shallower root systems and were plowed up every year, which made the soil A type of non-eroded sediment mixed with organic matter, used by plants. The plowed furrows were aligned in straight rows running downhill, which favored erosion and loss of topsoil. The local climate does not produce sufficient precipitation to support non- native grain crops, so the farmers drilled wells and over-pumped water from the underground aquifers.

The grain crops failed due to lack of water, leaving bare soil A type of non-eroded sediment mixed with organic matter, used by plants. Particles of midwestern prairie soil A type of non-eroded sediment mixed with organic matter, used by plants. Huge dust storms called black blizzards made life unbearable, and the once-hopeful homesteaders left in droves.

The lingering question is whether we have learned the lessons of the dust bowl, to avoid creating it again. Which of the following is NOT an example of chemical weathering? Oxidation rusting , dissolution , hydrolysis , and formation of soil A type of non-eroded sediment mixed with organic matter, used by plants. Exfoliation is an example of mechanical weathering. Which of these is NOT a component of soil A type of non-eroded sediment mixed with organic matter, used by plants.

How do chemical and mechanical weathering work together to create sediment? By increasing the surface area, mechanical weathering allows chemical weathering to take place more readily. What is the difference between weathering and erosion? Remember that weathering is breakdown of rocks, erosion is movement of resulting materials. Sedimentary rock is classified into two main categories: clastic and chemical. Clastic or detrital sedimentary rocks are made from pieces of bedrock , sediment , derived primarily by mechanical weathering.

Clastic rocks may also include chemically weathered sediment. Clastic rocks are classified by grain shape , grain size , and sorting. Chemical sedimentary rocks are precipitated from water saturated with dissolved minerals. Chemical rocks are classified mainly by composition of minerals in the rock. Lithification turns loose sediment grains, created by weathering and transported by erosion , into clastic sedimentary rock via three interconnected steps. Deposition happens when friction and gravity overcome the forces driving sediment transport, allowing sediment to accumulate.

Compaction occurs when material continues to accumulate on top of the sediment layer, squeezing the grains together and driving out water. The mechanical compaction is aided by weak attractive forces between the smaller grains of sediment.

Groundwater typically carries cementing agents into the sediment. These minerals , such as calcite , amorphous silica, or oxides , may have a different composition than the sediment grains.

Cementation is the process of cementing minerals coating the sediment grains and gluing them together into a fused rock. Diagenesis is an accompanying process to lithification and is a low- temperature form of rock metamorphism see Chapter 6, Metamorphic Rock. During diagenesis , sediments are chemically altered by heat and pressure.

A classic example is aragonite CaCO 3 , a form of calcium carbonate that makes up most organic shells. When lithified aragonite undergoes diagenesis , the aragonite reverts to calcite CaCO 3 , which has the same chemical formula but a different crystalline structure. In sedimentary rock containing calcite and magnesium Mg , diagenesis may transform the two minerals into dolomite CaMg CO 3 2. Diagenesis may also reduce the pore Empty space in a geologic material, either within sediments, or within rocks.

Can be filled by air, water, or hydrocarbons. The processes of cementation , compaction , and ultimately lithification occur within the realm of diagenesis , which includes the processes that turn organic material into fossils.

Detrital or clastic sedimentary rocks consist of preexisting sediment pieces that comes from weathered bedrock. Most of this is mechanically weathered sediment , although some clasts may be pieces of chemical rocks. This creates some overlap between the two categories, since clastic sedimentary rocks may include chemical sediments. Detrital or clastic rocks are classified and named based on their grain size.

Detrital rock is classified according to sediment grain size , which is graded from large to small on the Wentworth scale see figure. Grain size is the average diameter of sediment fragments in sediment or rock. Grain sizes are delineated using a log base 2 scale.

For example, the grain sizes in the pebble class are 2. These include, boulders, cobbles, granules, and gravel. Sand has a grain size between 2 mm and 0. Sediment grains smaller than sand are called silt. Silt is unique; the grains can be felt with a finger or as grit between your teeth, but are too small to see with the naked eye. Sorting describes the range of grain sizes within sediment or sedimentary rock.

It is important to note that soil A type of non-eroded sediment mixed with organic matter, used by plants. When reading the story told by rocks, geologists use sorting to interpret erosion or transport processes, as well as deposition energy. For example, wind-blown sands are typically extremely well sorted, while glacial deposits are typically poorly sorted.

These characteristics help identify the type of erosion process that occurred. Coarse-grained sediment and poorly sorted rocks are usually found nearer to the source of sediment , while fine sediments are carried farther away.

In a rapidly flowing mountain stream you would expect to see boulders and pebbles. In a lake fed by the stream , there should be sand and silt deposits. If you also find large boulders in the lake, this may indicate the involvement of another sediment transport process, such as rockfall caused by ice- or root-wedging.

Rounding is created when angular corners of rock fragments are removed from a piece of sediment due to abrasion during transport. Well-rounded sediment grains are defined as being free of all sharp edges. Very angular sediment retains the sharp corners.

More rounded grains imply a longer erosion time or transport distance, or more energetic erosional process. Mineral hardness is also a factor in rounding.

Composition describes the mineral components found in sediment or sedimentary rock and may be influenced by local geology, like source rock and hydrology. Other than clay, most sediment components are easily determined by visual inspection see Chapter 3, Minerals.

The most commonly found sediment mineral is quartz because of its low chemical reactivity and high hardness , making it resistant to weathering , and its ubiquitous occurrence in continental bedrock.

Other commonly found sediment grains include feldspar and lithic fragments. Lithic fragments are pieces of fine-grained bedrock , and include mud chips , volcanic clasts, or pieces of slate.

This is because the local rock is composed almost entirely of basalt and provides an abundant source of dark colored clasts loaded with mafic minerals. According to the Goldich Dissolution Series , clasts high in mafic minerals are more easily destroyed compared to clasts composed of felsic minerals like quartz.

Geologists use provenance to discern the original source of sediment or sedimentary rock. Provenance is determined by analyzing mineral composition and types of fossils present, as well as textural features like sorting and rounding.

In quartz sandstone , sometimes called quartz arenite SiO 2 , provenance may be determined using a rare, durable clast mineral called zircon ZrSiO 4. Zircon , or zirconium silicate , contains traces of uranium, which can be used for age-dating the source bedrock that contributed sediment to the lithified sandstone rock see Chapter 7, Geologic Time. Clastic rocks are classified according to the grain size of their sediment.

Coarse-grained rocks contain clasts with a predominant grain size larger than sand. Typically, smaller sediment grains, collectively called groundmass or matrix, fill in much of the volume between the larger clasts, and hold the clasts together.

Conglomerates are rocks containing coarse rounded clasts, and breccias contain angular clasts see figure. Both conglomerates and breccias are usually poorly sorted. Medium-grained rocks composed mainly of sand are called sandstone , or sometimes arenite if well sorted. Sediment grains in sandstone can having a wide variety of mineral compositions, roundness, and sorting. Quartz sandstone contains predominantly quartz sediment grains.

Sandstone that contains feldspar , which weathers more quickly than quartz , is useful for analyzing the local geologic history. Greywack e is a term with conflicting definitions. Greywacke may refer to sandstone with a muddy matrix, or sandstone with many lithic fragments small rock pieces. Fine-grained rocks include mudstone , shale , siltstone , and claystone.

Mudstone is a general term for rocks made of sediment grains smaller than sand less than 2 mm. Rocks that are fissile , meaning they separate into thin sheets, are called shale. Rocks exclusively composed of silt or clay sediment , are called siltstone or claystone , respectively. These last two rock types are rarer than mudstone or shale. Rock types found as a mixture between the main classifications, may be named using the less-common component as a descriptor.

For example, a rock containing some silt but mostly rounded sand and gravel is called silty conglomerate. Sand-rich rock containing minor amounts of clay is called clayey sandstone. Chemical sedimentary rocks are formed by processes that do not directly involve mechanical weathering and erosion.

Chemical weathering may contribute the dissolved materials in water that ultimately form these rocks. Biochemical and organic sediments are clastic in the sense that they are made from pieces of organic material that is deposited, buried, and lithified; however, they are usually classified as being chemically produced. Inorganic chemical sedimentary rocks are made of minerals precipitated from ions dissolved in solution , and created without the aid of living organisms. Inorganic chemical sedimentary rocks form in environments where ion An atom or molecule that has a charge positive or negative due to the loss or gain of electrons.

Biochemical sedimentary rocks are formed from shells and bodies of underwater organisms. The living organisms extract chemical components from the water and use them to build shells and other body parts. The components include aragonite, a mineral similar to and commonly replaced by calcite , and silica.

Organic sedimentary rocks come from organic material that has been deposited and lithified, usually underwater. The source materials are plant and animal remains that are transformed through burial and heat, and end up as coal Former swamp-derived plant material that is part of the rock record. Inorganic chemical sedimentary rocks are formed when minerals precipitate out of an aqueous solution , usually due to water evaporation. The precipitate minerals form various salts known as evaporites.

For example, the Bonneville Salt Flats in Utah flood with winter rains and dry out every summer, leaving behind salts such as gypsum and halite. The deposition order of evaporites deposit is opposite to their solubility order, i. The deposition order and saturation percentages are depicted in the table, bearing in mind the process in nature may vary from laboratory derived values.

Table after. Calcium carbonate - saturated water precipitates porous masses of calcite called tufa Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires.

Waterfalls downstream of springs often precipitate tufa Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires. Saline lakes concentrate calcium carbonate from a combination of wave action causing degassing, springs in the lakebed, and evaporation. In salty Mono Lake in California, tufa Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires.

Cave deposits like stalactites and stalagmites are another form of chemical precipitation of calcite , in a form called travertine. Calcite slowly precipitates from water to form the travertine , which often shows banding. This process is similar to the mineral growth on faucets in your home sink or shower that comes from hard mineral rich water.

Oxygenation of the atmosphere and oceans caused free iron ions, which are water-soluble, to become oxidized and precipitate out of solution. The iron oxide was deposited, usually in bands alternating with layers of chert. Chert , another commonly found chemical sedimentary rock, is usually produced from silica SiO 2 precipitated from groundwater. Silica is highly insoluble on the surface of Earth, which is why quartz is so resistant to chemical weathering.

Water deep underground is subjected to higher pressures and temperatures, which helps dissolve silica into an aqueous solution. As the groundwater rises toward or emerges at the surface the silica precipitates out, often as a cementing agent or into nodules. For example, the bases of the geysers in Yellowstone National Park are surrounded by silica deposits called geyserite or sinter.

The silica is dissolved in water that is thermally heated by a relatively deep magma source. Chert can also form biochemically and is discussed in the Biochemical subsection. Chert has many synonyms, some of which may have gem value such as jasper, flint, onyx, and agate, due to subtle differences in colors, striping, etc. Ooid refers to the sphere, oolite the rock with the spheres. When water is oversaturated with calcite , the mineral precipitates out around a nucleus, a sand grain or shell fragment, and forms little spheres called ooid Spheres of calcite that form in saline waters with slight wave agitation.

As evaporation continues, the ooid Spheres of calcite that form in saline waters with slight wave agitation. Biochemical sedimentary rocks are not that different from chemical sedimentary rocks; they are also formed from ions dissolved in solution.

However, biochemical sedimentary rocks rely on biological processes to extract the dissolved materials out of the water. Most macroscopic marine organisms use dissolved minerals , primarily aragonite calcium carbonate , to build hard parts such as shells.

When organisms die the hard parts settle as sediment , which become buried, compacted and cemented into rock. This biochemical extraction and secretion is the main process for forming limestone , the most commonly occurring, non- clastic sedimentary rock. Solid calcite reacts with hydrochloric acid by effervescing or fizzing. Dolomite only reacts to hydrochloric acid when ground into a powder, which can be done by scratching the rock surface see Chapter 3, Minerals.

Limestone occurs in many forms, most of which originate from biological processes. Entire coral reef A topographic high found away from the beach in deeper water, but still on the continental shelf.

Typically, these are formed in tropical areas by organisms such as corals. Fossiliferous limestone contains many visible fossils. A type of limestone called coquina originates from beach sands made predominantly of shells that were then lithified. Coquina is composed of loosely-cemented shells and shell fragments. You can find beaches like this in modern tropical environments, such as the Bahamas. Chalk contains high concentrations of shells from a microorganism called a coccolithophore.

Micrite , also known as microscopic calcite mud, is a very fine-grained limestone containing microfossils that can only be seen using a microscope. Biogenetic chert forms on the deep ocean floor , created from biochemical sediment made of microscopic organic shells.

This sediment , called ooze, may be calcareous calcium carbonate based or siliceous silica-based depending on the type of shells deposited.

For example, the shells of radiolarians zooplankton and diatoms phytoplankton are made of silica, so they produce siliceous ooze. Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record.

Although not derived from sediment , this lithified organic material is associated with sedimentary strata and created by similar processes—burial, compaction , and diagenesis. C Deposits of these fuels develop in areas where organic material collects in large quantities. Lush swamplands can create conditions conducive to coal Former swamp-derived plant material that is part of the rock record.

Shallow-water, organic material-rich marine sediment can become highly productive petroleum and natural gas deposits. See Chapter 16, Energy and Mineral Resources, for a more in-depth look at these fossil -derived energy sources. In contrast to detrital sediment , chemical, biochemical , and organic sedimentary rocks are classified based on mineral composition.

Most of these are monomineralic, composed of a single mineral , so the rock name is usually associated with the identifying mineral. Factors Affecting Landforms. Types of Desert Soil. What Factors Determine the Rate of Weathering? What Causes Weathering of an Arch? Landforms of a Savanna. The Topography of Deserts. What Are the Two Types of Weathering?

What Factors Cause Mechanical Weathering? Marble Vs. The Different Types of Landforms. References Forest Ecosytems; David A. Perry, et al.