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Wednesday, October 5, 2022

Middle Stone Age

From Wikipedia, the free encyclopedia

Middle Stone Age tool from Blombos Cave

The Middle Stone Age (or MSA) was a period of African prehistory between the Early Stone Age and the Late Stone Age. It is generally considered to have begun around 280,000 years ago and ended around 50–25,000 years ago. The beginnings of particular MSA stone tools have their origins as far back as 550–500,000 years ago and as such some researchers consider this to be the beginnings of the MSA. The MSA is often mistakenly understood to be synonymous with the Middle Paleolithic of Europe, especially due to their roughly contemporaneous time span, however, the Middle Paleolithic of Europe represents an entirely different hominin population, Homo neanderthalensis, than the MSA of Africa, which did not have Neanderthal populations. Additionally, current archaeological research in Africa has yielded much evidence to suggest that modern human behavior and cognition was beginning to develop much earlier in Africa during the MSA than it was in Europe during the Middle Paleolithic. The MSA is associated with both anatomically modern humans (Homo sapiens) as well as archaic Homo sapiens, sometimes referred to as Homo helmei. Early physical evidence comes from the Gademotta Formation in Ethiopia, the Kapthurin Formation in Kenya and Kathu Pan in South Africa.

Regional development

There are MSA archaeological sites from across the African continent, conventionally divided into five regions: northern Africa, comprising parts of the modern countries of Morocco, Algeria, Tunisia, and Libya; eastern Africa, stretching roughly from the highlands of Ethiopia to the southern part of Kenya; central Africa, stretching from the borders of Tanzania and Kenya to include Angola; southern Africa, which includes the numerous cave sites of South Africa; and western Africa.

In northern and western Africa, the wet-dry cycles of the modern Sahara desert has led to fruitful archaeological sites followed by completely barren soil and vice versa. Preservation in these two regions can vary, yet the sites that have been uncovered document the adaptive nature of early humans to climatically unstable environments.

Eastern Africa represents some of the most reliable dates, due to the use of radiocarbon dating on volcanic ash deposits, as well as some of the earliest MSA sites. Faunal preservation, however, is not spectacular, and standardization in site excavation and lithic classification was, until recently, lacking. Unlike northern Africa, shifts between lithic technologies were not nearly as pronounced, likely due to more favorable climatic conditions that would have allowed for more continuous occupation of sites. Central Africa reflects similar patterning to eastern Africa, yet more archaeological research of the region is certainly required.

Southern Africa consists of many cave sites, most of which show very punctuated starts and stops in stone tool technology. Research in southern Africa has been continuous and quite standardized, allowing for reliable comparisons between sites in the region. Much of the archaeological evidence for the origins of modern human behavior is traced back to sites in this region, including Blombos Cave, Howiesons Poort, Still Bay, and Pinnacle Point.

Transition from Acheulean

The Awash Valley

The term "Middle Stone Age" (MSA) was proposed to the African Archaeological Congress by Goodwin and Van Riet Lowe in 1929. The use of these terms was officially abandoned in 1965. although the term remains in use in the context of sub-Saharan Africa, beginning with a transitional late Acheulean period known as the Fauresmith industry. The Fauresmith industry is poorly dated, according to Herries (2011) beginning around 511–435 kya. This time, rather than the actual end of the Achaeulean around 130 kya is taken as the beginning of the MSA. The MSA so defined is associated with the gradual replacement of archaic humans by anatomically modern humans.

In a different convention, MSA refers to sites characterized by the use of Levallois methods for flake production, to the exclusion of Acheulean sites with large cleavers or handaxes. Following McBrearty and Tryon (2006), the term "early MSA" (EMSA) refers to sites predating the 126 kya interglacial, and "later MSA" (LMSA) refers to site younger than 126 kya. In this convention, Fauresmith sites of 500 to 300 kya are within the ESA, and the MSA begins after about 280 kya and is largely associated with H. sapiens, the earliest reliably dated MSA site in East Africa being Gademotta in Ethiopia, at 276 kya. The Middle Awash valley of Ethiopia and the Central Rift Valley of Kenya constituted a major center for behavioural innovation. It is likely that the large terrestrial mammal biomass of these regions supported substantial human populations with subsistence and manufacturing patterns similar to those of ethnographically known foragers.

Archaeological evidence from eastern Africa extending from the Rift Valley from Ethiopia to northern Tanzania represents the largest archaeological evidence of the shift from the Late Acheulian to the Middle Stone Age tool technologies. This transition is characterized by stratigraphic layering of Acheulian stone tools, a bifacial handaxe technology, underneath and even contemporaneous with MSA technologies, such as Levallois tools, flakes, flaked tools, pointed flakes, smaller bifaces that are projectile in form, and, on rare occasions, hafted tools. Evidence of the gradual displacement of Acheulian by MSA technologies is further supported by this layering and contemporaneous placement, as well as by the earliest appearance of MSA technologies at Gademotta and the latest Acheulian technologies at the Bouri Formation of Ethiopia, dated to 154 to 160 kya. This suggests a possible overlap of 100–150 thousand years.

Late Acheulean artefacts associated with Homo sapiens have been found in South African cave sites. The Cave of Hearths and Montague Cave in South Africa contain evidence of Acheulian technologies, as well as later MSA technologies, however there is no evidence of crossover in this region.

ESA Acheulean sites are well documented across West Africa (except from the most tropical regions) but mostly remain undated. A few late Acheulean sites ("MSA" in the sense of late Acheulean, not Levallois) have been dated. Middle Pleistocene (pre 126 kya) sites are known form the northern Sahelian zones, while Late Pleistocene (post 126 kya) sites are known both from northern and southern West Africa. Unlike elsewhere in Africa, MSA sites appear to persist until very late, down to the Holocene boundary (12 kya), pointing to the possibility of late survival of archaic humans, and late hybridization with H. sapiens in West Africa. Furthermore, such results highlight significant spatiotemporal cultural variability and suggest that long inter-group cultural differences played a major role in later stages of human evolution in Africa.

Lithic technology

2009 excavations at the Diepkloof Rock Shelter

Early blades have been documented as far back as 550–500,000 years in the Kapthurin Formation in Kenya and Kathu Pan in South Africa. Backed pieces from the Twin Rivers and Kalambo Falls sites in Zambia, dated at sometime between 300 and 140,000 years, likewise indicate a suite of new behaviors. A high level of technical competence is also indicated for the c. 280 ka blades recovered from the Kapthurin Formation, Kenya.

The stone tool technology in use during the Middle Stone Age shows a mosaic of techniques. Beginning approximately 300 kya, the large cutting tools of the Achuelian are gradually displaced by Levallois prepared core technologies, also widely used by Neanderthals during the European Middle Palaeolithic. As the MSA progresses, highly varied technocomplexes become common throughout Africa and include pointed artifacts, blades, retouched flakes, end and side scrapers, grinding stones, and even bone tools. However, the use of blades (associated mainly with the Upper Palaeolithic in Europe) is seen at many sites as well. In Africa, blades may have been used during the transition from the Early Stone Age to the Middle Stone Age onwards. Finally, during the later part of the Middle Stone Age, microlithic technologies aimed at producing replaceable components of composite hafted tools are seen from at least 70 ka at sites such as Pinnacle Point and Diepkloof Rock Shelter in South Africa.

Artifact technology during the Middle Stone Age shows a pattern of innovation followed by disappearance. This occurs with technology such as the manufacture of shell beads, arrows and hide working tools including needles, and gluing technology. These pieces of evidence provide a counterpoint to the classic "Out of Africa" scenario in which increasing complexity accumulated during the Middle Stone Age. Instead, it has been argued that such technological innovations "appear, disappear and re-appear in a way that best fits a scenario in which historical contingencies and environmental rather than cognitive changes are seen as main drivers".

Hominin evolution and migration

Homo erectus skull, Museum of Natural History, Ann Arbor
 

There have been two migration events out of Africa. The first was the expansion of H. erectus into Eurasia approximately 1.9 to 1.7 million years ago, and the second, by H. sapiens began during the MSA by 80 – 50 ka MSA out of Africa to Asia, Australia and Europe. Perhaps only in small numbers initially, but by 30 ka they had replaced Neanderthals and H. erectus. Each of these migrations represent the increased flexibility of the genus Homo to survive in widely varied climates. Based on the measurement of a large number of human skulls a recent study supports a central/southern African origin for Homo sapiens as this region shows the highest intra-population diversity in phenotypic measurements. Genetic data supports this conclusion. However, there is genetic evidence to suggest that dispersal out of Africa began in eastern Africa. Sites such as the Omo Kibish Formation, the Herto Member of the Bouri Formation, and Mumba Cave contain fossil evidence to support this conclusion as well.

Evidence for modern human behavior

There have been a number of theories proposed regarding the development of modern human behavior, but in recent years the mosaic approach has been the most favored perspective in regards to the MSA, especially when taken in consideration with the archaeological evidence. Some scholars including Klein have argued for discontinuity, while others including McBrearty and Brooks have argued that cognitive advances can be detected in the MSA and that the origin of our species is linked with the appearance of Middle Stone Age technology at 250–300 ka. The earliest remains of Homo sapiens date back to approximately 300 thousand years ago in Africa. the continent was mainly populated by groups of hunter-gatherers. In the archaeological record of both eastern Africa and southern Africa, there is immense variability associated with Homo sapiens sites, and it is during this time that we see evidence of the origins of modern human behavior. According to McBrearty and Brooks, there are four features that are characteristic of modern human behavior: abstract thinking, the ability to plan and strategize, "behavioral, economic and technological innovativeness," and symbolic behavior. Many of these aspects of modern human behavior can be broken down into more specific categories, including art, personal adornment, technological advancement, yet these four overarching categories allow for a thorough, albeit significantly overlapping, discussion of behavioral modernity.

Possible cultural complexes

Aterian stone tool

As early Homo sapiens began to diversify the ecological zones that they inhabited during the MSA, the archaeological record associated with these zones begins to show evidence for regional continuities. These continuities are significant for a number of reasons. The expansion of Homo sapiens into various ecological zones demonstrates an ability to adapt to a variety of environmental contexts including marine environments, savanna grasslands, relatively arid deserts, and forests. This adaptability is reflected in MSA artifacts found in these zones. These artifacts display stylistic variability depending on zone. During the Acheulian, which spanned from 1.5 million years ago to 300 thousand years ago, lithic technology displayed incredible homogeneity throughout all ecological niches. MSA technologies, with their evidence for regional variability and continuity, represent a remarkable advance. These data have been used to support theories of social and stylistic development throughout the MSA.

In southern Africa, we see the technocomplexes of Howiesons Poort and Stillbay, named after the sites at which they were first discovered. Several others have not been dated or have been dated unreliably; these include the Lupemban technocomplex of central Africa, the Bambatan in southeast Africa, 70–80ka, and the Aterian technocomplex of northern Africa, 160–90ka.

Abstract thinking

Evidence of abstract thinking can be seen in the archaeological record as early as the Acheulean–Middle Stone Age transition, approximately 300,000–250,000 years ago. This transition involves a shift in stone tool technology from Mode 2, Acheulean tools, to Mode 3 and 4, which include blades and microliths. The manufacture of these tools requires planning and the understanding of how striking a stone will produce different flaking patterns. This requires abstract thought, one of the hallmarks of modern human behavior. The shift from large cutting tools in the Acheulian to smaller and more diversified toolkits in the MSA represents a better cognitive and conceptual understanding of flintknapping, as well as the potential functional effects of distinct tool types.

Planning depth

The ability to plan and strategize, much like abstract thinking, can be seen in the more diversified toolkit of the Middle Stone Age, as well as in the subsistence patterns of the period. As MSA hominins began to migrate into a range of different ecological zones, it became necessary to base hunting strategies around seasonally available resources. Awareness of seasonality is evident in the faunal remains found at temporary sites. In less forgiving ecological zones, this awareness would have been essential for survival and the ability to plan subsistence strategies based on this awareness demonstrates an ability to think beyond the present tense and act upon this knowledge.

This planning depth is also seen in the presence of exotic raw materials at a variety of sites throughout the MSA. Procurement of local raw materials would have been a simple task to accomplish, yet MSA sites regularly contain raw materials that were obtained from sources over 100 km away, and sometimes farther than 300 km. Obtaining raw materials from this distance would require an awareness of the resources, a perceived value in the resources, whether it be functional or symbolic, and, possibly, the ability to organize an exchange network in order to obtain the materials.

Innovation

The ability to expand into new environments throughout Africa and, ultimately, the world, displays a level of adaptability and, consequently, innovativeness that is often seen as characteristic of behavioral modernity.[1] Middle Stone Age sites are found in a wide range of environments, including coastal and inland areas of southern and eastern Africa, and in at least one case MSA foragers were exploiting high-altitude glaciated environments, at Fincha Habera in Ethiopia. This, however, is not the only evidence of innovativeness that can be seen in early Homo sapiens. The development of new, regionally relevant tools, such as those used for the collection of marine resources seen at Abdur, Ethiopia, Pinnacle Point Cave, South Africa, and Blombos Cave, South Africa. The use of fire demonstrates another innovative aspect of human behavior when it is used in order to create stronger tools, such as the heated silcrete at Blombos, Howiesons Poort and Still Bay, and the heat treated bone tools from Still Bay.

Hafted tools are further representative of human innovation. The large cutting tools of the Acheulian technocomplex become smaller, as more complex tools are better suited towards the needs of highly diversified environments. Composite tools represent a new level of innovation in their increased efficacy and more complex manufacturing process. The ability to conceptualize beyond the mere reduction of stone cores demonstrates cognitive flexibility, and the use of glue, which was often processed with ochre, to attach flakes to hafts demonstrates an understanding of chemical changes that can be utilized beyond the simple use of color. Adhesives were used to construct hafted tools by 70ka at Sibudu Cave in South Africa.

Other technological innovations of the period include specialized projectile weapons found at various sites in Middle Stone Age Africa such as: bone and stone arrowheads at South African sites such as Sibudu Cave (along with an early bone needle also found at Sibudu) dating approximately 60,000–70,000 years ago, and bone harpoons at the Central African site of Katanda dating to about 90,000 years ago. Evidence also exists for the systematic heat treating of silcrete stone to increase its flake-ability for the purpose of toolmaking, beginning approximately 164,000 years ago at the South African site of Pinnacle Point and becoming common there for the creation of microlithic tools at about 72,000 years ago.

Characteristically modern human behaviors, such as the making of shell beads, bone tools and arrows, and the use of ochre pigment, are evident at Panga ya Saidi in Kenya by 78,000–67,000 years ago. Evidence of early stone-tipped projectile weapons (a characteristic tool of Homo sapiens), the stone tips of javelins or throwing spears, were discovered in 2013 at the Ethiopian site of Gademotta, and date to around 279,000 years ago.

Evidence was found in 2018, dating to about 320,000 years ago, at the Kenyan site of Olorgesailie, of the early emergence of innovations and behaviors including: long-distance trade networks (involving goods such as obsidian), the use of pigments, and the possible making of projectile points. It is observed by the authors of three 2018 studies on the site, that the evidence of these behaviors is approximately contemporary to the earliest known Homo sapiens fossil remains from Africa (such as at Jebel Irhoud and Florisbad), and they suggest that complex and modern behaviors had already begun in Africa around the time of the emergence of Homo sapiens.

Symbolic behavior

Zoomorphic pictogram on stone slab from the MSA of Apollo 11 Cave, Namibia

Symbolic behavior is, perhaps, one of the most difficult aspects of modern human behavior to distinguish archaeologically. When searching for evidence of symbolic behavior in the MSA, there are three lines of evidence that can be considered: direct evidence reflecting concrete examples of symbols; indirect evidence reflecting behaviors that would have been used to convey symbolic thought; and technological evidence reflecting the tools and skills that would have been used to produce art. Direct evidence is difficult to find beyond 40ka, and indirect evidence is essentially intangible, thus technological evidence is the most fruitful of the three.

Today there is widespread agreement among archaeologists that the world's first art and symbolic culture dates to the African Middle Stone Age. Some of the most striking artifacts, including engraved pieces of red ochre, were manufactured at Blombos Cave in South Africa 75,000 years ago. Pierced and ochred Nassarius shell beads were also recovered from Blombos, with even earlier examples (Middle Stone Age, Aterian) from the Taforalt Caves. In addition, ostrich egg shell containers engraved with geometric designs dating to 60,000 years ago were found at Diepkloof, South Africa, beads and other personal ornamentation have been found from Morocco which might be as much as 130,000 years old, and the Cave of Hearths in South Africa has yielded a number of beads dating from significantly prior to 50,000 years ago. At Panga ya Saidi in Kenya, marine shell beads appear perhaps as early as 67,000 years ago and certainly by 33,000 years ago, and engraved ochre by 48,500 years ago. Arrows and hide working tools (including a needle-like tool) have been found at Sibudu Cave dating between about 70–60,000 years ago. as evidence of making weapons with compound heat treated gluing technology. Evidence for the making of paints by a complex process also exists dating to 100,000 years ago in South Africa, and for the use of pigments in Kenya dating to about 320,000 years ago.

Complex cognition

A series of innovations have been documented by 170–160,000 years ago at the site of Pinnacle Point 13B on the southern Cape coast of South Africa. This includes the oldest confirmed evidence for the utilization of ochre and marine resources in the form of shellfish exploitation for food. Based on his analysis of the MSA bovid assemblage at Klasies, Milo reports MSA people were formidable hunters and that their social behavior patterns approached those of modern humans. Deacon maintains that the management of plant food resources through deliberate burning of the veld to encourage the growth of plants with corms or tubers in the southern Cape during the Howiesons Poort (c. 70–55 ka) is indicative of modern human behavior. A family basis to foraging groups, color symbolism and the reciprocal exchange of artifacts and the formal organization of living space are, he suggests, further evidence for modernity in the MSA.

Lyn Wadley et al. have argued that the complexity of the skill needed to process the heat-treated compound glue (gum and red ochre) used to haft spears would seem to argue for continuity between modern human cognition and that of humans 70,000 BP at Sibudu Cave.

In 2008, an ochre processing workshop likely for the production of paints was uncovered dating to ca. 100,000 years ago at Blombos Cave, South Africa. Analysis shows that a liquefied pigment-rich mixture was produced and stored in the two abalone shells, and that ochre, bone, charcoal, grindstones and hammer-stones also formed a composite part of the toolkits. Evidence for the complexity of the task includes procuring and combining raw materials from various sources (implying they had a mental template of the process they would follow), possibly using pyrotechnology to facilitate fat extraction from bone, using a probable recipe to produce the compound, and the use of shell containers for mixing and storage for later use.

Evidence for language

Ochre is reported from some early MSA sites, for example at Kapthurin and Twin Rivers, and is common after c. 100 ka. Barham argues that even if some of this ochre was used in a symbolic, color-related role then this abstraction could not have worked without language. Ochre, he suggests, could be one proxy for trying to find the emergence of language.

Formal bone tools are frequently associated with modern behaviour by archaeologists. Sophisticated bone harpoons manufactured at Katanda, West Africa at c. 90 ka[61][62] and bone tools from Blombos Cave dated at c. 77 ka may then also serve as examples of material culture associated with modern language.

Language has been suggested to be necessary to maintain exchange networks. Evidence of some form of exchange networks during the Middle Stone Age is presented in Marwick (2003) in which the distance between the source of raw material and location in which a stone artifact was found was compared throughout sites containing early stone artifacts. Five Middle Stone Age sites contained distances between 140–340 km and have been interpreted, when compared with ethnographic data, that these distances were made possible through exchange networks. Barham also views syntactic language as one aspect of behavior that in fact allowed MSA people to settle in the tropical forest environments of what is now the Democratic Republic of the Congo.

Many authors have speculated that at the core of this symbolic explosion, and in tandem, was the development of syntactic language that evolved through a highly specialized social learning system providing the means for semantically unbounded discourse. Syntax would have played a key role in this process and its full adoption could have been a crucial element of the symbolic behavioral package in the MSA.

Brain change

Although the advent of anatomical physical modernity cannot confidently be linked with palaeoneurological change, it does seem probable that hominid brains evolved through the same selection processes as other body parts. Genes that promoted a capacity for symbolism may have been selected for, suggesting that the foundations for symbolic culture may well be grounded in biology. However, behavior that was mediated by symbolism may have only come later, even though this physical capacity was already in place much earlier. Skoyles and Sagan, for example, argue that human brain expansion by increasing the prefrontal cortex would have created a brain capable of symbolizing its previously non-symbolic cognition, and that this process, slow to begin with, increasingly accelerated during the last 100,000 years. Symbolically mediated behavior may then feed back upon this process by creating a greater ability to manufacture symbolic artifacts and social networks. According to the research team in Jebel Irhoud, the discovery means that Homo sapiens—not members of a rival or ancestor species (Homo heidelbergensis, Homo naledi)—were the ones who left behind Middle Stone Age hand tools that have since been unearthed all over Africa.

Sites

Excavations at Pinnacle Point, South Africa

Numerous sites in southern Africa reflect the four characteristics of behavioral modernity. Blombos Cave, South Africa contains personal ornaments and what are presumed to be the tools used for the production of artistic imagery, as well as bone tools. Still Bay and Howieson's Poort contain variable tool technologies. These different types of assemblages allow researchers to extrapolate behaviors that would likely be associated with such technologies, such as shifts in foraging behaviors, which are further supported by faunal data at these sites.

Erosion

From Wikipedia, the free encyclopedia

An actively eroding rill on an intensively-farmed field in eastern Germany

Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

Agents of erosion include rainfall; bedrock wear in rivers; coastal erosion by the sea and waves; glacial plucking, abrasion, and scour; areal flooding; wind abrasion; groundwater processes; and mass movement processes in steep landscapes like landslides and debris flows. The rates at which such processes act control how fast a surface is eroded. Typically, physical erosion proceeds fastest on steeply sloping surfaces, and rates may also be sensitive to some climatically-controlled properties including amounts of water supplied (e.g., by rain), storminess, wind speed, wave fetch, or atmospheric temperature (especially for some ice-related processes). Feedbacks are also possible between rates of erosion and the amount of eroded material that is already carried by, for example, a river or glacier. The transport of eroded materials from their original location is followed by deposition, which is arrival and emplacement of material at a new location.

While erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. At agriculture sites in the Appalachian Mountains, intensive farming practices have caused erosion at up to 100 times the natural rate of erosion in the region. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes) ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, this leads to desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide.

Intensive agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils.

A natural arch produced by the wind erosion of differentially weathered rock in Jebel Kharaz, Jordan
 
A wave-like sea cliff produced by coastal erosion, in Jinshitan Coastal National Geopark, Dalian, Liaoning Province, China

Physical processes

Rainfall and surface runoff

Soil and water being splashed by the impact of a single raindrop

Rainfall, and the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four).

In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles. The distance these soil particles travel can be as much as 0.6 m (two feet) vertically and 1.5 m (five feet) horizontally on level ground.

If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs. If the runoff has sufficient flow energy, it will transport loosened soil particles (sediment) down the slope. Sheet erosion is the transport of loosened soil particles by overland flow.

A spoil tip covered in rills and gullies due to erosion processes caused by rainfall: Rummu, Estonia

Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of the order of a few centimetres (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers.

Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth. A gully is distinguished from a rill based on a critical cross-sectional area of at least one square foot, i.e. the size of a channel that can no longer be erased via normal tillage operations.

Extreme gully erosion can progress to formation of badlands. These form under conditions of high relief on easily eroded bedrock in climates favorable to erosion. Conditions or disturbances that limit the growth of protective vegetation (rhexistasy) are a key element of badland formation.

Rivers and streams

Dobbingstone Burn, Scotland, showing two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the stream's banks are glacial till that was left behind as ice age glaciers flowed over the terrain.
 
Layers of chalk exposed by a river eroding through them

Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside, creating head cuts and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V-shaped cross-section and the stream gradient is relatively steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles, and boulders can also act erosively as they traverse a surface, in a process known as traction.

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as scour. Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.

Thermal erosion is the result of melting and weakening permafrost due to moving water. It can occur both along rivers and at the coast. Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials. Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the Arctic coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a 100-kilometre (62-mile) segment of the Beaufort Sea shoreline averaged 5.6 metres (18 feet) per year from 1955 to 2002.

Most river erosion happens nearer to the mouth of a river. On a river bend, the longest least sharp side has slower moving water. Here deposits build up. On the narrowest sharpest side of the bend, there is faster moving water so this side tends to erode away mostly.

Rapid erosion by a large river can remove enough sediments to produce a river anticline, as isostatic rebound raises rock beds unburdened by erosion of overlying beds.

Coastal erosion

Wave cut platform caused by erosion of cliffs by the sea, at Southerndown in South Wales
 
Erosion of the boulder clay (of Pleistocene age) along cliffs of Filey Bay, Yorkshire, England

Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role.

Hydraulic action takes place when the air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrasion is caused by waves launching sea load at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with corrosion). Corrosion is the dissolving of rock by carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind of erosion. Attrition is where particles/sea load carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is boring, scraping and grinding of organisms, a process termed bioerosion.

Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the upcurrent supply of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form as a result of deposition. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a buildup of eroded material occurs forming a long narrow bank (a spit). Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.

Erosion of a coastal surface, followed by a fall in sea level, can produce a distinctive landform called a raised beach.

Chemical erosion

Chemical erosion is the loss of matter in a landscape in the form of solutes. Chemical erosion is usually calculated from the solutes found in streams. Anders Rapp pioneered the study of chemical erosion in his work about Kärkevagge published in 1960.

Formation of sinkholes and other features of karst topography is an example of extreme chemical erosion.

Glaciers

The Devil's Nest (Pirunpesä), the deepest ground erosion in Europe, located in Jalasjärvi, Kurikka, Finland
 

Glaciers erode predominantly by three different processes: abrasion/scouring, plucking, and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Scientists have shown that, in addition to the role of temperature played in valley-deepening, other glaciological processes, such as erosion also control cross-valley variations. In a homogeneous bedrock erosion pattern, curved channel cross-section beneath the ice is created. Though the glacier continues to incise vertically, the shape of the channel beneath the ice eventually remain constant, reaching a U-shaped parabolic steady-state shape as we now see in glaciated valleys. Scientists also provide a numerical estimate of the time required for the ultimate formation of a steady-shaped U-shaped valley—approximately 100,000 years. In a weak bedrock (containing material more erodible than the surrounding rocks) erosion pattern, on the contrary, the amount of over deepening is limited because ice velocities and erosion rates are reduced.

Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield. Differences in the height of mountain ranges are not only being the result tectonic forces, such as rock uplift, but also local climate variations. Scientists use global analysis of topography to show that glacial erosion controls the maximum height of mountains, as the relief between mountain peaks and the snow line are generally confined to altitudes less than 1500 m. The erosion caused by glaciers worldwide erodes mountains so effectively that the term glacial buzzsaw has become widely used, which describes the limiting effect of glaciers on the height of mountain ranges. As mountains grow higher, they generally allow for more glacial activity (especially in the accumulation zone above the glacial equilibrium line altitude), which causes increased rates of erosion of the mountain, decreasing mass faster than isostatic rebound can add to the mountain. This provides a good example of a negative feedback loop. Ongoing research is showing that while glaciers tend to decrease mountain size, in some areas, glaciers can actually reduce the rate of erosion, acting as a glacial armor. Ice can not only erode mountains but also protect them from erosion. Depending on glacier regime, even steep alpine lands can be preserved through time with the help of ice. Scientists have proved this theory by sampling eight summits of northwestern Svalbard using Be10 and Al26, showing that northwestern Svalbard transformed from a glacier-erosion state under relatively mild glacial maxima temperature, to a glacier-armor state occupied by cold-based, protective ice during much colder glacial maxima temperatures as the Quaternary ice age progressed.

These processes, combined with erosion and transport by the water network beneath the glacier, leave behind glacial landforms such as moraines, drumlins, ground moraine (till), kames, kame deltas, moulins, and glacial erratics in their wake, typically at the terminus or during glacier retreat.

The best-developed glacial valley morphology appears to be restricted to landscapes with low rock uplift rates (less than or equal to 2mm per year) and high relief, leading to long-turnover times. Where rock uplift rates exceed 2mm per year, glacial valley morphology has generally been significantly modified in postglacial time. Interplay of glacial erosion and tectonic forcing governs the morphologic impact of glaciations on active orogens, by both influencing their height, and by altering the patterns of erosion during subsequent glacial periods via a link between rock uplift and valley cross-sectional shape.

Floods

The mouth of the River Seaton in Cornwall after heavy rainfall caused flooding in the area and cause a significant amount of the beach to erode
The mouth of the River Seaton in Cornwall after heavy rainfall caused flooding in the area and cause a significant amount of the beach to erode; leaving behind a tall sand bank in its place

At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called rock-cut basins. Examples can be seen in the flood regions result from glacial Lake Missoula, which created the channeled scablands in the Columbia Basin region of eastern Washington.

Wind erosion

Árbol de Piedra, a rock formation in the Altiplano, Bolivia sculpted by wind erosion
 

Wind erosion is a major geomorphological force, especially in arid and semi-arid regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as deforestation, urbanization, and agriculture.

Wind erosion is of two primary varieties: deflation, where the wind picks up and carries away loose particles; and abrasion, where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) surface creep, where larger, heavier particles slide or roll along the ground; (2) saltation, where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3) suspension, where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50-70%) of wind erosion, followed by suspension (30-40%), and then surface creep (5-25%).

Wind erosion is much more severe in arid areas and during times of drought. For example, in the Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.

Mass wasting

A wadi in Makhtesh Ramon, Israel, showing gravity collapse erosion on its banks
 

Mass wasting or mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity.

Mass wasting is an important part of the erosional process and is often the first stage in the breakdown and transport of weathered materials in mountainous areas. It moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-wasting processes are always occurring continuously on all slopes; some mass-wasting processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a scree slope.

Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show a spoon-shaped isostatic depression, in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence.

Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm (0.02 to 0.04 in) in diameter by wind along the soil surface.

Submarine sediment gravity flows

Bathymetry of submarine canyons in the continental slope off the coast of New York and New Jersey

On the continental slope, erosion of the ocean floor to create channels and submarine canyons can result from the rapid downslope flow of sediment gravity flows, bodies of sediment-laden water that move rapidly downslope as turbidity currents. Where erosion by turbidity currents creates oversteepened slopes it can also trigger underwater landslides and debris flows. Turbidity currents can erode channels and canyons into substrates ranging from recently deposited unconsolidated sediments to hard crystalline bedrock. Almost all continental slopes and deep ocean basins display such channels and canyons resulting from sediment gravity flows and submarine canyons act as conduits for the transfer of sediment from the continents and shallow marine environments to the deep sea. Turbidites, which are the sedimentary deposits resulting from turbidity currents, comprise some of the thickest and largest sedimentary sequences on Earth, indicating that the associated erosional processes must also have played a prominent role in Earth's history.

Factors affecting erosion rates

Climate

The amount and intensity of precipitation is the main climatic factor governing soil erosion by water. The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by vegetation. This might be during periods when agricultural activities leave the soil bare, or in semi-arid regions where vegetation is naturally sparse. Wind erosion requires strong winds, particularly during times of drought when vegetation is sparse and soil is dry (and so is more erodible). Other climatic factors such as average temperature and temperature range may also affect erosion, via their effects on vegetation and soil properties. In general, given similar vegetation and ecosystems, areas with more precipitation (especially high-intensity rainfall), more wind, or more storms are expected to have more erosion.

In some areas of the world (e.g. the mid-western USA), rainfall intensity is the primary determinant of erosivity with higher intensity rainfall generally resulting in more soil erosion by water. The size and velocity of rain drops is also an important factor. Larger and higher-velocity rain drops have greater kinetic energy, and thus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops.

In other regions of the world (e.g. western Europe), runoff and erosion result from relatively low intensities of stratiform rainfall falling onto the previously saturated soil. In such situations, rainfall amount rather than intensity is the main factor determining the severity of soil erosion by water. According to the climate change projections, erosivity will increase significantly in Europe and soil erosion may increase by 13-22.5% by 2050 

In Taiwan, where typhoon frequency increased significantly in the 21st century, a strong link has been drawn between the increase in storm frequency with an increase in sediment load in rivers and reservoirs, highlighting the impacts climate change can have on erosion.

Vegetative cover

Vegetation acts as an interface between the atmosphere and the soil. It increases the permeability of the soil to rainwater, thus decreasing runoff. It shelters the soil from winds, which results in decreased wind erosion, as well as advantageous changes in microclimate. The roots of the plants bind the soil together, and interweave with other roots, forming a more solid mass that is less susceptible to both water and wind erosion. The removal of vegetation increases the rate of surface erosion.

Topography

The topography of the land determines the velocity at which surface runoff will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes (especially those without adequate vegetative cover) are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes. Steeper terrain is also more prone to mudslides, landslides, and other forms of gravitational erosion processes.

Tectonics

Tectonic processes control rates and distributions of erosion at the Earth's surface. If the tectonic action causes part of the Earth's surface (e.g., a mountain range) to be raised or lowered relative to surrounding areas, this must necessarily change the gradient of the land surface. Because erosion rates are almost always sensitive to the local slope (see above), this will change the rates of erosion in the uplifted area. Active tectonics also brings fresh, unweathered rock towards the surface, where it is exposed to the action of erosion.

However, erosion can also affect tectonic processes. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. Because tectonic processes are driven by gradients in the stress field developed in the crust, this unloading can in turn cause tectonic or isostatic uplift in the region. In some cases, it has been hypothesised that these twin feedbacks can act to localize and enhance zones of very rapid exhumation of deep crustal rocks beneath places on the Earth's surface with extremely high erosion rates, for example, beneath the extremely steep terrain of Nanga Parbat in the western Himalayas. Such a place has been called a "tectonic aneurysm".

Development

Human land development, in forms including agricultural and urban development, is considered a significant factor in erosion and sediment transport, which aggravate food insecurity. In Taiwan, increases in sediment load in the northern, central, and southern regions of the island can be tracked with the timeline of development for each region throughout the 20th century. The intentional removal of soil and rock by humans is a form of erosion that has been named lisasion.

Erosion at various scales

Mountain ranges

Mountain ranges are known to take many millions of years to erode to the degree they effectively cease to exist. Scholars Pitman and Golovchenko estimate that it takes probably more than 450 million years to erode a mountain mass similar to the Himalaya into an almost-flat peneplain if there are no major sea-level changes. Erosion of mountains massifs can create a pattern of equally high summits called summit accordance. It has been argued that extension during post-orogenic collapse is a more effective mechanism of lowering the height of orogenic mountains than erosion.

Examples of heavily eroded mountain ranges include the Timanides of Northern Russia. Erosion of this orogen has produced sediments that are now found in the East European Platform, including the Cambrian Sablya Formation near Lake Ladoga. Studies of these sediments indicate that it is likely that the erosion of the orogen began in the Cambrian and then intensified in the Ordovician.

Soils

If the rate of erosion is higher than the rate of soil formation the soils are being destroyed by erosion. Where soil is not destroyed by erosion, erosion can in some cases prevent the formation of soil features that form slowly. Inceptisols are common soils that form in areas of fast erosion.

While erosion of soils is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes) ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the eventual end result is desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems.

In the United States, farmers cultivating highly erodible land must comply with a conservation plan to be eligible for certain forms of agricultural assistance.

Consequences of human-made soil erosion

Butane

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