Dune in the context of Sandstone

A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the land surface of the Earth is arid or semi-arid. This includes much of the polar regions, where little precipitation occurs, and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.

Deserts are formed by weathering processes as large variations in temperature between day and night strain the rocks, which consequently break into pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter, and the resulting fragments and rubble strewn over the desert floor are further eroded by the wind. This picks up particles of sand and dust, which can remain airborne for extended periods – sometimes causing the formation of sand storms or dust storms. Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones, often forming desert pavements, and little further erosion occurs. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground water sources in the form of springs and seepages from aquifers. Where these are found, oases can occur.

At a flat coast or flat shoreline, the land descends gradually into the sea. Flat coasts can be formed either as a result of the sea advancing into gently sloping terrain or through the abrasion of loose rock. They may be basically divided into two parallel strips: the shoreface and the beach.

Flat coasts consist of loose material such as sand and gravel. Wind transports finer grains of sand inland over the dunes. The sea washes pebbles and sand away from the coast and dumps it at other locations.

Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.

The composition of sand varies, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide, or SiO₂), usually in the form of quartz.

Barrier islands are a coastal landform, a type of dune system and sand island, where an area of sand off the coast has been formed by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen, and are subject to change during storms and other action. They protect coastlines by absorbing energy, and create areas of protected waters where wetlands may flourish. A barrier chain may extend for hundreds of kilometers, with islands periodically separated by tidal inlets. The longest barrier island in the world is Padre Island of Texas, United States, at 113 miles (182 km) long. Sometimes an important inlet may close permanently, transforming an island into a barrier peninsula, often including a barrier beach. Though many are long and narrow, the length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls. The amount of vegetation on the barrier has a large impact on the height and evolution of the island.

There are chains of barrier islands along approximately 13 to 15% of the world's coastlines. They display different settings, suggesting that they can form and be maintained in a variety of environments. Numerous theories have been proposed to explain their formation.

In geography and geology, fluvial sediment processes or fluvial sediment transport are associated with rivers and streams and the deposits and landforms created by sediments. It can result in the formation of ripples and dunes, in fractal-shaped patterns of erosion, in complex patterns of natural river systems, and in the development of floodplains and the occurrence of flash floods. Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity. In typical rivers the largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders.When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used, as in periglacial flows and glacial lake outburst floods. Fluvial sediment processes include the motion of sediment and erosion or deposition on the river bed.

The Curonian Spit, sometimes called Courish Split (Lithuanian: Kuršių nerija; Russian: Ку́ршская коса́ / Kurshskaya kosa, German: Kurische Nehrung), is a 98-kilometre (61 mi) long, thin, curved sand-dune spit that separates the Curonian Lagoon from the Baltic Sea. It is a UNESCO World Heritage Site shared by Lithuania and Russia. Its southern portion lies within Kaliningrad Oblast of Russia, and its northern within southwestern Klaipėda County of Lithuania.

Coexistence is the property of things existing at the same time and in a proximity close enough to affect each other, without causing harm to one another. The term is often used with respect to people of different persuasions existing together, particularly where there is some history of antipathy or violence between those groups.

Coexistence can be observed to a property of all systems in which different aspects capable of interacting with each other exist at the same time. As one source asserts, "even at the molecular level, existence is always already coexistence". Nonliving things can also be characterized as coexisting where multiple kinds of such things exist in the same space, with the term having been used for things as disparate as different kinds of dunes on Mars, and black holes existing in the same region of space as dense nuclear star clusters. Other examples of coexistence include:

An erg (also sand sea or dune sea, or sand sheet if it lacks dunes) is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The word is derived from the Arabic word ʿirq (عرق), meaning 'dune field'. Strictly speaking, an erg is defined as a desert area that contains more than 125 km (48 sq mi) of aeolian or wind-blown sand and where sand covers more than 20% of the surface. Smaller areas are known as "dune fields". The largest hot desert in the world, the Sahara, covers 9 million square kilometres (3.5×10^ sq mi) and contains several ergs, such as the Chech Erg and the Issaouane Erg in Algeria. Approximately 85% of all the Earth's mobile sand is found in ergs that are greater than 32,000 km (12,355 sq mi), the largest being the Rub' al Khali, the Empty Quarter of the Arabian Peninsula. Ergs are also found on other celestial bodies, such as Venus, Mars, and Saturn's moon Titan.

The Great Sand Sea is an approximately 72,000 km (28,000 sq mi) sand desert (erg) in the Sahara that stretches from western Egypt to eastern Libya, in North Africa. Most of the area is covered by sand dunes.

Maranhão (Brazilian Portuguese pronunciation: [maɾɐˈɲɐ̃w] ) is a state in Brazil. Located in the country's Northeast Region, it has a population of about 7 million and an area of 332,000 km (128,000 sq mi) and it is divided into 217 municipalities. Clockwise from north, it borders on the Atlantic Ocean for 2,243 km and the states of Piauí, Tocantins and Pará. The people of Maranhão have a distinctive accent within the common Northeastern Brazilian dialect. Maranhão is described in literary works such as Exile Song by Gonçalves Dias and Casa de Pensão by Aluísio Azevedo.

The dunes of Lençóis are an important area of environmental preservation. Also of interest is the state capital of São Luís, which is a UNESCO World Heritage Site. Another important conservation area is the Parnaíba River delta, between the states of Maranhão and Piauí, with its lagoons, desert dunes and deserted beaches or islands, such as Caju island, which shelters rare birds.

The African humid period (AHP; also known by other names) was a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, trees and lakes was caused by changes in the Earth's axial tilt, changes in vegetation and dust in the Sahara which strengthened the African monsoon, and increased greenhouse gases.During the preceding Last Glacial Maximum, the Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, and its lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of Heinrich event 1, simultaneously to the Bølling–Allerød warming. Rivers and lakes such as Lake Chad formed or expanded, glaciers grew on Mount Kilimanjaro and the Sahara retreated. Two major dry fluctuations occurred; during the Younger Dryas and the short 8.2 kiloyear event. The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the Sahel, Arabia and East Africa, the end of the period appears to have taken place in several steps, such as the 4.2-kiloyear event.

The AHP led to a widespread settlement of the Sahara and the Arabian Desert, and had a profound effect on African cultures, such as the birth of the Ancient Egyptian civilization. People in the Sahara lived as hunter-gatherers and domesticated cattle, goats and sheep. They left archaeological sites and artifacts such as one of the oldest ships in the world, and rock paintings such as those in the Cave of Swimmers and in the Acacus Mountains. Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. When the period ended, humans gradually abandoned the desert in favour of regions with more secure water supplies, such as the Nile Valley and Mesopotamia, where they gave rise to early complex societies.

Death Valley National Park is a national park of the United States that straddles the California–Nevada border, east of the Sierra Nevada. The park boundaries include Death Valley, the northern section of Panamint Valley, the southern section of Eureka Valley and most of Saline Valley.

The park occupies an interface zone between the arid Great Basin and Mojave deserts, protecting the northwest corner of the Mojave Desert and its diverse environment of salt-flats, sand dunes, badlands, valleys, canyons and mountains.

The Nafud desert or simply The Nafud (Arabic: صحراء النفود, romanized: ṣahrā' an-nafūd) is a desert in the northern part of the Arabian Peninsula at 28°18′N 41°00′E / 28.30°N 41.00°E / 28.30; 41.00, occupying a great oval depression. It is 290 kilometres (180 mi) long and 225 kilometres (140 mi) wide, with an area of 65,000 square kilometres (25,000 sq mi).

The Nafud is an erg, a desert region located in north-central Saudi Arabia. It is noted for its sudden violent winds, which account for the large crescent-shaped dunes. The sand in the Nafud is a brick-reddish color. Rain comes once or twice per year. In some lowland areas, namely those near the Hejaz Mountains, there are oases where dates, vegetables, barley, and fruits are grown. The Nafud is connected to the Rub' al Khali by the Dahna, a corridor of gravel plains and sand dunes, 800 mi (1,287 km) long and 15 to 50 miles (24.1 to 80.5 km) wide.

The Ramlat al-Sab'atayn (Arabic: رملة السبعتين, lit. 'Sand of the Two Sevens') is a desert region that corresponds with the northern deserts of modern Yemen (Al-Jawf, Marib, Shabwah governorates) and southwestern Saudi Arabia (Najran province).

It comprises mainly transverse and seif dunes and covers an area of about 100 by 240 kilometres (60 by 150 mi), roughly 26,000 square kilometres (10,000 sq mi).