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Landforms are specific physical features of the land, located in a particular place. Landform types are classes of landforms. Thus, Italy, the Italian peninsula, is a landform, and peninsula is a landform type. Crucially, landforms and landform types are almost never precise. At best, they are heuristic identifications, based on informed judgement. Even when we can define a landform or type based on key differences, of elevation for example, the definition is seldom better than vague and thus the boundaries of the landforms under that definition may not be clearly defined. Any such method is of course subject to the effects of conventionality.

Perhaps the only precise landform types are those dealing with surface waterlandmasses and drainage basins. Land-water boundaries may be imprecise below a certain level of measurement. If we try to place the coastline within meters, we will generally fail. But if we are dealing with larger scales, including the global scale, and are content with a coastline placed within kilometers, the uncertainty is not a factor.

Nonetheless, the identification of landforms, in tradition or heuristically, is normal and useful. The shape of the land and its elevation have a direct impact on human life and on those elements of nature that we most interact with; they determine where we can live, what activities we can engage in, and our access to resources. While landforms often blend into each other, we can identify large and important differences between the central areas and average conditions of adjoining landforms.

Landforms are not determined by climate; a dry valley and a humid valley are both valleys, and we could even speak of types of valleys without resort to non-topographical facts. It is true, though, that other conditions can shape landforms, as canyons are carved by rivers, for example. Furthermore, landforms can determine local climate, as climates are colder at elevation, and the shape of the land changes the circulation of water and air and thus the heat they carry.

Landform types determined by elevation or relative elevation include the broad types highland and lowland, either of which can be defined in absolute elevation or in elevation relative to surrounding areas. More specific elevation landform types include:
mountain — a high point, or highly-localized highland. Applied only to the highest group.
hill — a high point, or highly-localized highland, shorter than a mountain.
ridge — a linear highland.
mountain range — a highland region comprising multiple mountains and/or mountain ridges
plateau — a flat highland region
plain — a flat lowland region
basin — a relative low region surrounded by ridges or highlands (the generic meaning of ‘basin’ is “bowl”).
valley — a relative low region defined by surrounding ridges or highlands (but smaller than a basin), and usually linear, and thus functioning, by nature, as a drainage path. The term is sometimes simply the radial region of a drainage path.
drainage basin — a basin from which precipitation flows into a particular drainage path or settlement basin (see Surface water). By definition this includes the inner slopes of the watersheds determining its borders; a drainage basin is thus a mix of lowlands and highlands.
floodplain — a flat lowland region adjacent to a drainage path, flooded during a high-water event.

Landform types determined by the land-water border:
landmass — a continuous area of land surrounded entirely by water.
island — any landmass other than those conventionally designated continents or comprising multiple designated continents.
peninsula — an region of a larger landmass mostly surrounded by water.
isthmus — a narrow area of land joining two larger areas of land.

Humans tend to live in lowlands, provided they are not arid (like, for example, the Tarim Basin, or the Great Basin of North America). Hence we see major concentrations of population in the Indo-Gangetic Plain, the North China Plain, Sichuan (the basin, that is), the Ferghana Valley, the Po-Venetian lowland, Burma (the Irrawaddy Valley), the Chao Phraya lowland, and the Red River-Hanoi lowland. In North America, these lowland concentrations exist mainly along the Cordillera (see below): the Valley of México, the Los Angeles Basin and adjacent San Fernando Valley, the Salish Sea lowland, the Willamette Valley, the California Central Valley, and the Snake River Plain; the Great Valley of Appalachia is an exception, while in the rest of (non-desert) North America, the rural population density is much greater.

The identification of individual highland ranges is typically a matter of tradition and convention; the American Cordillera is perhaps best thought of as a single range extending the entire length of the New World, including the Rockies, the Coastal Ranges, and the Andes. While highlands are generally less populous, there are some, such as the Plateau of Iran and the South American Altiplano, with significant populations. Often peoples will specialize in highland habitation. Speakers of Iranian dialects — the Persians, Kurds, Pashtuns, and Baluchis — occupy the Iranian plateau and surrounding highlands; the adjacent lowlands to the west are inhabited by Arabs, to the east by the Indo-Aryans, and in each case the border between peoples is approximately the topographical transition. The Tibetans have made their home in a very high and inhospitable peninsula, Tibet, and until recently occupied it almost exclusively.

Among the peninsulas that are typically identified are the Malay peninsula, Anatolia, Iberia, the Balkan peninsula (with Greece as a peninsula on that peninsula), Arabia, the Horn of Africa, Baja California, the Yucatan, and Labrador. Alaska as a whole is a peninsula, but the expression ‘Alaska Peninsula’ generally applies to a smaller peninsula extending from its south. Two landforms that could be called peninsulas have, for cultural reasons, been classified as something more important: Europe, which has been called a continent, and India, which is “the Subcontinent”, a term used almost exclusively for India. The imprecision of the peninsula concept is such that it requires an arbitrary quantitative line; a peninsula must be notably smaller than the remainder of its landmass, which is why we wouldn’t identify as a peninsula all of Sulawesi aside from Minahasa. That reasoning, and that arbitrary distinction, would apply as well to Africa and North and South America.

The existence of an isthmus is the apparent justification for the divisions of the Old World and New World landmasses into multiple continents. It is possible to define an isthmus empirically; if we take the curve of all possible distances across a landmass, measured from sea to sea and weighted by (divided by) distance along the coast, an isthmus exists where there is a local minimum of the shortest overland distance divided by coastal distance, which for round “continents” would be (2r sin(θ/2))/θ. But in the area of the Sinai in the Old World, for example, this comes not to one nadir but two, making the choice between them arbitrary.



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