A tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing rock outcrop that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest. In the South West of England, the term is commonly also used for the hills themselves – particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall. [1]
Although English topographical names often have a Celtic etymology, the Oxford English Dictionary lists no cognates to the Old English word in either the Breton or Cornish languages (the Scottish Gaelic tòrr is thought to derive from the Old English word). It is therefore accepted that the English word Tor derives from the Old Welsh word tẁrr or twr, [2] meaning a cluster or heap. [3]
Tors are landforms created by the erosion and weathering of rock; most commonly granites, but also schists, dacites, dolerites, ignimbrites, [4] coarse sandstones and others. [5] Tors are mostly less than 5 meters (16 ft) high. Many hypotheses have been proposed to explain their origin and this remains a topic of discussion among geologists and geomorphologists, and physical geographers. It is considered likely that tors were created by geomorphic processes that differed widely in type and duration according to regional and local differences in climate and rock types. [1] [6]
For example, the Dartmoor granite was emplaced around 280 million years ago. When the cover rocks eroded away it was exposed to chemical and physical weathering processes. [7] Where joints are closely spaced, the large crystals in the granite readily disintegrate to form a sandy regolith known locally as growan. This is readily stripped off by solifluction or surface wash when not protected by vegetation, notably during prolonged cold phases during the Quaternary ice ages – periglaciation.
Where joints happen to be unusually widely spaced, core blocks can survive and remain above the weathering surface, developing into tors. These can be monolithic, as at Haytor and Blackingstone Rock, but are more usually subdivided into stacks, often arranged in avenues. Each stack may include several tiers or pillows, which may become separated: rocking pillows are called logan stones. These stacks are vulnerable to frost action and often collapse leaving trails of blocks down the slopes called clitter or clatter. Weathering has also given rise to circular "rock basins" formed by the accumulation of water and repeated freezing and thawing. An example is found at Kes Tor on Dartmoor.
Dating of 28 tors on Dartmoor showed that most are surprisingly young, less than 100,000 years of surface exposure, with none over 200,000 years old. [8] They probably emerged at the start of the last major ice age (Devensian). By contrast the Scottish Cairngorms, which is the other classic granite tor concentration in Britain, the oldest tors dated are between 200 and 675 thousand years of exposure, with even glacially-modified ones having dates of 100–150,000 years. [9]
A drumlin, from the Irish word droimnín, first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine. Assemblages of drumlins are referred to as fields or swarms; they can create a landscape which is often described as having a 'basket of eggs topography'.
Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features generated by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform and terrain history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling. Geomorphologists work within disciplines such as physical geography, geology, geodesy, engineering geology, archaeology, climatology, and geotechnical engineering. This broad base of interests contributes to many research styles and interests within the field.
In geomorphology and geology, a peneplain is a low-relief plain formed by protracted erosion. This is the definition in the broadest of terms, albeit with frequency the usage of peneplain is meant to imply the representation of a near-final stage of fluvial erosion during times of extended tectonic stability. Peneplains are sometimes associated with the cycle of erosion theory of William Morris Davis, but Davis and other workers have also used the term in a purely descriptive manner without any theory or particular genesis attached.
An inselberg or monadnock is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa a similar formation of granite is known as a koppie, an Afrikaans word from the Dutch diminutive word kopje. If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, though not all bornhardts are inselbergs. An inselberg results when a body of rock resistant to erosion, such as granite, occurring within a body of softer rocks, is exposed by differential erosion and lowering of the surrounding landscape.
Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite. When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Within saprolite, spheroidal weathering often creates rounded boulders, known as corestones or woolsack, of relatively unweathered rock. Spheroidal weathering is also called onion skin weathering,concentric weathering,spherical weathering, or woolsack weathering.
In glaciology, a roche moutonnée is a rock formation created by the passing of a glacier. The passage of glacial ice over underlying bedrock often results in asymmetric erosional forms as a result of abrasion on the "stoss" (upstream) side of the rock, and plucking on the "lee" (downstream) side. Some geologists limit the term to features on scales of a metre to several hundred metres and refer to larger features as crag and tail, though they are formed in essentially the same way.
The geographic cycle, or cycle of erosion, is an idealized model that explains the development of relief in landscapes. The model starts with the erosion that follows uplift of land above a base level and ends, if conditions allow, in the formation of a peneplain. Landscapes that show evidence of more than one cycle of erosion are termed "polycyclical". The cycle of erosion and some of its associated concepts have, despite their popularity, been a subject of much criticism.
Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the weathering and erosion of pieces of bedrock, especially large "joint blocks". This occurs in a type of glacier called a "valley glacier". As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock. The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice/water phase transition, gradually loosening the rock between the joints. This produces large chunks of rock called joint blocks. Eventually these joint blocks come loose and become trapped in the glacier.
David Leslie Linton was a British geographer and geomorphologist, was professor of geography at Sheffield and Birmingham, best remembered for his work on the landscape development of south-east England with S. W. Wooldridge, and on the development of tors.
Marjorie Mary Sweeting, was a British geomorphologist specializing in karst phenomena. Sweeting had gained extensive knowledge on various topographies and landscapes by travelling around the world to places such as Greece, Australia, Czechoslovakia, United States, Canada, South Africa, Belize, and most notably China. She published Karst Landforms, and Karst in China: its Geomorphology and Environment. The latter is the first comprehensive Western account of China's karst, and one of the first western published works on the karst found within China, despite being a male dominated field.
A weathering rind is a discolored, chemically altered, outer zone or layer of a discrete rock fragment formed by the processes of weathering. The inner boundary of a weathering rind approximately parallels the outer surface of the rock fragment in which it has developed. Rock fragments with weathering rinds normally are discrete clasts, ranging in size from pebbles to cobbles or boulders. They typically occur either lying on the surface of the ground or buried within sediments such as alluvium, colluvium, or glacial till. A weathering rind represents the alteration of the outer portion of a rock by exposure to air or near surface groundwater over a period of time. Typically, a weathering rind may be enriched with either iron or manganese, and silica, and oxidized to a yellowish red to reddish color. Often a weathering rind exhibits multiple bands of differing colors.
In geomorphology, cryoplanation or is a term used to both describe and explain the formation of plains, terraces and pediments in periglacial environments. Uncertainty surrounds the term, and the effectiveness of the cryoplanation process is held to be limited meaning it can only produce small terraces. Instead, many of so-called cryoplanation terraces are likely an expression of the underlying lithology and rock structure rather than being unique products of cold-climate processes.
The Gornaya Shoria megaliths, meaning Mountain Shoria megaliths, are rock formations found within the Mountain Shoria (Russian: Горная Шория) region that comprises the southern part of Kemerovo Oblast in southern Siberia, Russia. The Gornaya Shoria megaliths, also known as the Surak-Kuilum megalithic complex and by other names, form the ridgecrests and summit of Mount Kuylyum (Russian: Горна Куйлюм), 1,203 m (3,947 ft) in elevation. The base of this mountain is located 8 km away from the village of Orton. Mount Kuylyum is part of the Kulyum-Surak granite massif in the Shoria Mountains. The ridgecrests and summits of this massif ranges in elevation between 700 and 1,203 m.
In geology and geomorphology a planation surface is a large-scale land surface that is almost flat with the possible exception of some residual hills. The processes that form planation surfaces are labelled collectively planation and are exogenic. Planation surfaces are planated regardless of bedrock structures. On Earth, they constitute some of the most common landscapes. Geological maps indicate that planation surfaces may comprise 65% of the landscapes on Saturn's largest moon, Titan, which hosts a hydrological cycle of liquid methane. Peneplains and pediplains are types of planation surfaces planated respectively by "peneplanation" and "pediplanation". In addition to these there are planation surfaces proposed to be formed by cryoplanation, marine processes, areal glacial erosion and salt weathering. The term planation surface is often preferred over others because some more specific planation surface types and processes remain controversial. Etchplains are weathered planation surfaces.
An etchplain is a plain where the bedrock has been subject to considerable "etching" or subsurface weathering. Etchplanation is the process forming etchplains. Contrary to what the name might suggest, etchplains are seldom completely flat and usually display some relief, as weathering of the bedrock does not advance uniformly. This means that weathering is unrelated to the flatness which might be derivative of various other processes of planation including peneplanation and pediplanation. Erosion of etchplains can result in the exposure of inselbergs such as bornhardt and tors. Generally the topography exposed at a stripped etchplain, that is an etch surface, after erosion of regolith is one with many irregularities as result of structurally defined areas of rock strength.
In geomorphology a nubbin is a small and gentle hill consisting of a bedrock core dotted with rounded residual blocks. The blocks derive from disintegrated and weathered bedrock layers. In particular it is assumed that the boulders of the nubbins are the remnants of the outer one or two exfoliation shells that weathered underground, albeit some weathering can continue to occur once the boulders are exposed on surface.
Climatic geomorphology is the study of the role of climate in shaping landforms and the earth-surface processes. An approach used in climatic geomorphology is to study relict landforms to infer ancient climates. Being often concerned about past climates climatic geomorphology considered sometimes to be an aspect of historical geology. Since landscape features in one region might have evolved under climates different from those of the present, studying climatically disparate regions might help understand present-day landscapes. For example, Julius Büdel studied both cold-climate processes in Svalbard and weathering processes in tropical India to understand the origin of the relief of Central Europe, which he argued was a palimpsest of landforms formed at different times and under different climates.
Hillslope evolution is the changes in the erosion rates, erosion styles and form of slopes of hills and mountains over time.
The Sub-Mesozoic hilly peneplains or Sub-Mesozoic hilly relief is a landscape in Scandinavia made up of undulating hills and joint valleys and occasional kaolinized bedrock in valley bottoms. The landscape formed in the Mesozoic Era and was eventually drowned by the sea during the Campanian transgression and covered by a thick blanket of Cretaceous sedimentary rocks. Later erosion of the cover rocks partly re-exposed this landscape. During the Quaternary epoch the re-exposed Mesozoic hilly relief escaped major glacier erosion being only surficially scoured in parts.
The geology of national parks in Britain strongly influences the landscape character of each of the fifteen such areas which have been designated. There are ten national parks in England, three in Wales and two in Scotland. Ten of these were established in England and Wales in the 1950s under the provisions of the National Parks and Access to the Countryside Act 1949. With one exception, all of these first ten, together with the two Scottish parks were centred on upland or coastal areas formed from Palaeozoic rocks. The exception is the North York Moors National Park which is formed from sedimentary rocks of Jurassic age.
