Huggett Structural Landforms
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SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 129 (a ) Simple impact structure Sedimentary fill Overturned Overturned Allochthonous fallback breccia flap flap D Fallout ejecta Ap pa da ren dt I t crater mpact melt and melt fragments T Strongly shocked target rocks ru e c rater Fractured and brecciated target rocks Fault 1km ()b Complex impact structure Faulted Breccia and impact Central peak Breccia and impact Faulted rim melt annulus and rings melt annulus rim Figure 5.11 Simple and complex impact structures. new impact sites every year. Researchers have also found horizontal once the sea has retreated or after they have several impact structures in the seafloor. been uplifted form characteristic landforms (Table 5.2). The spatial distribution of terranean impact structures If the beds stay flat and are not dissected by river val- reveals a concentration on the Precambrian shield areas leys, they form large sedimentary plains (sediplains). of North America and Europe (Figure 5.12). This con- Many of the flat riverine plains of the Channel Country, centration reflects the fact that the Precambrian shields in south-western Queensland, Australia, are of this type. North America and Europe have been geologically stable If the beds stay flat but are dissected by river valleys, they for a long time, and that the search for, and study of, form plateaux, plains, and stepped topography (Colour impact craters has been conducted chiefly in those areas. Plate 1, inserted between pages 208 and 209). In sedi- It is not a reflection of the impaction process, which mentary terrain, plateaux are extensive areas of low relief occurs at random over the globe. that sit above surrounding lower land, from which they are isolated by scarps (see Figure 5.16, p. 135). A bed of hard rock called caprock normally crowns them. A LANDFORMS ASSOCIATED mesa or table is a small plateau, but there is no fine WITH FOLDS dividing line between a mesa and a plateau. A butte is a very small plateau, and a mesa becomes a butte Flat beds when the maximum diameter of its flat top is less than Stratified rocks may stay horizontal or they may be its height above the encircling plain. When eventually folded. Sedimentary rocks that remain more or less the caprock is eroded away, a butte may become an 130 STRUCTURE Impact structure diameter (km) 300 200 100 50 10 1 Figure 5.12 The distribution of known impact craters. SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 131 Table 5.2 Landforms associated with sedimentary rocks Formative conditions Landform Description Horizontal beds Not dissected by rivers Sediplain Large sedimentary plain Dissected by rivers with Plateau Extensive flat area formed on caprock, surrounded by thin caprock lower land, and flanked by scarps Mesa or table Small, steep-sided, flat-topped plateau Butte Very small, steep-sided, flat-topped plateau Isolated tower, rounded peak, Residual forms produced when caprock has been jagged hill, domed plateau eroded Stepped scarp A scarp with many bluffs, debris slopes, and structural benches Ribbed scarp A stepped scarp developed in thin-bedded strata Debris slope A slope cut in bedrock lying beneath the bluff and covered with a sometimes patchy veneer of debris from it Dissected by rivers with Bluffs, often with peculiar Straight bluffs breached only by major rivers. thick caprock weathering patterns Weathering patterns include elephant skin weathering, crocodile skin weathering, fretted surfaces, tafoni, large hollows at the bluff base Folded beds Primary folds at various Anticlinal hills or Jura-type Folded surfaces that directly mirror the underlying stages of erosion relief geological structures Inverted relief Structural lows occupy high areas (e.g. a perched syncline) and structural highs low areas (e.g. an anticlinal valley) Planated relief Highly eroded folds Appalachian-type relief Planated relief that is uplifted and dissected, leaving vestiges of the plains high in the relief Differential erosion of Ridge and valley topography Terrain with ridges and valleys generally following the folded sedimentary strike of the beds and so the pattern of folding sequences (includes breached anticlines and domes) Cuesta Ridge formed in gently dipping strata with an asymmetrical cross-section of escarpment and dip-slope Homoclinal ridge or strike Ridge formed in moderately dipping strata with just ridge about asymmetrical cross-section Hogback Ridge formed in steeply dipping strata with symmetrical cross-section Escarpment (scarp face, scarp The side of a ridge that cuts across the strata. Picks out slope) lithological variations in the strata Dip-slope The side of a ridge that accords with the dip of the strata Flatiron (revet crag) A roughly triangular facet produced by regularly spaced streams eating into a dip-slope or ridge (especially a cuesta or homoclinal ridge) Source: Partly after discussion in Twidale and Campbell (1993, 187–211) 132 STRUCTURE isolated tower, a jagged peak, or a rounded hill, depend- from other directions. Domes are also termed periclines. ing on the caprock thickness. In stepped topography, An example is the Chaldon pericline in Dorset, England, scarps display a sequence of structural benches, produced in which rings of progressively younger rocks – Wealden by harder beds, and steep bluffs where softer beds have Beds, Upper Greensand, and Chalk – outcrop around been eaten away (see Colour Plate 10, inserted between a core of Upper Jurassic Portland and Purbeck beds. pages 208 and 209). Domed structures also form where the crust is thrust upwards, although these forms are usually simpler than those formed by more complex pressure distributions. Folded beds Domes are found, too, where plugs of light material, Anticlines are arches in strata, while synclines are such as salt, rise through the overlying strata as diapirs. troughs (Figure 5.13). In recumbent anticlines, the beds Folds may be symmetrical or asymmetrical, open or are folded over. Isoclinal folding occurs where a series tight, simple or complex. Relief formed directly by folds is of overfolds are arranged such that their limbs dip in rare, but some anticlinal hills do exist. The 11-km-long the same direction. Monoclines are the simple folds in Mount Stewart–Halcombe anticline near Wellington, which beds are flexed from one level to another. An New Zealand, is formed in Late Pleistocene sediments example is the Isle of Wight monocline, England, which of the coastal plain. It has an even crest, the surfaces of runs from east to west across the island with Creta- both its flanks run parallel to the dip of the underlying ceous rocks sitting at a lower level to the north than beds (Box 5.1), and its arched surface replicates the fold to the south. In nearly all cases, monoclines are very (Ollier 1981, 59). Even anticlinal hills exposed by ero- asymmetrical anticlines with much elongated arch and sion are not that common, although many anticlinal hills trough limbs. Anticlines, monoclines, and synclines form in the Jura Mountains remain barely breached by rivers. through shearing or tangential or lateral pressures applied The commonest landforms connected with folding to sedimentary rocks. Domes, which may be regarded as are breached anticlines and breached domes. This is double anticlines, and basins, which may be regarded because, once exposed, the crest of an anticline (or the as double synclines, are formed if additional forces come top of a dome) is subject to erosion. The strike ridges on ()a Anticline (b ) Syncline ()c Asymmetrical folding (d ) Monocline (e ) Isoclines (f ) Recumbent fold ()Domeg ()h Basin Figure 5.13 Structures formed in folded strata. SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 133 Box 5.1 DIP, STRIKE, AND PLUNGE In tilted beds, the bedding planes are said to dip. The An anticlinal axis that is tilted is said to pitch or dip or true dip of a bed is given as the maximum angle plunge (Figure 5.14b). The angle of plunge is the between the bed and the horizontal (Figure 5.14a). angle between the anticlinal axis and a horizontal plane. The strike is the direction at right angles to the dip Plunging anticlines can be thought of as elongated measured as an azimuth (compass direction) in the domes. Synclinal axes may also plunge. horizontal plane. ()a ()b Strike Plunge Direction of dip Angle of dip Figure 5.14 Terms relating to sedimentary structures. (a) Dip and strike. (b) Plunge. each side tend to be archetypal dip and scarp slopes, with beds dipping gently, perhaps up to 5 degrees. They a typical drainage pattern, and between the streams that are asymmetrical forms characterized by an escarpment cross the strike the dipping strata have the characteris- or scarp, which normally forms steep slopes of cliffs, tic forms of flatirons, which are triangular facets with crowned by more resistant beds, and a dip slope, which their bases parallel to the strike and their apices pointing runs along the dip of the strata. Homoclinal ridges,or up the dip of the rock. The strike ridges are very long strike ridges, are only just asymmetrical and develop where the folds are horizontal, but they form concentric in more steeply tilted strata with a dip between 10 rings where the folds form a dome. The scarp and vale and 30 degrees. Hogbacks are symmetrical forms that sequence of the Kentish Weald, England, is a classic case develop where the strata dip very steeply at 40 degrees of a breached anticline (Figure 5.15). Strike ridges may plus. They are named after the Hog’s Back, a ridge of surround structural basins, with the flatirons pointing in almost vertically dipping chalk in the North Downs, the opposite direction. England. Where strata of differing resistance are inclined over On a larger scale, large warps in the ground surface a broad area, several landforms develop according to form major swells about 1,000 km across.