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The landforms of the study area have been divided into the following groups occupying the base of the morphologica3. maps (figs. 12a, 13a, 14a and 15a):
(a) Denudational landforms which include features such as ridges, residual hills, inselbergs, cuestas and tablelands) that have been eroded in solid rocks into dissected regions such as uplands and mountain ranges;
(b) Fluvio-denudational landforms that have been created by denudational mass movements and/or sheet wash and other fluvial agents (such forms
include pediments and erosional teri'aces);
(c) ffluvial depositlonal landforms that have been developed by the activity of running water such as wadi beds, alluvial accumulation terraces, and alluvial fans, and
(d) Eolian landforms which have been developed by wind action such as sand dunes, sand sheets, and blow-out hollows.
Individual landform features such as breaks of slope, water courses, fault lines, crest lines, summits, and fault line scarps are presented in separate sheets (overlays for figs. 12a, 13a, 14a, 15a, and underlays for figs. 12b, 13b, 14b and 15b).
The morpho-lithological units have been presented in separate sheets (figs. 12b, 13b, 14b and 15b) in the following manner according to the categories above:
(a) denudational units that include the denudational landforms as distributed in lithological-geological areal distribution,
(b) fluvio-denudational units which are the same as those in figures 12a, 13a,/
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13a, 14a and 15a,
(o) alluvial units that include certain lithological information, and (d) eolian units which in addition to the morphological units include features such as ergs, regs and hamadas.
Abbreviated chronological data are presented in the morpho-lithological maps (figs.12a, 13a, 14a and 15a). The abbreviation used has been derived from the geological maps and modified to denote the geological period and epoch.
Interpretation and the ne^l for landform classification
Interpretation: For any practical purpose, it has been argued that the maps of the landforms should be relatively easy to read, interpret, and apply; that such maps must provide short but expressive identifications to satisfy the requirements of simplicity and allow small areas to be properly mapped, and that the maps should include enough information concerning an area to allow some evaluation of the geomorphological
characteristics of that area qualitatively and quantitatively. Such maps should produce areal units that can also be evaluated for assessments and other practical purposes. The geomorphological maps of the study area do not satisfy these requirements. They are difficult to read for non specialists, require skill to interpret, and perhaps may not be easy to apply. The reason is that such maps have been designed to give sufficient information about the geomorphology of the study area to those who are very much interested in such information. The maps include details that are necessary/
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necessary for other practical fields so that they need a specialist to
read and interpret* Contrary to other landform maps the legend of the geomorphological maps is concerned vzith the characterization of the
morphogenesis, morphology and sedimentation of the study area. That is to say that the maps provide much more information than needed for many
practical purposes. The maps are also concerned with the morphochro no logical <
data that are needed for the evaluation of the development of the landforms
in time. This is another fact that serves the special purpose of these maps rather than other practical purposes. The essential reason for the development of the geomorphological maps of the study area is, however,
that the landforms are characterized hy different environments. The effect of such factors as rock types, tectonic movements, type of processes,
topographic positions, profile developments, and other geological factors
causes internal and external modifications that caused significant differences^ in the morphological character!sties of these landforms. It is very clear that the geomorphological maps will not facilitate the practical usage of the maps by non-specialist. However, such difficulties can he largely overcome hy other methods of mapping such as the landform classification method. ■ ■■ftft
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si •WThe need for the landform classification method: It has been stated
earlier in this chapter that the first step in outlining .the geomorphological tgl
maps was the recognition, identification, and mapping the major landform
systems or units. This facilitates the idea of using the Land System Classification method for landform classification and mapping. A method
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that/
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that has been successfully used by other v/orkers (see chapter 3) in the field of applied geomorphology. This method 'is discussed in the following chapter and has been made the basis of summary maps (figs. 16 - 20 and tables 9 - ll) which are derived from the geomorphological maps of the study area (figs. 12a - 15b).
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PART ONE : GEOMOItPHOLOGICAL MAPPING LANDFOHM CLASSIFICATION
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LANDFOHM CLASSIFICATION
?£ * ;f«SB ■.. a Land evaluation, land classification and the land system approachInterest in the uses of the land, both actual and potential, characterize^
the work of several groups and organizations. Among these are: the Division . -■ ;Wf
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of Land Research and Regional Survey of the Commonwealth, Scientific and
,^'’S Industrial Research Organization of Australia (C.S.I.R.O.), the Land Resources:^ Division of the Directorate of Overseas Surveys (DOS), the Military
Engineering Experimental Establishment (MEXE) and the joint group known as MSXE-OXFORD CROUD (see Beckett & Webster, 1965a).
The attempt to classify the land (land classification, including land evaluation) was made to establish scientific research and to develop techniques that bridge the gap between methods of detailed field analysis and methods of reconnaissance in order to meet the requirements of the land utilization and/or planning programmes launched for immediate development.
The term land classification is used to identify and record the land (for definition of land, see Christian and Stev/art 1968, Mabbutt 1968)
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characteristics which are of potential significance to man such ass landforms, W- *5 •< _ i»£^s •■D-i
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soil, vegetation and water and to map the area of occurrence. Hence the land system approach is to classify the information obtained about a particular area and to store and/or use this information for evaluation.
The mapping unit of the C.S.I.R.O. is the land system which is an area with a recurring pattern of landforms, soils and vegetation, mappable from aerial photographs, The role of geomorphology in providing the physical basis/
basis for land system mapping is evident throughout the reports of the C.S.I.R.O., the DOS and the MEXE-OXFORD groups* It appears to be
that the basis for land system mapping is the pattern of the landscape which . is made up of groupings of the landforms, soils and vegetation. If this is the case then in arid lands, where vegetation is scarce and the soil is poorly developed, the land classification based on landform and sometimes soil patterns should be regarded as comparable to those of the tropics (which include vegetation). It follows that the landform classification would provide adequate information for integrated surveys of the desert terrain aiming at practical application. Since the main grouping of the land system here will be the physiographic or the geomorphic units, it has been designed to take into consideration the scheme of the joint group
(MEXE-OXFORD) and the Cambridge Study group for desert land classification (see Hughes, et al., l%5s Perrin, et al., 1%9)*
The need for classification here is to illustrate the relationship between different landforms in the study area. One should admit, however, that even when attempting a land evaluation scheme it requires a team of scientists rather than one geomorphologist. It should be noted here that the landform classification for the study area is presented for the special purpose rather than the general purpose (which is practical). Nevertheless the scheme could be used for more practical programmes as the necessary preliminary information. One could add that the question of landform classification arises when dealing with morphological mapping from aerial photographs.
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rapid geomorphological analysis of the elements of the whole region (playing an important role in its development) then the use of aerial
photographs should he included, whenever possible, in any mapping procedure designed for such purposes. This is because of many factors (mentioned before) involved in minimizing and economizing the efforts towards
regional development.
Landf orm cl as s if i c at i on
Dividing the landforms into units has been the concern of several
authors (see Bourne 1931? Hudson 1936? Linton 1951? Clark 1957? Lueder 1959? Beckett and Webster 1965, Hughes, et al., 1965? Savigear 1965? Mitchell,
et al., 1966, Perrin, et al., 1969a, Thomas 1969? Young 1969? and Mitchell 1971). Although most authors claim a practical definition and hierarchical ability of their classification, the problem remains controversial. In fact, there are three major factors contributing a great deal to the controversy. These factors are:
1. The validity of the conclusions derived from small areas concerning larger regions, provinces and zones in terms of prediction, uniformity, similarity and differentiation.
2. The validity and the existence of the units in nature according to their defined boundaries.
3. The scale in which the defined units are mappable with minimum costs to fulfill the map content and its use for other purposes.
The unit to be accepted is an area where the climate, landforms, soil and vegetation lie within the limits significant for a particular form of land/
land use, and should he sufficiently uniform in its physical property to enable a farmer to menage the whole extent of one facet in the same way (Beckett and Webster 19&5, see also Gibbons, et a'l. 1964). The
complexity of the land and its gradational properties have been simplified. This is because the classification of the landforms is always subjective
and boundaries have to be drawn somewhere. However, the landform classification is less complicated and more applicable than the land evaluation which involves other factors such as soil and vegetation.
It could be argued that the landform classification is a systematic arrangement of different kinds of landforms according to those properties that determine their size, shape and texture in order to establish
similarities and differences. Therefore, it should provide the preliminary boundaries for those more extensive units which include other factors in order to determine the land capability for permanent sustained production.
If the site (Linton 1951), the unit landform (Lueder 1957), the morphoIogioal unit (Savigear 19&5), the land element (Brinks et al. 1966) or the relief unit (Young 1969) indicate certain uniformity among the small scale landforms, the major landform units vary even within one area to such an extent that no strict rules can be applied (see Linton 1951, P* 215, and Thomas 19&9, P* 115)•
In dealing with landform classification it is not always so easy to formulate an hierarchy of the landform units. This is because of the complexity and the nature of certain types of landforms. It 5 s as Thomas (1969) pointed out ’’certain landforms occur’ more often as complexes than as simple/
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simple unitsan example of these landform complexes is ”... a two (or more) cycle-river valley .In fact, it could be argued that a pediment or a bahada with its size and sometimes indefinite boundaries and surface texture cannot be regarded as a simple mappable landform unit because a bahada for example is in itself a combination of smaller units and a pediment might be glacis or structural form. First because of the scale problem and second because it would not be manageable in the same way as the simple unit. It is, therefore, more acceptable to use the term landform complex which was introduced by Thomas (1969, pp. 118-119) for those units which ha/e a more complicated nature (see chs. 5 and 6).
It should be added here that the landform complexes (or the landform complex types as it would be used in the landform classification of some parts of Central Saudi Arabia) could be used to indicate those landforms which are comparable in the way that;
a) - they should have been developed during the same period of time, b) - they should have morphological similarity, and
c) - they might have a similar genesis and d) - a similar constituent material.
Therefore, these units should be distinguished on the strength of
morphological consideration and not on the basis of topography, lithology combined together. This is because a landform complex may be the result of the operation of two or more climatic sequences acting on a particular lithology during a period in which that particular form or farms has been
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The scale problem
It is obvious that the smaller the scale of the units to he mapped the greater the number of forms has to be generalized (or vice versa). It follows that the map’s content is dependant on the scale to a large extent. The scale is also important for the purpose of the map in terms of the degree of precision needed in the map. For regional purposes, for example, a scale between 1:250,000 and 1:50,000 is sufficient for indicating the general direction of development. But when the need is for detailed information in terms of land capability the upper limit of the scale becomes inadequate. Furthermore, using the upper limit it is very difficult, in practice, to show every potential unit specially when size and shape plays an important role. The problem of classification within the above mentioned scale limits is which class of the landform
should be within the upper limit and which one falls into the lower limit. If the aim is to classify the landforms in terms of their morphogenetic elements, then the complicated nature of such forms would result in a great variety and number of morphogenetic combinations, so that when represented it has to be generalized with either different or combined forms.
In spite of all the difficulties, the medium scale (1:250,000 -
1:50,000) is sufficient to show the major mappable units (see tables 9 - ll) of the desert landform. This is only applicable when the preliminary work has been done on the scale 1:50,000 to 1:80,000. Then the final map could be reduced to the desired scale. For our purpose the range of scale between 1:250,000 for landform systems and 1:100,000 for the landform complex types seems/
: . (1-60) ' '
seems to be reasonable. It must be considered, here that the availability of material has so far determined the scale (these are; aerial photographs at scale approximately 1:60,000, photo mosaics at scale 1:50,000 and
geological and topographical maps at scale 1:500,000).
It could be argued that although the smaller scale maps are more
readily appreciated in handling, it is more practical to produce these maps on a medium scale. It is because such ma,ps would be used for more
detailed work and by other interested groups in planning and development (e.g. for underground water), Future work, however, should bridge this gap in terms of maps for general, applied and special purposes.
landform Classification for the Study Area
Hughes and others (1965) have classified the terrain in an evaluation scheme where they include examples from various parts of the arid lands. Mitchell and Perrin (1967) subdivided the deserts of the world into
physiographic units. Perrin and Mitchell (1969a), attempted ”to evaluate the extent to which a new physiographic classification of landscape can be used as the basis for the storage of practical information about terrain from which predictions can be made about conditions at unvisited sites”,
(p. l). Mitchell (1971) claims that in testing the ’‘practical value of facets by assessing their internal homogeneity and mutual distinctiveness over this whole climatic zone...”, the facets were found ’’adequately
homogeneous and mutually distinct to be useful units for sorting scientific and land-use data within single land systems...” (p. 69). But he also found that ’’over wider expanses of territory...” the value of the facets ’’diminished/
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"diminished io unacceptably low levels for most purposes” (p. 69)* In applying this system for the landform classification of the study area it has been found that although it could be applied anywhere in arid lands it has to be modified according to the local variation, complexity and scale.
The classification of the landforms for the study area has been developed as a result of the aerial photographic interpretation and the morphological mapping which is the subject of the previous chapter. It
seems that the most valuable method of identifying landform units and for classifying them is the use of aerial photographic interpretation. This is not only because such a technique reveals more information about the land surface (such as micro-relief, drainage pattern, etc.), but also because it shov/s clearly the relationship between all facets involved .in designating the landscape. As a result a visual comparison (which will lead later to detailed analysis) between the units can be made.
It should be noted here that when classifying the landforms one should pay careful attention to the micro-relief and the morphologically discordant lines. These lines have been delineated on aerial photographs first and identified and named later by a synthesis of morphological characteristics. This should be the case when it was stated that * a landform unit is the
result of the operation of climate upon a particular lithology in which such forms have been developed1.
If we accept the argument which defines the smallest mapping unit as the genetically homogeneous surface which is geometrically a simple area which does not contain any breaks of slope, then we would exclude some parts of the arid lands where some of the elements are not simple. It has been argued/
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argued, that even the genetically homogeneous surface, no matter how simple or complicated, is the result of complex processes acting in certain directions. It follows that by a landform complex type we delineate a complex of forms in a more or less distinctly limited area.
Unlike other units it is not a concrete unit but an abstract unit
established in a, deductive way. Accordingly, this type of landform may be divided into two groups according to their inclination. The first group are the highly inclined surfaces and the second are the flat surfaces.
It has been found more practical and convenient to set up the
morphological regions first and complete the geomorphological maps, then work out the subdivision of these regions from larger to smaller units with the aid of the semi-detailed geomorphological maps and field work. The degree of the division of these units will depend upon the degree of
the relief development. It is always easier to determine the accumulation surfaces (which are mostly the simple units) than to group the erosion- denudation surfaces (which are mostly the complex units).
The maps shown in figures 12b, 13b, 14b, 15b and 16-20 have been constructed according to the foregoing concepts and compiled from aerial