David K. C. Jones

Large scale geomorphological mapping and Highway engineering design

Summary Geomorphological mapping in site investigations for highway engineering is proving to be a rapid means of obtaining much relevant information about ground conditions. Such mapping requires the recognition of both the origin of surface features and the geological processes that still influence them. In addition information about materials is recorded and inferences made about their extent. The paper identifies eight aims of a geomorphological survey for highway engineering and discusses these in the context of small-scale maps and large-scale plans. The latter are illustrated by case studies from Nepal and South Wales. The review identifies the established applications of geomorphological mapping to site investigation and concludes that further work can usefully be directed towards a broadening of the geomorphological content of site investigation.

The morphology of degraded landslide slopes in South West Dorset

Summary Geomorphological techniques, including morphological mapping and profiling have been used to investigate the distribution of old landslides on the southern slopes of the Char valley in West Dorset. It is shown that these field techniques provide data which can be usefully employed to precede and supplement geotechnical investigations. The spatial pattern of landslide morphology is described and a tentative chronology suggested. The survey reveals a similar, although more extensive and complete, pattern of landslide degradation to that reported for slides on London Clay and the Lias Clay of the East Midlands.

Rapid geomorphological assessments for engineering

Summary Geomorphological mapping is being used increasingly in engineering projects (especially at the reconnaissance and site-investigation stages) as a rapid, highly cost-effective means of assessing both potential hazards and resources. This is normally achieved through a field mapping programme which is dependent on the availability of aerial photographs, and which involves the correct interpretation of landforms as to their origins, material composition, and associated present-day geomorphological processes. The results of such a survey need to be presented in a form which is of direct use to an engineering geologist and/or engineer. This is best achieved, in most cases, by the production of problem or site-orientated maps derived from the primary geomorphological survey. The case studies, drawn from several investigations in arid land environments, were made by a small team of (3-5) geomorphologists and geologists, working for short but concentrated periods of time in the field. A search for fine aggregates in Bahrain was closely associated with the identification and mapping of contributing drainage networks and source areas for alluvial fan deposits. Thelocation of trial pits and boreholes at a site in Dubai was rationalised in terms of a reconnaissance map of both landforms and their constituent materials; while at Suez, extrapolation between existing trial pit and borehole sites was made possible from the geomorphological mapping of landform boundaries. Hazards to engineering from flooding (at Suez) and dune migration (in Dubai) were identified, assessed, and mapped on the basis of a knowledge of their place in the overall geomorphological system.

The Character and Possible Origins of the Aeolian Sediments of the Wahiba Sand Sea, Oman

The Wahiba sand sea covers some 12 500 square kilometres and contains a great variety of dune forms. There is evidence of at least three major periods of dune-building. The earliest deposits form the largest field of aeolianite yet reported, and these are covered by two types of unlithified sands: ancient megaridges with reddish sand; and presently very active, pale sands. Observations of sand colour on the ground and on satellite imagery lead to the hypothesis that the dune sands are derived primarily from pale coastal sands and secondarily from dark wadi sediments in the north-east and centre-west. In the north-west a cycling of sand between wadis and the sand sea can also be hypothesized. Samples of lithified and unlithified sands were analysed for grain-size characteristics, roundness, colour and carbonate content. The sands are very similar texturally to sands from other sand seas. The unusually high carbonate content, and the discovery of the remains of marine organisms in the sand, confirm the hypothesis of a primarily marine origin. It is suggested that there would be a rich source of carbonate sand on the coast because of the existence of a warm sea covering a wide offshore shelf, and strong onshore summer winds. The carbonate would be conserved in the very dry climate. Trend surfaces fitted to the spatial patterns of some of the sedimentary properties produced patterns which partially confirm these hypotheses of origin. The most important source of sand is again shown to be marine, and a north-western terrestrial source appears to be confirmed. The hypothesis of recycling of sand in this area also has some support in the trend-surface patterns.

Evolving models of the Tertiary evolutionary geomorphology of southern England, with special reference to the Chalklands

Abstract This paper examines recent advances in understanding of the Tertiary evolutionary geomorphology of the southern England Chalklands, a subject of continuing controversy despite over a century of investigation. It begins by briefly outlining the classic Wooldridge and Linton model and discussing the many lines of criticism. Particular attention is paid to the explanation of discordant drainage and numerous lines of evidence are presented to explain why the ‘superimposition model’ has been rejected in favour of an explanation involving the development of anteconsequents. Attention is then directed to the new models of evolution advanced in the early 1980s which placed heavier emphasis on Palaeogene denudation and pulsed tectonism. These models are compared and contrasted, and attention focused on the growing recognition that the fundamental erosional surface is an etchplanated Summit Surface originating in the Palaeogene rather than a peneplanated surface developed during the Miocene and Pliocene. Significant developments of the last fifteen years are then reviewed — the establishment of inversion tectonics, lithostratigraphical division of the Chalk, apatite fission-track dating of uplift episodes and new information on the extent of the late Pliocene Red Crag incursion — and combined to produce a new evolutionary sequence which differs from previous interpretations in a number of regards. First, the concept of structural compartmentalization into morphotectonic regions removes the need for uniformity of evolution and indicates that uplift could have been variable in time and space, thereby removing the problems caused by continual adherence to the concept of mid-Tertiary tectonism. Second, the growing evidence for Pleistocene differential uplift of at least 200 m is considered. Third, an explanation of the so-called Plio-Pliocene marine bench involving modified etchplanation is advanced. The paper concludes with a discussion of remaining uncertainties.

A preliminary geomorphological assessment of part of the Karakoram Highway

Summary The Karakoram Highway has been constructed through extremely difficult high arid mountainous terrain characterized by long, steep slopes, highly fractured and heavily weathered rocks, extensive areas of debris accumulation and locally high rates of geomorphological activity. The problems for road construction imposed by difficulties of access and remoteness have been compounded by rockslope instability, debris flows, river and meltwater surges, and fluctuating glacier snouts. As a consequence, the employment of sophisticated engineering techniques was abandoned in favour of widespread blasting and an extended period of labour-intensive maintenance. This paper is concerned with an 147 km stretch of the Highway and essentially subdivides into two parts. The first part describes the terrain conditions and main hazards posed to road construction, and includes a geomorphological assessment of the chosen alignment. The second section presents the results of a detailed survey of 128.8 km of the Highway in terms of the patterns of disruption experienced during the first three years of its life, and the distribution of a variety of hazards which will continue to endanger the road. This survey clearly reveals the importance of a good appreciation of geomorphological processes to highway engineering in arid mountains, as well as indicating the potential for geomorphological inputs to highway design.

On the uplift and denudation of the weald

Abstract The evolution of the Weald has been the subject of continuing controversy over nearly two centuries of investigation. This paper reviews current knowledge of the area’s development during the Tertiary against this background and produces an evolutionary geomorphology focused on a cross-section reconstruction of the Weald stretching from the south coast to the Thames. Estimations of the original thickness of Chalk (400–460 m) and other Mesozoic strata indicate a complex fold structure which, if ever fully developed, would have had a crestal elevation of 1400 m. The additional accumulation of an estimated 60–220 m of Palaeogene sediments reveals gross denudation of up to 1350 m in 66 Ma or 20.4 m Ma−1, a low rate on relatively erodible strata indicating periods of relative morphostasis in the Tertiary. Four models of evolution are identified — ‘Neogene flexuring and erosion’, ‘pulsed tectonism focusing on Palaeogene denudation’, the classic Wooldridge and Linton interpretation, and the model detailed in this paper — and applied to the cross-sectional reconstruction to produce spatial and temporal patterns of gross denudation and denudation rates. The analysis reveals that all models are possible theoretically and that denudation rates lie in the lower half of the range attributed to ‘normal denudation’. The model detailed in this paper has a proposed pattern of evolution for the Central Weald that involves early Palaeogene uplift, warping and denudation, followed by slower denudation in the later Palaeogene, stimulated erosion in the Neogene due to tectonic activity, culminating with severe denudation and relief generation in the Quaternary due to uplift of up to 260 m. The Chalk downlands, by contrast, are considered to have developed from the Pleistocene exhumation and dissection of areas that evolved more slowly in the Tertiary and were dominated by low relief landsurfaces at no great elevation above contemporary summits that experienced lengthy periods of morphostasis.

Geomorphological Investigations for the Channel Tunnel Terminal and Portal

Extensive geomorphological investigations of the Channel Tunnel UK portal and terminal site were commissioned by TML during 1986/87, prior to the commencement of the detailed ground investigations. These studies were primarily concerned with identifying and delimiting the spatial extent of landforms, superficial materials and in particular the evidence for contemporary or relict slope instability. The investigations took the form of large-scale (1:500) geomorphological mapping of the proposed development area. This paper describes the results of the mapping and how the information was incorporated into the site investigation and subsequent construction programme. In addition, some conclusions are drawn about the geomorphological evolution of the landslides that had developed off the chalk escarpment in the study area.

Geomorphological investigation, for engineering purposes, of blowing sand and dust hazard

Summary Blowing sand and dust are increasingly common hazards associated with dryland urbanization, largely because of the disruption of desert soils by human activity. This paper reviews the characteristics and problems of moving sand and dust and discusses how the four main methods of analysis—remotely sensed imagery, meteorological data, geomorphological mapping and process monitoring—can be integrated so as to provide the basis for well informed management strategies. This is exemplified by description of a recent survey for a town referred to as ‘Alfa’, which involved an unusual methodology heavily dependent on the collection of information on ground conditions by means of geomorphological mapping. The resultant integration of data on erodibility (sand and dust supply potentials) and erosivity (potentials for wind transport using meterological records) provided a good framework for focusing attention on those topographic/sediment units posing the greatest potential hazard and therefore requiring the most careful management. The new methodologies put forward in this paper are considered to be potentially applicable in a wide variety of dryland environments and to be worthy of further development and refinement.

Global warming and geomorphology

literature, the potential geomorphological consequences have yet to be properly considered, especially in the context of the British Isles. This paper introduces the subject of global warming influences on landform evolution and in the process emphasizes that environmental change is typical of the Quaternary and that the consequences of change need not necessarily be detrimental. The difficulties of predicting change are discussed and the role of local factors, including human activity, emphasized. The main conclusion is that the potential influence of climate change on landforms in Britain, until at least 2050, will be sufficiently limited as to be wholly determined by management decisions, indicating the need to formulate informed policies at the earliest opportunity.