Scarp Retreat Research Articles

“Canons” revisited and reviewed: Lester Kingʼs views of landscape evolution considered 50 years later

Fifty years after its publication, Lester King9s Canons of Landscape Evolution is reviewed and is considered in light of subsequent trends and actual developments. Some of his ideas, such as the role of scarp recession and the antiquity of some surfaces, remain current. His broad view of the world and his interest in major relief anticipated trends that are now fashionable. Some of his interpretations, however, and in particular his downgrading of the importance of structural factors and his linking of scarp retreat and pedimentation, have not stood the test of time. Other concepts, such as the etch or two-stage origin of forms, which were mooted but not fully appreciated in King9s day, have come to the forefront, and technological advances in dating and survey, particularly of the ocean floors, have signaled new perspectives in landscape interpretation. Nevertheless, King9s was a courageous attempt to provide guidelines for landscape study.

Pattern and timing of the post-Caledonian denudation of northern Scandinavia constrained by apatite fission-track thermochronology

Abstract Apatite fission-track thermochronology has been used to study the post-Caledonian denudation history of northern Scandinavia. Post-orogenic denudation progressively shifted from the interior of the continent towards the North Atlantic margin. The present-day area of maximum elevation in the Northern Scandes mountain range has experienced continuous denudation at least since Jurassic time. In Jurassic-Cretaceous time, the area north and east of this region experienced either no denudation at all or some denudation followed by a transient thermal event with a peak temperature in late Cretaceous time. Final denudation of the area to the east of the Northern Scandes probably started in late Cretaceous-Paleogene time and possibly accelerated in Neogene time. The denudation history of northern Scandinavia can be explained by scarp retreat of an uplifted rift flank. The pattern and timing of denudation of the Northern Scandes is different from that of the Southern Scandes, which experienced domal-style, late-stage postrift uplift in Neogene time. Geomorphological observations, offshore data from the Atlantic and Barents Sea margins, and scarce stratigraphical information from the mainland are in general agreement with the new thermochronological data.

Terrestrial in situ cosmogenic nuclides: theory and application

The cosmogenic nuclide exposure history method is undergoing major developments in analytical, theoretical, and applied areas. The capability to routinely measure low concentrations of stable and radioactive cosmogenic nuclides has led to new methods for addressing long-standing geologic questions and has provided insights into rates and styles of surficial processes. The different physical and chemical properties of the six most widely used nuclides: 3He, 10Be, 14C, 21Ne, 26Al, and 36Cl, make it possible to apply the surface exposure dating methods on rock surfaces of virtually any lithology at any latitude and altitude, for exposures ranging from 10 2 to 10 7 years. The terrestrial in situ cosmogenic nuclide method is beginning to revolutionize the manner in which we study landscape evolution. Single or multiple nuclides can be measured in a single rock surface to obtain erosion rates on boulder and bedrock surfaces, fluvial incision rates, denudation rates of individual landforms or entire drainage basins, burial histories of rock surfaces and sediment, scarp retreat, fault slip rates, paleoseismology, and paleoaltimetry. Ages of climatic variations recorded by moraine and alluvium sediments are being directly determined. Advances in our understanding of how cosmic radiation interacts with the geomagnetic field and atmosphere will improve numerical simulations of cosmic-ray interactions over any exposure duration and complement additional empirical measurements of nuclide production rates. The total uncertainty in the exposure ages is continually improving. This article presents the theory necessary for interpreting cosmogenic nuclide data, reviews estimates of parameters, describes strategies and practical considerations in field applications, and assesses sources of error in interpreting cosmogenic nuclide measurements. TABLE OF CONTENTS 1. Introduction 1.1. Development of the TCN methods 1.2. Applications of TCN Exposure methods 1.3. Previous reviews 2. Glossary 2.1. Terminology 2.2. Notation 3. Principles 3.1. Introduction 3.1.1. Source of the primary radiation 3.1.2. Effects of the geomagnetic field on GCR 3.1.3. Trajectory models and models of secondary nuclide production rates 3.1.4. Recent numerical models of GCR particle production 3.1.5. Nuclide production from primary GCR 3.1.6. TCN production by energetic nucleons 3.1.7. TCN production by low-energy neutron 3.1.8. TCN production by muons 3.1.9. Factors limiting TCN applications 3.2. Numerical simulation of low-energy neutron behavior 3.3. Analytical equations for TCN production 3.3.1. Fast neutron (Spallation) production 3.3.2. Production by epithermal neutrons 3.3.3. Production by thermal neutrons 3.3.4. Production by muons and muon-derived neutrons 3.3.5. Total nuclide production 3.4. Energetic neutron attenuation length 3.5. Temporal variations in production rates 3.5.1. Variations in the primary GCR flux 3.5.2. Variations due to solar modulation of the magnetic field 3.5.3. Effects of the geomagnetic field 3.5.4. Variations in atmospheric shielding 3.5.5. Other sources of temporal variations in production 3.6. Estimation of production rates 3.6.1. Helium-3 3.6.2. Beryllium-10 3.6.3. Carbon-14 3.6.4. Neon-21 3.6.5. Aluminum-26 3.6.6. Chlorine-36 3.7. Scaling and correction factors for production rates 3.7.1. Spatial scaling 3.7.2. Topographic shielding 3.7.3. Surface coverage 3.7.4. Sample thickness 3.7.5. Thermal neutron leakage 3.8. Exposure dating with a single TCN 3.9. Exposure dating with multiple nuclides 3.10. Nuclide-specific considerations 3.10.1. Helium-3 3.10.2. Beryllium-10 3.10.3. Carbon-14 3.10.4. Neon-21 3.10.5. Aluminum-26 3.10.6. Chlorine-36 3.11. TCN dating of sediment 4. Sampling strategies 4.1. Field sampling considerations 4.1.1. Sample description 4.1.2. Sampling methodology 4.2. Other lithological considerations 4.3. How much sample is needed? 4.4. Strategies for concentration-depth profiles 5. Sample preparation and experimental data interpretation 5.1. TCN sample preparation 5.1.1. Preparation time 5.1.2. Physical and chemical pretreatment 5.1.3. Isotopic extraction 5.2. Experimental data interpretation 6. Uncertainty and sources of error 6.1. Sources of error 6.1.1. Sample characteristics 6.1.2. Sample preparation and elemental analyses 6.1.3. Mass spectrometry 6.1.4. Systematic errors 6.2. Reporting the uncertainty 6.2.1. Error propagation 6.2.2. Evaluating accuracy by intercomparison 6.2.3. Multiple sample measurements 6.2.4. Sensitivity analysis 7. Directions of future contributions Acknowledgements References

Multiple talus flatirons, variations of scarp retreat rates and the evolution of slopes in Almazán Basin (semi-arid central Spain)

We studied a group of talus flatirons situated at the foot of structural platforms capped by Miocene limestones in the Almazán Basin, a southeastern extension of the Tertiary Duero Basin, in northern Spain. This group of talus flatirons exhibits five different stages of slope evolution (S 1, S 2, S 3, S 4, and S 5). They are concave slopes separated from the limestone scarp by a distance that increases with the age of the slope. We measured their profiles and fitted them to mathematical functions. By extrapolating these functions, we obtained the intersection point with the scarp at every stage. This gives different distances retreated by the scarp during each interval between consecutive stages of slope evolution. Remains of charcoal found in the debris slopes corresponding to stages S 1, S 3, and S 4 have been dated by means of 14C to 1090±40, 3590±40, and 28,550±130 BP, respectively. These values allowed us to calculate the following rates of scarp retreat: 0.5 mm/year during the time interval between stages S 4 and S 3, and 10.5 mm/year for the interval between S 3 and S 1. There is a good correlation between the ages of the slope accumulations and cold periods. S 3 is correlated with the Middle Bronze Age cold epoch and S 4 may correspond to the Heinrich event H 3. These correlations seem to confirm that talus flatirons in arid environments are good climatic indicators.

Contribution to the Theory of Scarpland Development from Observations in Central Queensland, Australia

Abstract Theories of scarpland geomorphology developed over the past century have been characterized by divergent conceptual frameworks and have been hindered by language barriers. Here, we review the main theories and assess them with reference to field evidence from central Queensland. This region provides exceptional scope for such an assessment because of the detailed K‐Ar chronology available for basalt that is draped extensively over scarps and valleys. The basalt provides the key to deciphering the pattern and rate of denudation. Erosion began in the late Mesozoic and was greatly advanced when volcanism occurred during the middle Cenozoic. Postbasaltic erosion has been dominated by the incision of the basaltic caprock and by the lengthening of canyons, rather than by scarp retreat. Canyons have extended headward by 1–2 km/m.yr., widened by 150–250 m/m.yr., and deepened by 6–25 m/m.yr., while scarps between canyons have retreated by only 18–130 m/m.yr. Canyon cutting both on the scarp front and on t...

Tectonic and geomorphic controls on the fluvial styles of the Eslida Formation, Middle Triassic, Eastern Spain

The Eslida Fm is an alluvial unit of the Buntsandstein Facies of the Iberian Ranges deposited in a half-graben basin over a period of 2–2.5 Ma during the lower Anisian (Middle Triassic). It can be subdivided into six cycles that can be correlated along the basin. The overall architecture and cyclicity were controlled by the episodic activity of the basin boundary fault. Two proximal source areas of minor importance and a distal one of major importance are identified. The petrological composition of the sandstones is arkosic to subgrauwackic, indicating source areas containing both low-metamorphic rocks and gneiss, and granitic rocks. Alluvial fans, very common in rift basins, were not developed because of slow fault scarp retreat and the lack of large antecedent rivers cutting across the rift shoulders. Paleocurrents point to the SE, except for a zone in the middle part of the basin where interference with a NE–SW fault system deflected the channels to the S. The infilling of the basin is asymmetric in a transverse section, with channel facies near the basin boundary fault to the NE and overbank facies in the distal SW part.

Abyssal erosion and scarp retreat: Deep Tow observations of the Blake Escarpment and Blake Spur

Deep Tow surveys conducted at two sites at the base of the Blake Spur and Blake Escarpment serve to delineate the contact between Mesozoic platform carbonates of the Blake Plateau and adjacent abyssal strata. The continental margin in the surveyed areas consists of steep limestone cliffs and headlands with over 2 km of relief. This extreme topography distorts the hydrographic structure of the water column and intensifies abyssal current flow in the surveyed areas. Significant lateral scarp retreat is indicated by the presence of broad, nearly horizontal benches of Mesozoic limestone presently exposed at the base of the Blake Spur and Escarpment in 4.5–5 km water depth; and by angular, apparently truncated limestone ledges that crop out on the cliff face. At the Blake Spur, the contact between Mesozoic platformal limestones and basinal Tertiary strata does not occur directly at the foot of the cliff, but is exposed on the abyssal plain within a 50 m deep erosional moat. Here, strong (>2 knots) south-flowing bottom currents occur within 100 m of the sea floor. The coincidence of this vigorous current flowing within the moat near the base of the escarpment and the eroded character of the sea floor suggests that currents are an important mechanism in sculpting this abyssal headland and contributing to lateral scarp retreat. Similar erosional processes are indicated along the Blake Escarpment near 29°03′N, where the relatively straight cliff face is interrupted by occasional steep-walled box canyons. On the headland between the canyons, the Mesozoic outcrop projects into the Blake–Bahama Basin for several kilometers as a broad, arcuate, gently-sloping bench. These outcropping benches of Mesozoic strata at the base of the escarpment are analogous to larger-scale buried benches observed in the subsurface that were formed by pre-Miocene erosion. Neogene and Quaternary scarp retreat is suggested by the location of the contact between Mesozoic rocks and younger strata seaward of the steeper cliff face, and is probably the result of current-induced erosion. Focused erosion may be facilitated by the presence of a thin (∼50 m) filament of corrosive Antarctic Bottom Water (AABW) that flows vigorously along the base of the cliff at both sites. The recessed position of the canyons shelters them from the main flow of this abyssal current, but the steepness and morphology of the canyon walls suggest that additional unidentified erosional processes are focused at the canyon heads.

Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa

The onshore region of a passive margin forms an integral part of the geological evolution from continental break–up to later sedimentation in the offshore basins. The dominant surface process in the onshore region is denudation, which acts to remove any direct evidence of surface uplift. However, denudation can be constrained on geological time–scales through low temperature thermochronological data, such as that obtained through apatite fission track analysis. Here, we present a suite of such data from the Atlantic Margin of southern Africa. The data have been modelled in terms of their temperature histories since the Jurassic. These temperature histories have been combined with heat–flow data to estimate the equivalent depth of denudation over these time–scales. Average denudation rates are of the order of a few tens of metres per million years, but show considerable variations both temporally and spatially. These results demonstrate that passive margins experience complex patterns of denudation. Three landscape evolution models are considered. Our results imply the downwarping model is inappropriate. The other two models, scarp retreat and pinned drainage divide, predict trends similar to those observed but the complexities inherent in the data and the evolution of passive margin topography do not allow us to resolve one from the other. In practice, both models probably operate to some extent as a margin evolves. Estimates of palaeotopography have been made, assuming simple isostatic response models to denudational unloading. Flexural models with effective elastic thickness (EET) of 25 km predict elevations 2 km and more above the present day values, while models with EET of 0 km predict elevations up to 750 m higher than the present–day. These models ignore any post–break–up tectonic uplift and need independent constraints on surface elevation to assess their validity.

Denudation rates for the southern Drakensberg escarpment, SE Africa, derived from in-situ- produced cosmogenic 36 C1: initial results

Cosmogenic 36 C1 concentrations in basalt samples from the Drakensberg escarpment on the SE African passive margin give quantitative estimates of denudation and scarp retreat rates. Over the 10 4 –10 6 year timespan addressed by these data, the calculated escarpment retreat rate has been 50–95 m Ma -1 and the average summit denudation rate 6 m Ma -1 . The scarp retreat rate is an order of magnitude less than previously suggested. The rate of summit lowering is sufficient to prevent the long-term intact survival of erosion cycle surfaces formed in the Mesozoic that were previously inferred for this region.

Scarp retreat rates in semiarid environments from talus flatirons (Ebro Basin, NE Spain)

Triangular slope facets or talus flatirons are presented as a useful tool for the calculation of scarp retreat rates in arid regions. In the central sector of the Ebro Basin, sequences of talus flatirons have been developed around mesas capped by Miocene limestone. These talus flatirons are grouped in four stages of slope evolution (S 2, S 3, S 4, S 5). By the extrapolation of the relict talus profiles with the adjusted log-functions, the position of the scarp for each one of the stages is located. The mean values obtained indicate a scarp retreat of 23 m between stages S 3 and S 2, 8 m between S 4 and S 3 and 26 m between S 5 and S 4. The ages obtained by radiocarbon dating are 2529±52 B.P. and 2930±60 B.P. for stage S 2, 27 864±444 B.P. for S 3 and 35 570±490 B.P. for S 4. Consequently, the mean scarp retreat rates are 0.9 mm/yr for the S 2–S 3 interval and 1 mm/yr for S 3–S 4. The scarp retreat rate is related to the lithological and structural features of both the caprock and the underlying clay sediments and to the palaeoclimatic evolution of the region that controls the intensity of weathering and water erosion processes that affect the slope system.

Origem e evolução da Serra do Mar

The origin of the subparallel mountain systems which compose the coastal Serra do Mar and the inland Serra da Mantiqueira is related to the Upper Cretaceous. These features - distributed along a quite singular continental atlantic border - nave a complex evolution arising controversial questions. The answers to these problems are mainly dependentto the dating of the Japi erosional surface and other younger ones, a study still faced by the small amount of detailed-scale surveys. The scarp domains covered by dense vegetation often impose difficult trail access. Nevertheless, the present knowledge stage allows us to admit that during Tertiary the evolution of the local relief has affected into a considerable extent the synchronic oceanic phase sediments of the Santos Basin. There is a lot of evidence and proofs that the Serra do Mar has been developed from a very different geographical position as compared to the existing one. The hypothesis supported by the present authors is a long-term scarp retreat from southeast to northwest due to erosional processes approximately starting at the Santos fault line in the present continental platform.

Silica‐Cemented Terrace Edges, Central California Coast

AbstractThe morphologic development of soils on marine terraces is a function of terrace age and geomorphic evolution. In this study, interrelationships between terrace landscape evolution and pedogenic silica cementation were investigated. Soil morphology, micromorphology, selective dissolution, penetration resistance, and hydraulic conductivity were used to determine the nature and extent of the cementation and evaluate the effect of cementation on soil properties. Soils on the interior part of the terrace (coarse‐loamy, mixed, thermic Typic Epiaquolls) contain plinthite, continuous throughout the entire terrace, cemented to the extent that it is only slowly permeable. A seasonally perched water table develops above the plinthite, and water seeps out of soils at the terrace edges. Some of these soils are cemented by opaline silica, in both channels and interstitial voids. The silica cementation extends inward into the terrace soils only 4 m, but at the terrace edge is strong enough to result in block fall as the predominant mechanism of scarp retreat. Redoximorphic Fe depletions are prominent in deeper horizons at the edge, but are absent from the interior. These depletions are formed through seasonal saturation and Fe reduction, by water seeping laterally through preferential flow paths in and adjacent to shrinkage cracks above the regolith‐bedrock contact. Contrasting morphology and chemistry between terrace interior and edge both result from, and cause, the interaction of pedogenic processes and landscape evolution. Contrasts between interior and edge soils suggest that caution is essential in using edge exposures as representative of soils under a geomorphic surface.

Ground-water sapping processes, Western Desert, Egypt

Depressions of the Western Desert of Egypt (specifically, Kharga, Farafra, and Kurkur regions) are mainly occupied by shales that are impermeable, but easily erodible by rainfall and runoff, whereas the surrounding plateaus are composed of limestones that are permeable and more resistant to fluvial erosion under semiarid to arid conditions. Scallop-shaped escarpment edges and stubby-looking channels that cut into the plateau units are suggestive of slumping of limestones by ground-water sapping at the limestone-shale interfaces, removal of slump blocks by weathering and fluvial erosion, and consequent scarp retreat. Spring-derived tufa deposits found near the limestone escarpments provide additional evidence for possible ground-water sapping during previous wet periods. A computer simulation model was developed to quantify the ground-water sapping processes, using a cellular automata algorithm with coupled surface runoff and ground-water flow for a permeable, resistant layer over an impermeable, friable unit. Erosion, deposition, slumping, and generation of spring-derived tufas were parametrically modeled. Simulations using geologically reasonable parameters demonstrate that relatively rapid erosion of the shales by surface runoff, ground-water sapping, and slumping of the limestones, and detailed control by hydraulic conductivity inhomogeneities associated with structures explain the depressions, escarpments, and associated landforms and deposits. Using episodic wet pulses, keyed bymore » {delta}{sup 18}O deep-sea core record, the model produced tufa ages that are statistically consistent with the observed U/Th tufa ages. This result supports the hypothesis that northeastern African wet periods occurred during interglacial maxima. The {delta}{sup 18}O-forced model also replicates the decrease in fluvial and sapping activity over the past million years, as northeastern Africa became hyperarid. The model thus provides a promising predictive tool for studying long-term landform evolution that involves surface and subsurface processes and climatic change.« less

Wearing Down, Wearing Back, and Gorge Extension in the Long-Term Denudation of a Highland Mass: Quantitative Evidence from the Shoalhaven Catchment, Southeast Australia

The long standing issue of the dominance of scarp retreat versus summit lowering in the denudation of a highland mass is investigated with supporting evidence provided by Tertiary basalts throughout the Shoalhaven catchment in southeast Australia. Both of these forms of denudation are found to be insignificant compared to the role of fluvial gorge extension over the last 30 m.y. Headward advancement of the Shoalhaven Gorge has been occurring at approximately 15 times the rate of major escarpment retreat, 250 times the average rate of summit lowering, and over 500 times the rate of interfluve consumption. These estimates show that the headward erosion of gorges is the most important process denuding the highlands in the Shoalhaven region and possibly elsewhere in the highlands of eastern Australia. Over the long term, the highlands in this region will become considerably more dissected well before they decrease substantially in height or are narrowed. This conclusion has important implications for models predicting isostatic rebound from assumed character and rates of denudation.

Denudation, fission track analysis and the long-term evolution of passive margin topography: application to the southeast Brazilian margin

A detailed transect of fission track data is combined with present day elevation to constrain the evolution of topography across the continental margin of southeast Brazil since break-up. The fission track data show clearly that rocks on the lowlying coastal plain have been exhumed from temperatures of >110–120°C over the last 100 Ma, while the basalt capping the escarpment has not been at temperatures >50°C since its eruption ∼130 Ma. The fission track data are broadly consistent with a model of scarp retreat. Modelling the denudation history and associated isostatic rebound highlights the sensitivity of the predictions to the assumed effective flexural rigidity of the continental lithosphere. Estimates of denudation based on modelling the topography alone range from 2 to 6 km and the amount of denudation possible before elevation reaches sea-level is reduced for higher flexural rigidities (10 23–25 NM). Consequently high geothermal gradients (45–60°C/km) are required to predict the fission track data satisfactorily. The greater inferred levels of denudation for flexural rigidities of 10 19–21 Nm allow the fission track data to be predicted adequately with gradients of 15–30°C, values similar to present day estimates. An appropriate value for the flexural rigidity needs to be determined independently before we can confidently constrain the contributions of long term tectonic uplift and erosional rebound to the present day elevation.

Controls on erosional retreat of the uplifted rift flanks at the Gulf of Suez and northern Red Sea

The Gulf of Suez and Red Sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion. We find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy. The prerift strata are composed of cliff‐forming Eocene‐Cretaceous carbonates overlying the easily eroded Cretacous‐Cambrian “Nubian” sandstone. This lithologic succession promotes scarp retreat of the sedimentary section, followed by dissection of the underlying basement. The prerift section thins from over 2000 m at the northern end of the rift to less than 400 m at its junction with the Red Sea. Thus, at the northern part of the Gulf of Suez, the Nubian sandstone is minimally exposed, and the carbonates form a scarp at the rift border fault. Farther south, undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat. The escarpment cuts diagonally away from the border fault and is over 100 km inland from the border fault at the southernmost Gulf of Suez. The amount of retreat varies inversely with the sediment thickness. Exposure and erosion of basement are initiated by the retreat of the escarpment, and the depth of erosion, as indicated by fission track ages, increases with distance from the escarpment. These observations are explained by a model in which erosion along the Gulf of Suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones. Undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m.y. The average rate of scarp retreat has been 6 km/m.y. and the along‐strike propagation of the erosion has been 12 km/m.y. The diachronous erosion of the the rift flanks at the Gulf of Suez highlights the importance of distinguishing between the timing of uplift and of erosion. Both thermochronometric and stratigraphic data primarily indicate the timing of erosion, which may differ significantly from the timing of the uplift that initiates it. They must be interpreted carefully to avoid erroneous conclusions about rift tectonics.

Late cenozoic history of the eastern llanos of venezuela: Geomorphology and stratigraphy of the Mesa Formation

The study area corresponds to the Mapire River basin in the southern part of the State of Anzoátegui and constitutes a region of low relief covered by the Pleistocene Mesa Formation. This formation consists of horizontally bedded alluvial sediments, represented by light gray, coarse- to fine-grained quartz sand, with cross-bedding marked by ochre, red and pink alteration, and to a minor extent by marsh deposits, represented by laminated and spotted shale. The typical topography consists of mesas north of the left margin of the Orinoco River (the Musinacio and Mapire Mesas). To the north of the mesas there is a topography of low hills covered by blocks of ferruginous crusts. A typical tropical savanna climate exists in the region, with a strong dry season which lasts 4–6 months, followed by a rainy season with excess humidity. The dominant erosional process is scarp retreat, due to the action of the savanna climate on the lithology of the partly unconsolidated sediments; it has formed deep canyons in the headwaters, and is magnified by tectonic uplift and the lowering of the regional base level. However, some topographic forms have been preserved due to the hardening of the sands by iron oxides. Preliminary thermoluminescence dating of sand from two stratigraphic sections confirms the Pleistocene age of the Mesa Formation (0.5–1 Ma BP), previously suggested by its stratigraphic position and geomorphological interpretation.

The role of episodic plain formation and continuous etching and stripping processes in the End-Tertiary landform development of SE Kenya

Scarp retreat and related plain formation in the southeastern Kenyan landscape were episodic processes and associated with periods of high base-levels. Both sedimentation and erosion contributed to landscape leveling. Correlation of tectonic movements in the interior and regional base-level changes shows that a tectonic control over the Late Tertiary plain formation is more than likely. The proposed leveling process opposes the pediplanation concept in being triggered off by base level rise instead of base-level lowering. Moreover, it is not continuous, but seems to come to an almost stand still as soon as the regional base level is lowered. It further allows weathering and erosion processes to continue in areas outside the reach of base-level changes and can therefore solve the apparent paradox between plain formation and contemporaneous etching and stripping processes.

Miocene Glacial Stratigraphy and Landscape Evolution of the Western Asgard Range, Antarctica

40Ar/39Ar dated in-situ volcanic ashfall deposits indicate that the surficial stratigraphy of the western Asgard Range in the Dry Valleys region extends back at least to 15.0 Ma. The preservation of Miocene and Pliocene colluvium and drift on valley slopes shows that major bedrock landforms are relict and that little slope evolution has occurred during the last 15.0 Ma. Major bedrock landforms can be compared directly with buttes, mesas, box canyons, and escarpments of platform deserts. Although glaciers played a role in shaping the present topography of the western Asgard Range, the main landforms were probably cut by progressive scarp retreat, propagation of embayments with theater-shaped heads, and isolation of buttes and mesas prior to 15.0 Ma.We also recognize a phase of wet-based alpine glacier expansion in theater-headed embayments prior to 15.0 Ma ago, along with a late-Miocene phase of northeast-flowing ice-sheet overriding between 14.8/ 15.2 Ma and 13.6 Ma. However, there is no evidence for notable glacier expansion during the last 13.6 Ma. Instead, in-situ ashfall deposits indicate persistent hyper-arid, cold-desert conditions with only minor advance (less than 2.5 km) of rock glaciers and glacierets at valley heads. Our results are consistent with Pliocene stability rather than meltdown of the adjacent East Antarctic Ice Sheet.

King of the plains: Lester King's contributions to geomorphology

Lester King was one of the most influential geomorphologists of the twentieth century. He based his interpretations of landforms and landscapes firmly in the concept of parallel scarp retreat, for although he acknowledged the possibility of slope decline in certain lithological and vegetational environments, he used the mechanism little, if at all, in understanding landscape. Rather, pediments and inselbergs, for instance, as well as entire landscapes, were construed in terms of scarp recession. King appreciated and emphasised the evidence provided by plains for the interpretation of landscape. He considered pediplanation (scarp retreat and pedimentation) to be active in all regions where running water is responsible for shaping the land surface. King's studies in denudation chronology are consistent with his commitment to scarp retreat, implicit in which is the possibility of the survival of very old surfaces; though whether old enough to match the realities of landscape is doubtful. King challenged the Davisian scheme of landscape evolution and offered an alternative model that has proved viable in many parts of the world. Whether pedimentation and scarp retreat are necessarily related is however questionable. King devised original explanations for several well known landforms, and though many of his ideas and interpretations have fallen by the wayside he stimulated many of his contemporaries critically to review their assumptions and explanations.