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Evolution of natural rock arches: A realistic small-scale experiment
Abstract
People tend to admire well-developed rock arches and bridges as a symbol of perfection in nature. However, the origin of such arches still remains unclear, with different authors emphasizing different factors as essential. An authentic small-scale physical model of a perfect arch was created in situ from friable sandstone through a simulation of natural erosion. Based on three-dimensional photogrammetric and numerical modeling, we established three evolutionary stages of the modeled arch: the initial, mature, and senile stages. Erosion removed the material from stress shadows, thus exposing the compressed zone of the material, which spans over the discontinuity. From its original lintel thickness with a length ratio of 0.3, the modeled arch thinned out to a ratio of 0.03, which is close to the best-developed natural arches. The erosion rate of the arch surface was relatively low for 90% of its lifespan, but the last 10% was characterized by an abrupt increase in the erosion rate. Our results show that the only conditions essential to producing a delicate arch through erosion are a thin, vertically elongated rock body with an appropriate discontinuity, and a material prone to stress-controlled erosion.
... The Honey Waterfalls seem to be rare on the regional scale: on the one hand, somewhat similar geosites are known in other places in Russia [10]; on the other hand, the combination of phenomena (granitoids, weathering crust, Lower Cretaceous carbonates, active river erosion, and waterfalls) is unique to the southern Central Ciscaucasus and, probably, the entire Northern Caucasus. The Ring Mountain is rare on the national scale: natural arches and natural bridges are known in different places of the world [74][75][76][77][78][79][80], but they are not so frequent and tend to degrade or even disappear [81,82]; this means that natural arches are highly unique for each particular country, including one so large as Russia (especially in the case of such a typical, large, and famous feature The Honey Waterfalls seem to be rare on the regional scale: on the one hand, somewhat similar geosites are known in other places in Russia [10]; on the other hand, the combination of phenomena (granitoids, weathering crust, Lower Cretaceous carbonates, active river erosion, and waterfalls) is unique to the southern Central Ciscaucasus and, probably, the entire Northern Caucasus. The Ring Mountain is rare on the national scale: natural arches and natural bridges are known in different places of the world [74][75][76][77][78][79][80], but they are not so frequent and tend to degrade or even disappear [81,82]; this means that natural arches are highly unique for each particular country, including one so large as Russia (especially in the case of such a typical, large, and famous feature as the Ring Mountain). ...
... The Ring Mountain is rare on the national scale: natural arches and natural bridges are known in different places of the world [74][75][76][77][78][79][80], but they are not so frequent and tend to degrade or even disappear [81,82]; this means that natural arches are highly unique for each particular country, including one so large as Russia (especially in the case of such a typical, large, and famous feature The Honey Waterfalls seem to be rare on the regional scale: on the one hand, somewhat similar geosites are known in other places in Russia [10]; on the other hand, the combination of phenomena (granitoids, weathering crust, Lower Cretaceous carbonates, active river erosion, and waterfalls) is unique to the southern Central Ciscaucasus and, probably, the entire Northern Caucasus. The Ring Mountain is rare on the national scale: natural arches and natural bridges are known in different places of the world [74][75][76][77][78][79][80], but they are not so frequent and tend to degrade or even disappear [81,82]; this means that natural arches are highly unique for each particular country, including one so large as Russia (especially in the case of such a typical, large, and famous feature as the Ring Mountain). The Little Saddle is rare only on the local scale: it is highly suitable for distant observation of the Elbrus Mountain from the Kislovodsk area, but some other viewpoints can be found in other neighboring areas of the southern Central Ciscaucasus and the central Greater Caucasus. ...
Many geographical domains possess notable geological and geomorphological features, which are yet to be characterized comprehensively in terms of geoheritage. The present study focuses on the Kislovodsk area, which is situated in the southern part of the Central Ciscaucasus (post-Paleozoic platform), where the latter joins to the Greater Caucasus (late Cenozoic orogen). Three geosites are reported from there, and their qualitative description and semi-quantitative, score-based assessment are offered. The Honey Waterfalls represent an example of river erosion affecting Carboniferous granitoids with uppermost Jurassic weathering horizon and overlain by Lower Cretaceous carbonates. The Ring Mountain is a natural arch formed as result of wind erosion. The Little Saddle is a viewpoint offering a spectacular, panoramic view toward the Elbrus Mountain that is the highest peak of Russia and Europe and an impressive dormant stratovolcano. The Honey Waterfalls and the Ring Mountain are ranked nationally (the latter receives the highest total scores), and the Little Saddle is ranked regionally. These geosites are diverse in several aspects, and, particularly, different geoheritage types and forms are established. The Kislovodsk area bearing the reported geoheritage objects is a part of the Mineralnye Vody resort area, which is large and important for the national tourism and recreation industry. The related opportunities and challenges for geoheritage resource management have to be considered.
... Arches form and evolve over time in response to environmental and anthropogenic stresses whose weathering action can be aided by predisposing geological factors such as 40 weak rock layers or rock mass fractures (Ostanin et al., 2017). Rock fractures nucleate and propagate in orientations often reflecting principal stress directions or following pre-existing discontinuities (Blair Jr, 1987;Cruikshank and Aydin, 1994;Bruthans et al., 2014;Řihošek et al., 2018). As fractures grow to critical lengths, material is shed through partial failures at the arch lintel and abutments, sometimes resulting in the sculpture of ideal arch forms (i.e., the inverted catenary) but often contributing to reduced stability and ultimate collapse (Moore et al., 2020). ...
Fracture generation and propagation are primary mechanisms of structural degradation in natural rock arches and other freestanding rock landforms. However, methods to detect structural changes arising from fracturing are limited, particularly at sites with difficult access and high cultural value. Here we show how ambient vibration modal analysis can be used to identify fracture-controlled resonance modes at a sandstone arch in Utah (USA) aiding the selection of relevant modes for structural health monitoring. We characterized modal properties of Hunter Canyon Arch (i.e., resonance frequencies, damping ratios, and mode shapes) using spectral and cross-correlation analyses of data generated from an array of nodal geophones. Results revealed properties of nine resonance modes with frequencies between 1 and 12 Hz, damping ratios between 0.6 and 4.3 %, and an assortment of 3D mode shapes. Experimental data were then compared to numerical models implementing both homogeneous media and heterogeneous configurations generated through discretization of compliant zones in areas of mapped fractures. Results showed that all numerical solutions replicated the first two resonance modes of the arch, indicating these are insensitive to structural complexity derived from fractures and thus may be poor targets for monitoring. Meanwhile, heterogenous models with implemented fracture zones succeeded in matching the frequency and shape of one additional higher mode, indicating this mode is sensitive to fracture properties and thus most likely to respond to structural change from fracture propagation. Evolutionary crack damage modelling confirmed the sensitivity of this mode, and conversely the relative insensitivity of other modes, to simulated fracture propagation. While examination of fundamental modes is common in structural health monitoring studies, our results suggest that identifying changes in higher-order modes, i.e., those determined to be affected by fractured areas, may be more informative for characterizing structural damage in monitoring applications.
... For example, sandstone erosion rate constitutes several dozen millimeters per 1000 years [23]. There are successful laboratory experiments substantiating the formation of spectacular sandstone landforms such as arches, alcoves, pedestal rocks, pillars [28,29], and arcades [5] as a result of erosion. However, successful laboratory experiments illustrating the whole cycle of honeycomb formation are yet to be conducted. ...
Honeycomb weathering is a common phenomenon found on various rock surfaces all around the world. However, honeycomb formation mechanisms are still poorly understood. In this study, we propose a model describing moisture transport within the sandstone and erosion resulting from salt deposition during evaporation of moisture off the rock surface. The moisture transport model is based on the non-linear diffusion equation, where the volumetric moisture content is a combined parameter accounting for the moisture and gas (vapor) content. The moisture transport model accounts for the several-orders-of-magnitude decrease in moisture diffusivity, observed during drying. It was assumed that erosion occurs when the evaporation front is located close to the rock surface. The depth of erosion is proportional to the moisture flow rate through the drying surface. The ABAQUS finite-element software suite was used for numerical solution of the non-linear diffusion equation. The iterative scheme of erosion simulation for different drying cycles was implemented using the Python programming language. Computations were conducted in the 2D setting for the square model with dimensions of 50 mm × 50 mm. Simulation results demonstrate the possibility of obtaining various landform shapes (honeycombs, tafoni) by varying only the value of the distribution of moisture content at the bottom side, simulating the rate of internal wetting of rock.
... However, controls other than those related to rock properties must not be ignored. The role of stress field at the local scale has been recently made central in respect to medium-and small-scale sandstone landforms (Bruthans et al., 2014Ř ihošek et al., 2018;Moore et al., 2020), but is also applicable to the regional scale, bearing directly on the origin of regular orthogonal jointing patterns (Caputo, 1995;Mandl, 2005;Li and Ji, 2021), so evident in many sandstone and conglomerate terrains. Climatic controls are scale-dependent too and involve both regional climate characteristics, decisive for, among others, water availability in the system and the presence or absence of frozen ground that inhibits Fig. 13. ...
Coarse clastic sedimentary successions cover approximately-one fourth of the continental surface and give rise to distinctive landforms at a variety of scales. Rock-mass strength differences between members of layered successions account for the presence of escarpments, typically capped by thick sandstone or conglomerate beds, usually also with mid-slope cliffs and benches reflecting variable resistance of individual members of the succession. Depending on the dip of strata, two main types of regional landscapes are plateaus and plains, or homoclinal ridges (cuestas), but higher degree of deformation may occur in the vicinity of major faults, resulting in hogback morphology. Medium-scale landforms include residual hills of various types such as mesas and buttes, rock cities and assemblages of ruiniform relief, whereas canyons and slots are common valley forms. Escarpment retreat is usually considered as an overarching concept in geomorphology of layered successions, but it does not seem to be a universal pathway of landscape evolution and even retreat itself may occur in different ways. We propose that juxtaposition of strong and weak rocks in the vertical succession is the viable unifying theme, as it has profound geomorphological implications, influencing processes and patterns of evolution at a variety of spatial scales. However, depending on lithological characteristics, mechanical and hydrogeological properties, dominant processes may vary, explaining considerable landform diversity within tablelands, even though at the grand scale stepped topography becomes a repetitive theme. Synthetic graphical presentation of morphogenetic systems on coarse clastic successions is also presented.
... Based on knowledge related to the effect of physical fields on granular rocks (Bruthans et al., 2014Filippi et al., 2018;Řihošek et al., 2019;Moore et al., 2020;Safonov et al., 2020), we set a hypothesis that some other factor(s) must be responsible for the rapid disintegration of these portionsa factor missing or diminished in the main rock mass. ...
Weathering and disintegration of sandstone outcrops are a complex process involving loose material production, rockfall phenomena, and creation of picturesque natural sceneries. On historical monuments, they induce damage to building stone. Here we present a new look on sandstone weathering/recession the central aspect of which is rapid disintegration of portions of the rock massif, which are no longer physically connected with the main rock mass, though still in situ. A set of field and laboratory measurements including mechanical (tensile strength, drilling resistance) and hydraulic (permeability, surface moisture) methods were applied to compare the properties of the disconnected portions with those of the surrounding rock mass. Also, physical weathering experiments were performed to characterize the effect of confinement on the breakdown rate of several sandstone samples. The presence of disconnected portions is very common in dry climates where they are usually elongated parallel to horizontal surfaces. In humid temperate climates, however, they are less abundant, being elongated mostly vertically. The surfaces of detachment follow bedding planes, planar elements of other sedimentary structures, and subhorizontal fractures, and stress shadows on subvertical cliff faces. Weathered surfaces of the disconnected portions show reduced tensile strength and drilling resistance values, a faster capillary water absorption and a higher surface moisture compared to the much less weathered surfaces of the surrounding rock mass. Physical experiments demonstrated that a confinement by the surrounding rock mass may considerably delay the loosening of rock during weathering. The much faster disintegration rates of the disconnected portions of rock compared to their surroundings are explained by the fact that they are not confined and have a larger surface area. The recession rates of sandstone surfaces with disconnected portions are highly variable both in space and time and their genesis can be demonstrated by two suggested conceptual models.
... The development of the elasticity theory in the 19th century allowed to better understand the mechanics of arch construction. It was noticed that the most effective shape of the arch reflects the pressure lines in the element (Bovo, Mazzotti, and Savoia 2012;Cecchi 2012;Foraboschi 2004;Řihošek et al. 2018). Based on that assumption several different shapes of arch lintels were developed. ...
Lintels have more than a thousand-year-old history as one of the most important structural elements of historical buildings. In the 20th century, however, masonry lintels have been replaced almost completely by steel or reinforced concrete. The problem of assessing the technical condition, repair and reconstruction methods, and modernization of masonry buildings is still actual and depends on the real load-bearing capacity of masonry lintels. The article features the analysis of defects and damages of lintels and stresses occurring in them. Different types of laboratory models and analysis of their tests results are presented in the article. Laboratory material tests are presented. The additional strength effect provided from brick layers over lintel is analysed with laboratory tests.
Progressive fracturing contributes to structural degradation of natural rock arches and other freestanding rock landforms. However, methods to detect structural changes arising from fracturing are limited, particularly at sites with difficult access and high cultural value, where non-invasive approaches are essential. This study aims to determine how fractures affect the dynamic properties of rock arches, focusing on resonance modes as indicators of structural health conditions. We hypothesize that damage resulting from fracture propagation may influence specific resonance modes that can be identified through ambient vibration modal analysis. We characterized the dynamic properties (i.e., resonance frequencies, damping ratios, and mode shapes) of Hunter Canyon Arch, Utah (USA), using spectral and cross-correlation analyses of data generated from an array of nodal geophones. Results revealed properties of nine resonance modes with frequencies between 1 and 12 Hz. Experimental data were then compared to numerical models with homogeneous and heterogeneous compositions, the latter implementing weak mechanical zones in areas of mapped fractures. All numerical solutions replicated the first two resonance modes of the arch, indicating these modes are insensitive to structural complexity derived from fractures. Meanwhile, heterogenous models with discrete fracture zones succeeded in matching the frequency and shape of one additional higher mode, indicating this mode is sensitive to the presence of fractures and thus most likely to respond to structural change from fracture propagation. An evolutionary crack damage model was then applied to simulate fracture propagation, confirming that only this higher mode is sensitive to structural damage resulting from fracture growth. While examination of fundamental modes is common practice in structural health monitoring studies, our results suggest that analysis of higher-order resonance modes can be more informative for characterizing fracture-driven structural damage.
The Ennedi plateau region of north-east Chad is a spectacular sandstone erosional landscape comprising arches, pillars, mushroom rocks and other weathering forms. These landforms are found in associated with isolated sandstone hills (buttes and inselbergs), relict and sand-filled alluvial wadi channels, and in exceptional cases permanent water-filled pools (gueltas). Although little is known about these different landforms, with reference to similar sandstone landscapes elsewhere it is likely that rock structures and episodic surface availability have influenced spatial and temporal variations of erosional processes and thus landform development in the region. A model is proposed to explain Ennedi landscape evolution, set in a regional climatic and environmental context.
Natural arches are culturally valued rock landforms common in sedimentary rocks of the Colorado Plateau and additionally occur broadly around the world. Recent notable collapses of some of these landforms have highlighted the need to better understand the mechanics of their failure. While environmentally driven weathering has been the focus of most previous studies of arch collapse, comparably little attention has been given to anthropogenic vibration sources and how these often slight- to moderate-magnitude shaking events might steadily weaken arches over time. We collected 12–15 months of continuous ambient vibration data from arches and nearby bedrock in both anthropogenically ‘noisy’ and ‘quiet’ locations and used these datasets to develop an annual model of arch peak ground velocity based on magnitude-cumulative frequency distributions. Working from these models, we added vibration events of varying magnitude or frequency of occurrence, informed by field data, imitating arch vibration in response to different anthropogenic activities such as helicopter flights or induced earthquakes. We then applied subcritical fracture mechanics principles to predict annual crack growth rates in an idealized arch under these different vibration conditions. Our results demonstrate that in a single year, cracks grow minimally longer (∼1%) in ‘noisy’ environments than in areas not experiencing anthropogenic vibration energy. Few (1+) 30-s moderate-magnitude events (∼15 mm/s) or many (>37,000) 30-s low-magnitude events (∼2 mm/s) cause markedly increased crack growth. Our approach provides a valuable new framework for assessing the range and frequency of occurrence of vibrations experienced by an arch, and for predicting arch damage. Our results, in turn, yield important new outputs applicable in support of conservation management of these and similar landforms world-wide under exposure to a range of human-induced vibration activity.
The Azure Window was a natural arch situated in the west coast of Gozo (Maltese Archipelago) that collapsed in March 2017. We employ a Diver Propulsion Vehicle-mounted camera system to capture data for the 3D-modelling of this collapsed arch via photogrammetry. We demonstrate use of this method to document complex underwater geomorphology spread across a large area, and draw up a geomorphic assessment of the site and collapse event on the basis of this 3D-model. The methodology enables a reconstruction and understanding of the collapse event. On account of the high-resolution attained, we are able to cross-match the principal submerged components with their pre-collapse location; this enables an understanding of the dynamics of the collapse, confirmation of rock break-up along existing joints, and mapping the distribution of the rock and debris from the collapse event. We conclude that the key stages in the collapse of the Azure Window entailed erosion at the base of the pillar, leading to the latter's collapse in the southwest direction, breaking into two main sections that separated along the lithological boundary. We also find clear evidence that separation of some sections of the pillar followed pre-existing joints. The bridge collapsed vertically upon loss of support from the pillar, breaking into two main components and many other fragments. We also document further changes at the site post-collapse. We show that this approach can be utilised to understand and characterise such events even when significant time has elapsed since collapse, and rocks have already undergone erosion and significant marine growth.
The Azure Window was a natural arch situated in the west coast of Gozo (Maltese Archipelago) that collapsed in March 2017. We employ a Diver Propulsion Vehicle-mounted camera system to capture data for the 3D-modeling of this collapsed arch via photogrammetry. We demonstrate use of this method to document complex underwater geomorphology spread across a large area, and draw up a geomorphic assessment of the site and collapse event on the basis of this 3D-model. The methodology enables a reconstruction and understanding of the collapse event. On account of the high-resolution attained, we are able to cross-match the principal submerged components with their pre-collapse location; this enables an understanding of the dynamics of the collapse, confirmation of rock break-up along existing joints, and mapping the distribution of the rock and debris from the collapse event. We conclude that the key stages in the collapse of the Azure Window entailed erosion at the base of the pillar, leading to the latter's collapse in the southwest direction, breaking into two main sections that separated along the lithological boundary. We also find clear evidence that separation of some sections of the pillar followed pre-existing joints. The bridge collapsed vertically upon loss of support from the pillar, breaking into two main components and many other fragments. We also document further changes at the site post-collapse. We show that this approach can be utilised to understand and characterise such events even when significant time has elapsed since collapse, and rocks have already undergone erosion and significant marine growth.
This paper provides a review of recent publications dealing with different issues of sandstone geomorphology, which was long an under-researched theme. Nevertheless, considerable advances have been made recently regarding the origin of various characteristic landforms and landscapes developed in sandstones. The structure of this review reflects the diversity of spatial scales, so that it begins with minor landforms and moves towards regional landscapes, considering both the form and the process. It first focuses on the origin and evolution of mi-croforms on rock surfaces, especially honeycombs and arcades, which have been demonstrated to reflect the distribution of stress within the rock mass and the hydraulic field. A distinction is made between 'organizing' and 'decay' factors, with the latter varying depending on local circumstances, whereas the overarching role of the former explains equifinality. Then the concept of negative feedback between stress and erosion is reviewed, with reference to medium-scale landforms, mainly pedestal rocks and rock arches. In the next part new evidence and ideas about the role of subsurface processes in shaping medium-size and major landforms is presented, with emphasis on chemical decay (arenization), mechanical erosion and sapping. Larger landforms discussed are rock cities and ruiniform relief, canyons and mesas. It is argued that subsurface processes play an important part in the evolution of rock cities and boulder-filled canyons, whereas recent work on residual hills shows that plateau ➔ mesa ➔ butte ➔ pinnacle trajectory is but one possible pathway of geomorphic evolution of tablelands. In the following part a new concept of in situ disintegration of sandstone-capped escarpments is presented as complementary to the widely accepted model emphasizing large scale, often catastrophic mass movements. Finally , recent attempts to use geomorphometry tools to characterize rugged sandstone relief are presented. The paper is closed with an outline of challenges for the future.
Arcades, i.e. lenticular and other specifically shaped hollows controlled by discontinuities, have recently been recognized as a weathering form typical for sandstones, weathered quartzites, granites, or tuffs. They are produced by accelerated weathering and erosion in stress shadows related to the redistribution of gravity-induced stress along planar discontinuities in the rock. These forms occur worldwide in various settings (inland humid, arid, and coastal). The origin of arcades has been demonstrated via physical experiments and supported by a relatively simplistic numerical modeling. However, details on their shaping and the evolution of related forms have not been explained. We performed an advanced numerical modeling to produce various shapes of arcades and rock pillars during the erosion of rock masses dissected by discontinuities. We demonstrate that the erosion model, in which erosion takes place when the maximum principal stress is below a certain critical value, can adequately describe the formation of arcades. In the modeling, we set higher critical values for stresses at discontinuities than in a homogeneous material (representing a rock mass) to represent the higher tendency for disintegration of the discontinuity material, which was weakened by the discontinuity formation processes. By applying various discontinuity geometries and values of critical stresses, we were able to reproduce the formation of various arcade shapes and complex-three-dimensional clusters of arcade cavities with rock pillars. Discontinuities and stress-controlled erosion/weathering are the only necessary conditions for arcade formation.
The self-weight of freestanding natural arches imparts stresses on the lintel and abutments, setting the conditions for rock fracture and erosion. Rock is weak in tension, therefore an ideal arch form is one that minimizes tensile stresses and supports the weight of the lintel in compression, e.g. an inverted catenary. However, rock mass structure, including bedding, cross-bedding and other discontinuities, often imparts strong control on the geometry of arches, leading to forms that are less favorable from a stress perspective. Here we analyze a suite of nineteen arch models created from ground- and drone-based photogrammetry to assess the three-dimensional static stress field under gravitational loading. The models represent a range of arch lengths from 4 to 88 m, as well as a variety of forms, and use material properties previously calibrated from dynamic analysis of ambient vibrations. Our results demonstrate that arches shaped like beams have relatively high tensile stresses and a nearly symmetrical statistical distribution of principal stresses, while those with convex forms have comparably lower tensile stresses and statistical distributions favoring compressive stresses. In-situ observations of tensile cracks frequently correspond to the location of predicted tensile stresses in our models. We calculated the ratio of mean principal stresses for each arch, which is theoretically 1 for a flat prismatic beam and approaches infinity for an ideal inverted catenary. We found several arches with mean principal stress ratio around 1 and that values increased with lintel convexity. These results indicate that while self-sculpture might attempt to create ideal stress-based forms, discontinuities can control lintel geometry and natural arches evolve with forms that may be less favorable for long-term stability. The mean principal stress ratio is a simple metric to classify and compare arches, which may be useful for assessment of arch stability supporting conservation and hazard analyses.
Natural arches, pillars and other exotic sandstone formations have always been attracting attention for their unusual shapes and amazing mechanical balance that leave a strong impression of intelligent design rather than the result of a stochastic process. It has been recently demonstrated that these shapes could have been the result of the negative feedback between stress and erosion that originates in fundamental laws of friction between the rock’s constituent particles. Here we present a deeper analysis of this idea and bridge it with the approaches utilized in shape and topology optimisation. It appears that the processes of natural erosion, driven by stochastic surface forces and Mohr-Coulomb law of dry friction, can be viewed within the framework of local optimisation for minimum elastic strain energy. Our hypothesis is confirmed by numerical simulations of the erosion using the topological-shape optimisation model. Our work contributes to a better understanding of stochastic erosion and feasible landscape formations that could be found on Earth and beyond.
Weathering and erosion of sandstone produces unique landforms1, 2 such as arches, alcoves, pedestal rocks and pillars. Gravity-induced stresses have been assumed to not play a role in landform preservation3 and to instead increase weathering rates4, 5. Here we show that increased stress within a landform as a result of vertical loading reduces weathering and erosion rates, using laboratory experiments and numerical modelling. We find that when a cube of locked sand exposed to weathering and erosion processes is experimentally subjected to a sufficiently low vertical stress, the vertical sides of the cube progressively disintegrate into individual grains. As the cross-sectional area under the loading decreases, the vertical stress increases until a critical value is reached. At this threshold, fabric interlocking of sand grains causes the granular sediment to behave like a strong, rock-like material, and the remaining load-bearing pillar or pedestal landform is resistant to further erosion. Our experiments are able to reproduce other natural shapes including arches, alcoves and multiple pillars when planar discontinuities, such as bedding planes or fractures, are present. Numerical modelling demonstrates that the stress field is modified by discontinuities to make a variety of shapes stable under fabric interlocking, owing to the negative feedback between stress and erosion. We conclude that the stress field is the primary control of the shape evolution of sandstone landforms.
The Golden Gate Highlands National Park (GGHNP) is well known for its impressive sandstone formations. While previous geoscience research in the park has focused on geology, palaeontology, slope forms and the prominent lichen weathering, remarkably little has been written on the diversity and possible origins of sandstone phenomena in the region. The objectives of this study were (1) to present a geomorphological map of prominent and interesting landforms for particular portions of the park and (2) to document the variety of macro- and microscale sandstone formations observed. During field work, we undertook global positioning system measurements to map landforms and, in addition, measured the dimensions of several landform types. A Schmidt hammer was used to conduct rock hardness tests at a variety of localities and lithologies for comparative purposes. We indentified and mapped 27 macro- and microscale sandstone landforms, of which 17 are described in detail. It is demonstrated that for the most part, the landforms are a likely product of surface lithological reactions to a regional climate characterised by pronounced multitemporal temperature and moisture shifts, recently and in the past. However, many of the geomorphological processes producing landforms are controlled by microclimates set up by factors such as macro- and microtopography.
Conservation implications: The GGHNP is best known for its geological, geomorphological and palaeontological heritage. This paper highlights the diversity of sandstone geomorphological phenomena, many of them rare and ‘unique’ to the region. Not only are these landforms of aesthetic interest to tourists, but they also provide microhabitats for biota. Thus, conservation of biota requires associated conservation of geo-environments where they are established.
Weathering patterns on sandstone cliff faces and sloping surfaces were studied from the viewpoint of gravity-induced stress combined with the presence of planar discontinuities in the rock. Critical evaluation of hundreds of photos taken in various regions around the world, numerical modelling using finite element method (including automatic stress-controlled element removal technique of erosion modelling), and physical in situ modelling of friable sandstone revealed the formation of a specific sub-group of weathering forms related to planar discontinuities. Due to their striking similarity with elements known from architecture, these pits are introduced as “arcades” or arcade-like forms. These pits and cavities vary in size but they are similar in shape, coalesced into “trainsˮ along the discontinuities, and separated from each other by hourglass-shaped pillars. In their origin and geometry, these forms differ from widely known cavernous weathering forms such as honeycombs and tafoni. We have shown that arcade shape and size are controlled by the redistribution of gravity-induced stress along planar discontinuities and the resulting stress shadows. The stress shadow zones along the discontinuities are characterized by material loss owing to various weathering and erosional mechanisms. Although overlooked as yet, stress-controlled weathering along discontinuities has a strong effect on rock surface morphology, and arcades can be taken as its principal example.
Those factors controlling the weathering and erosion of sandstone on the field scale are still not well understood. In this study, a specific sandstone overhang (and its surroundings) with artificially induced and extremely high erosion rates was subjected to a complex investigation. Contrast between the erosion rate of the wet and dry portions of the same cliff enabled isolation of the factors responsible for rapid sandstone retreat. Erosion rates, moisture, and salt content, as well as suction were monitored in the field. Mineral phases and water chemistry were analyzed. The measurement of tensile strength, laboratory frost weathering tests, and numerical modeling of stress were performed. The acquired data show that an increase of moisture content in pores in the area of the studied overhang decreased tensile strength of the sandstone to 14 % of its dry value, and increases the sandstone weathering and erosion rate, by nearly 4 orders of magnitude, compared to the same sandstone under natural moisture condition outside on the cliff area. Consequently, frost weathering, in combination with wetting weakening was found to play a major role in weathering/erosion of the sandstone cliff and overhang. Frost weathering rate in both the laboratory and field increases up to 15 times with decreasing gravity-induced stress. The results also indicate that sandstone landforms in temperate climates may potentially develop very rapidly if the pore space is nearly saturated with water, and will later remain relatively stable when the moisture content decreases. As a general implication, it is suggested that overhangs in Central Europe (and elsewhere) might be the result of rapid frost weathering of nearly saturated sandstone during the Last Glacial. This article is protected by copyright. All rights reserved.
Recent work has shown that gravity-induced stress within a landform due to vertical loading reduces weathering and erosion rates, contrary to commonly held hypotheses. The purpose of this investigation is to evaluate the negative feedback between stress and weathering of sandstone monuments at the Petra World Heritage Site in Jordan via field observations, salt weathering experiments, and physical and numerical modeling. Previous studies on weathering of Petra monuments have neglected the impact of stress, but the ubiquitous presence of stress-controlled landforms in Petra suggest that it has a substantial effect on weathering and erosion processes on manmade monuments and natural surfaces. Laboratory salt weathering experiments with cubes of Umm Ishrin sandstone from Petra demonstrated the inverse relationship between stress magnitude and decay rate. Physical modeling with Střeleč locked sand from the Czech Republic was used to simulate weathering and decay of Petra monuments. Sharp forms subjected to water erosion decayed to rounded shapes strikingly similar to weathered tombs in Petra. The physical modeling results enabled visualization of the recession of monument surfaces in high spatial and temporal resolution and indicated that the recession rate of Petra monuments was far from constant both in space and time. Numerical modeling of stress fields confirmed the physical modeling results. This novel approach to investigate weathering clearly demonstrates that increased stress decreases the decay rate of Petra monuments. To properly delineate the endangered zones of monuments, the potential damage caused by weathering agents should be combined with stress modeling and verified by documentation of a real damage.
This article gives the mathematical solutions of deflection, bending moment, and longitudinal fibre stress of seven different configurations of single- and multilayer roofs in horizontally bedded rock. The work, based on the theory of elastic beams on elastic supports, shows the importance of abutment compression in contributing to the deflection of the roof layers. When the ratio of the thickness of the individual layers, H, to the span of the working room, l, exceeds l/2 the theory becomes inexact. By using the finite element method it was found that the extreme fibre tensile stress is equal to the uniformly distributed load on the roof layer for H/l greater than 1.5. For ratios between these limits the actual tensile stresses can be determined from stress concentration diagrams.The theories for single- and double-layer roofs have been verified by model experiments. Two types of experimental equipment set-ups were used: (1) involving a uniform load on the top surface of the model; and (2) involving runs under controlled conditions in a high-capacity centrifuge. The model experiments in a centrifuge together with finite element theories display the difference in failure mode between thin and thick roof layers. Vertical to subvertical fissures in regions with high bending moments, that is in the roof centre and above the abutment, precede the collapse of a thin roof layer. High shear stresses appear above the abutment in thick roof layers (H > l/2) which give rise to arching over the opening.The theories and results from model experiments are now being put into practice at the lead mine of Laisvall, northern Sweden. Results from present precise leveling and differential roof sag (sag differences from layer to layer) measurements in the face excavations are in accordance with results from proposed theories.
Spectacular rock fins on the flanks of Salt Valley anticline in southeast Utah are formed by erosion along zones of joints. Within a rock fin, arches form where intense fracturing is localized. Fracture localization is controlled by shear displacement along existing horizontal or vertical discontinuities. Horizontal discontinuities may be shale layers, shale lenses, or bedding planes, whereas vertical discontinuities are usually preexisting joint segments. The roof and overall shape of an arch is controlled by existing shale layers, interfaces between sandstones of different properties, or secondary fractures due to shear on vertical joints. Joints that bound rock fins are related to the formation of the diapir-cored Salt Valley anticline. Shear displacement along existing discontinuities, which localizes intense fracturing, is probably related to the growth of Salt Valley anticline and its subsequent collapse due to dissolution of the anticlines salt core. 31 refs., 11 figs.
Canyonlands Country: Four Corners Geological Society Guidebook
- R W Blair
- Jr
- J N Mann
- C Mcfee
- G A Rothwell
- L M Thenhaus
- P C Thenhaus
- C Wyant
Blair, R.W., Jr., Mann, J.N., McFee, C., Rothwell,
G.A., Thenhaus, L.M., Thenhaus, P.C., and Wyant, C., 1975, Origin and Classification of Natural
Arches in Southern Utah, in Fasett, J.E., ed., Canyonlands Country: Four Corners Geological Society Guidebook, 8th Field Conference, p. 81-86.
Geologic map of Arches National Park and vicinity
- H H Doelling
Doelling, H.H., 1985, Geologic map of Arches National Park and vicinity, Grand County, Utah, with
accompanying text: Salt Lake City, Utah, Utah
Geological and Mineralogical Survey Map 74,
scale 1:50 000, 15 p. text.
The Arches of Arches National Park; A Comprehensive Study
- D J Stevens
- J E Mccarrick
Stevens, D.J., and McCarrick, J.E., 1988, The Arches
of Arches National Park; A Comprehensive
Study: Moab, Utah: Salt Lake City, Utah, Mainstay Publishing, 169 p.
Some aspects of the geology of Arches National Monument
- W L Stokes
Stokes, W.L., 1951, Some aspects of the geology of
Arches National Monument, Grant County, Utah:
Utah Academy of Sciences, Arts and Letters Proceedings 1948-1949, v. 26, p. 151.
Origin and Classification of Natural Arches in Southern Utah
- Blair
Geologic map of Arches National Park and vicinity, Grand County, Utah, with accompanying text
- Doelling
Some aspects of the geology of Arches National Monument, Grant County, Utah: Utah Academy of Sciences
- Stokes
The Arches of Arches National Park; A Comprehensive Study: Moab, Utah
- Stevens