Rock Benches Research Articles

Reconstraining the Quaternary glacial history of Lahaul Himalaya, northern India

The study reports new progress in the glacial chronology from new sites of the Chandrabhaga basin (CBB), in the transition between the monsoon-dominated Pir-Panjal and westerly-dominated Trans-Himalaya, in Lahaul Himalaya, northern India. In Lahaul, asynchronous timing and extent of late Quaternary glaciation have been reported (Owen et al., 1996, 1997, 2001; Eugster et al., 2016). The established oldest and most extensive Chandra glacial stage remains undated and speculative. We reconstrain the late Quaternary glacial history of the CBB using geomorphological mapping, Infrared Stimulated Luminescence (IRSL), and 10Be Terrestrial Cosmogenic Nuclide (TCN) surface exposure dating. Four local glacial stages, the Chandra, Batal, Dali, and Mayar, are dated to 152 ± 37 ka, 43 ± 3 ka, 21 ± 2 ka, and Little Ice Age (LIA), respectively. The Chandra glacial stage occurred during the late Marine Isotope Stage 6 (MIS 6; 186 ± 30 – 113 ± 10 ka), characterized by thick lacustrine deposits, rock benches, and ice-polished bedrock along the trunk valleys. We provide the first account of the numerical dating for the much-awaited timing of the oldest Chandra glacial stage (Local Last Glacial Maxima; LLGM) in the region (Owen et al., 1996, 1997, 2001). The LLGM in the CBB is much older than previously thought, consistent with some north-western Himalayan glacial records (Owen et al., 2006; Dortch et al., 2010; Hedrick et al., 2011; Orr et al., 2017, 2018). The Batal glacial stage occurred during the early MIS 3 and is dated from 47 ± 2 to 41 ± 2 ka, represented by the thick lacustrine deposits and impressive terraces. The Dali glacial stage occurred during the Global Last Glacial Maxima (GLGM), characterized by a series of latero-frontal recession moraines, and is dated from 33 ± 3 to 13 ± 1 ka. The Mayar advance is characterized by confined tributary valley glaciers, possibly linked to the LIA advance reported in the Kulti valley (Saha et al., 2019). The equilibrium line altitude (ELA) rose by ∼486 m from the Chandra stage to the present day. Reconstructed ELA altitudes and glacial landforms demonstrate that the glaciations have become progressively less extensive through time for this part of the western Himalayas. The change of glaciation pattern during the later part of the Quaternary is like other regions of the orogen, such pattern of glaciation reflects regional climatic forcing.

Mechanism of excavation-induced cracking of the protective layer of a rock bench in a large underground powerhouse under high tectonic stress

Excavation-induced unloading fractures in the rock mass can easily cause rock engineering structural failure under high stress. This research work investigated the unloading fracture characteristics of the protective layer of the rock bench in Shuangjiangkou underground powerhouse in detail, thereby revealing the unloading fracture mechanisms via numerical simulation and true triaxial unloading tests, and proposed an optimal method for protective layer excavation. The study found that unloading cracking failure in the rock mass is asymmetric in Shuangjiangkou underground powerhouse, with the fracturing degree of rock mass on the downstream protective layer being greater than that on the upstream side. The dip angle of the unloading fracture plane in the downstream protective layer increased with the aspect ratio (k) of the protective layer and gradually coincided with the diagonal of the protective layer. The stress concentration in the downstream protective layer was caused by the tectonic stress, the stress changed in the form of multistep loading σ1 and unloading σ3 during layered excavation. Tensile-shear unloading cracks were formed in the rock under the excavation stress path, becoming steeper and denser, and gradually parallel to σ1 as the unloading amount of σ3 increased. As k increases, the σ1 in the protective layer becomes steeper, causing the unloading fracture plane to steepen and eventually coincided with the diagonal of the protective layer. A formula for designing the protective layer width was proposed based on the relationship of the dip angle of the fracture plane with k. The findings lend themselves to implications to engineering geology in that engineers should consider the asymmetric failure characteristics of the tunnel caused by high tectonic stress, and formulate asymmetric excavation and support schemes during the construction process of rock benches or other similar structures.

ПРИМЕНЕНИЕ АНАЛИЗА КИНЕМАТИЧЕСКОЙ УСТОЙЧИВОСТИ ПРИ ОПРЕДЕЛЕНИИ РАЦИОНАЛЬНЫХ ПРОЕКТНЫХ ПАРАМЕТРОВ УСТУПОВ КАРЬЕРА

The main factor that determines the stability of the pit benches in rocky and semi-rocky rocks is geological and structural since potential collapse prisms of a particular configuration are formed by differently oriented ruptured faults (cracks) with certain spatial relationships between themselves and the pit surface. Evaluation of possible planar, wedge-shaped, and overturning collapses of individual benches is carried out by analyzing the parameters of structural disturbances identified within the rock mass of the studied section of the open pit relative to the orientation of open pit edges. The article presents the results of the analysis of the kinematic stability of the optimal parameters of the benches on the example of the Kurzhunkul open pit, which ensures their stability on the limiting contour. A sequence of actions is proposed for determining the stability parameters of rock benches in a quarry. Performing a kinematic analysis based on the identified systems of cracks allows us to determine the probable failure patterns of the slopes of the benches and select the calculation schemes for their stability to substantiate their optimal parameters and make decisions on their stabilization. Geological and structural zoning of the quarry space according to this method allows analyzing the holding capacity of safety berms to control detected collapses, to highlight benches that need to be brought into a safe state with the determination of their stable parameters, and to recommend options for optimizing the design profile.

BLOCK MODEL APPROACH FOR ANALYSIS OF ROCK BENCH STABILITY IN OPEN PIT MINES

Wedge failure analysis conventionally adopted for the design of global angle of open pit mines is not always applicable when the area consists of jointed rock benches. In this analysis, the unstable wedge is assumed intact while, in fact, the rock bench consists of an assemblage of blocks generated by the intersections of well-defined discontinuities. Block model approach adopted in Resoblok provides an alternative solution for the design and stability analysis of a rock bench. The model analyzes the stability of individual block by limit equilibrium method while the discontinuity aspect is considered statistically based on geometrical modelling of the individual fractured rock mass. This paper presents the stability analysis of five rock benches from two mining sites. The analysis shows that Resoblok model provides a good indication of the distribution, the number and the volume of unstable blocks; therefore, it is useful for the design of open pit mines. The result of this study is in good agreement with field observation.

Mineralogical and Geochemical Compositions of the Lopingian Coals and Carbonaceous Rocks in the Shugentian Coalfield, Yunnan, China: with Emphasis on Fe-Bearing Minerals in a Continental-Marine Transitional Environment

This paper presents the mineralogical and geochemical compositions of coal benches and non-coal (carbonaceous rock benches, parting, roof and floor) samples from the No. 1 Coal in the Longtan Formation of the Permian-Lopingian epoch from the Shugentian Coalfield, eastern Yunnan Province, southwestern China. The coal is rich in Nb, Ta, Zr, and Hf, which were derived from the Kangdian Upland with the dominant compositions of the Emeishan basalt. The minerals identified in the samples include mixed-layer illite-smectite, kaolinite, quartz, siderite, and minor calcite, pyrite, anatase and ankerite. Albite and chamosite occur in the roof and floor samples. The parting sample (SGT1-2p) is characterized by abundant siderite (64.9%) and calcite (20.1%), and one carbonaceous rock sample SGT1-11 contained a large amount of pyrite (26.1%). Four factors were responsible for the geochemical and mineralogical compositions in the samples; namely, the terrigenous detrital materials transported from the Kangdian Upland, direct volcanic ash inputs, multi-stage inputs of hydrothermal fluids, and marine influences. The co-existence of siderite and pyrite was attributed to a continental-marine transitional environment.

A 20‐year data set of species replacement patterns in the middle‐intertidal zone of Tatoosh Island, Washington, USA

AbstractThis data set documents changes in the sessile species occupying several rock benches on wave‐exposed shores of Tatoosh Island, Washington, USA from 1993 to 2012. Plots and transects were located within the middle intertidal zone dominated by the mussel Mytilus californianus. Data were taken in 14 60 × 60 cm quadrats positioned at two corners with permanent marking screws. A 10 × 10 grid defined by the intersection of equally spaced monofilament lines yielded a set of 100 fixed points per quadrat per census. Plots were generally located initially in sites that had undergone natural wave disturbance in the past 0–3 years, to better document transient successional dynamics, and most plots have experienced one or two disturbance/succession cycles over the course of the data collection. A further set of points was monitored on 11 permanent transects 9.1 m long with 30 initially randomly placed points, which cover a broader span of shoreline than the quadrats. The data collection was implemented to parameterize Markov Chain models and use these to make predictions about the effects of local species extinction that could subsequently be tested in an experimentally tractable ecosystem. To date, the data have been used in (1) a parameterization and analysis of a basic multi‐species Markov Chain model, (2) a spatially explicit cellular automata, (3) a reformulation, parameterization, and experimental test of the Neutral Theory of Biodiversity, (4) a comparative analysis of Markov Chain models across different marine habitats, (5) development of an approach to link global change to multi‐species interactions using an environment‐dependent (ocean acidification) series of Markov Chain models, (6) analysis of changes in system dynamics following experimental species extinction, and (7) parameterization of population dynamic models of mussels revealing density‐linked stochastic patterns. The Markov Chain and Neutral models have subsequently been tested in independent experiments. These data may be of further use in analyzing detailed patterns of species transitions, as well as more standard analyses of spatial and temporal patterns of species abundance and richness.

A 20-year dataset of species replacement patterns in the middle-intertidal zone of Tatoosh Island, Washington, USA

This data set documents changes in the sessile species occupying several rock benches on wave-exposed shores of Tatoosh Island, Washington, USA from 1993-2012. Plots and transects were located within the middle intertidal zone dominated by the mussel Mytilus californianus. Data were taken in 14 60 x 60 cm quadrats positioned at two corners with permanent marking screws. A 10 x 10 grid defined by the intersection of equally-spaced monofilament lines yielded a set of 100 fixed points per quadrat per census. Plots were generally located initially in sites that had undergone natural wave disturbance in the past 0-3 years, to better document transient successional dynamics, and most plots have experienced 1-2 disturbance/succession cycles over the course of the data collection. A further set of points was monitored on 11 permanent transects 9.1 m long with 30 initially randomly placed points, which cover a broader span of shoreline than the quadrats. The data collection was implemented to parameterize Markov Chain models and use these to make predictions about the effects of local species extinction that could subsequently be tested in an experimentally tractable ecosystem. To date, the data have been used in 1) a parameterization and analysis of a basic multi-species Markov Chain model, 2) a spatially-explicit cellular automata, 3) a reformulation, parameterization and experimental test of the Neutral Theory of Biodiversity, 4) a comparative analysis of Markov Chain models across different marine habitats, 5) development of an approach to link global change to multi-species interactions using an environment-dependent (ocean acidification) series of Markov Chain models, 6) analysis of changes in system dynamics following experimental species extinction, and 7) parameterization of population dynamic models of mussels revealing density-linked stochastic patterns. The Markov Chain and Neutral models have subsequently been tested in independent experiments. These data may be of further use in analyzing detailed patterns of species transitions, as well as more standard analyses of spatial and temporal patterns of species abundance and richness. This article is protected by copyright. All rights reserved.

Solar radiation plays a role in habitat selection by the sea star Pisaster ochraceus

In order to avoid intense abiotic stresses at low tide, motile animals in the intertidal zone must identify and move into 'safe' microhabitats before the tide drops. We experimentally investi- gated the role of solar radiation in the microhabitat selection of the sea star Pisaster ochraceus, an ecologically important predator in the Pacific Northwest. Specifically, we tested the response of P. ochraceus to UVB radiation (290 to 315 nm), UVA radiation (315 to 400 nm), and photosynthetically active radiation (PAR; 400 to 700 nm) to determine whether specific wavelength ranges prompted specific responses. Sea stars were exposed to ambient summer light levels in shallow water in out- door aquaria in which one-half of the tank was covered with a light-filtering material. We established 4 light filtration treatments: filter control (+UVB +UVA +PAR), no UVB (-UVB +UVA +PAR), no UVR (-UVB -UVA +PAR), and shade (-UVB -UVA -PAR), plus a no-filter control treatment (+UVB +UVA +PAR). P. ochraceus did not avoid UVB or UVA, but they did avoid PAR: individuals in tanks with shaded areas spent significantly more time under the filters than did individuals in the tanks with fil- ters that blocked UV radiation. In field surveys, 85% of emersed P. ochraceus encountered during low tide were located in areas completely shaded from both PAR and UV radiation (by rocks or by the intertidal kelp Hedophyllum sessile). This study demonstrates that the presence of PAR in shallow water conditions prompts P. ochraceus to seek out shaded microhabitats that have more benign low- tide abiotic conditions than exposed rock benches.

Algal zonation in tidepools: experimental evaluation of the roles of physical disturbance, herbivory and competition

Tidepool algal community structure and organization is poorly understood relative to our understanding of communities on emergent intertidal substrata. The patterns and causes of algal zonation in small, steeply sided, mid-intertidal pools (potholes) were investigated on the Oregon coast. From top to bottom, pools were characterized by a foliose algal zone (pool edges), a crustose coralline algal zone (pool sides), and a bare zone (pool bottoms). These zonation patterns were extremely persistent. Erect fleshy algae fluctuated seasonally, with peaks in abundance during the summer. Seasonal variation was consistent between years, not only among all pools but also in individual pools. The relative importances of physical disturbance and biotic interactions in determining tidepool community structure were evaluated along a gradient of physical harshness. The pools studied here offered the opportunity to experimentally manipulate physical disturbance which is often difficult, rarely done, and essential to our understanding of community organization. Using a factorial experimental design, cobbles, limpets and coralline algae, a major space occupier in tidepools, were manipulated. Identically designed experiments were performed at both wave-exposed and wave-protected sites using identical replicate pools chiseled out of rock benches. Scouring by cobbles generally reduced the abundance of organisms in pools, limited them to higher areas in pools, or both. The effects of cobbles were especially apparent on pool bottoms or on the depth distribution of algae, suggesting that there was a strong vertical gradient of scouring where cobbles were present. This scouring gradient was confirmed in natural pools by measuring scouring rates with painted plates, and scouring evidently led to the zonation patterns observed in natural pools. Herbivores had no effect on algal distributions within tidepools, but evidently influenced the relative abundance of fleshy algae at different wave exposures. Abundant herbivorous snails at protected sites may have resulted in a high proportion of herbivore-resistant coralline algae and more bare space, while at exposed sites, herbivorous snails were scarce and fleshy algae were more abundant. Removal of all corallines resulted in an increase in the abundance of fleshy crustose algae at both sites, suggesting that corallines pre-empt space from fleshy crusts. Competition among fronds of erect algal groups was evaluated by reciprocally removing the fronds of erect fleshy and erect coralline algae. After the removal of erect fleshy or erect coralline algal fronds, only slight changes were observed over 18 months, suggesting that competition between algal fronds is unimportant in determining algal distribution and abundance. For these intertidal potholes, algal distributions within pools are largely determined by the physical process of scouring, but herbivores may cause between site variations in pool communities.

Indirect Effects and Habitat Use in an Intertidal Community: Interaction Chains and Interaction Modifications

Species may indirectly affect populations of other species in two basic ways. Interac- tion chains result from linked direct interactions between species pairs. Interaction modifica- tions, emergent effects of a multispecies assemblage, result when a third species changes how a pair of other species interacts. Experiments and patterns of abundance in the upper zone of a rocky intertidal community provide examples of both types of indirect effects. By consuming limpets (Lottia digitalis), bird predators indirectly enhance acorn barnacle (Balanus glandula) abundance on horizontal rock benches because limpets dislodge or inadvertently consume young acorn barnacles, an example of an interaction chain. Because L. digitalis has a light-colored shell that is similar in color to barnacle shells, barnacles alter the bird-limpet interaction by changing the ability of birds to find limpets, an example of an interaction modification. Distin- guishing between interaction chains and interaction modifications is important because interac- tion chains can be predicted with only a knowledge of pairwise species interactions while de- scriptions of interaction modifications require a knowledge of all species combinations involved, precluding a priori prediction. By classifying empirically determined indirect effects, the degree to which indirect effects can be predicted from a knowledge of pairwise interactions can be determined, and the mechanisms that cause interaction modifications can be identified to aid in future prediction.

DEVELOPMENT OF THE CASPIAN SHORELINE UNDER THE PRESENT INCREASE IN ITS LEVEL

The authors, questioning the implications for the Caspian of a conventional model of shoreline dynamics on a flat, sandy coast upon a rise in sea level, present the findings of field observations made along the Dagestan shore during the period (post-1977) of the recent rise of the Caspian Sea level. They argue that changes in coastal morphology have varied largely according to the steepness of the offshore slope and, in general, sharply accelerated erosion was observed only on more steeply sloping sections of coast and in areas previously protected by rock benches (translated by Jay K. Mitchell; PlanEcon, Inc.; Washington, DC 20005).

Feeding Rates and Prey Selection of Oystercatchers in the Pearl Islands of Panama

The feeding pattern of the American oystercatcher (Haematopus palliatus) was examined in the Pearl Archipelago, on the Pacific coast of Panama. Twenty species of mobile molluscs from intertidal rock benches formed more than 97 percent of the diet, but most species were underrepresented as prey relative to their abundance in nature. At middle to high shore levels, oystercatchers avoided three abundant but small-bodied snails, and fed primarily upon larger size classes of two other, large-bodied herbivorous molluscs. Pairs of birds averaged more than 80 prey items per low tide. This predation rate significantly affected the local abundance and size structure of the two preferred species. Lower on the shore foraging was limited, and relatively few prey of any species were taken. Prey utilization by oystercatchers resulted from the behavior of prey species, their position on the shore, and active choice by birds. Local mortality of gastropods due to predators such as oystercatchers may contribute to observed, intersite variability in the distribution and abundance of intertidal organisms in Panama. RARE, PATCHILY DISTRIBUTED, OR INTERMITTENTLY PRESENT PREDATORS play important roles in a number of habitats, including coral reefs (Glynn 1976), subtidal kelp beds (Duggins 1980), tropical forests (Gradwohl & Greenberg 1982), and tropical rocky intertidal shores (Garrity & Levings 1981). Such predators may be difficult to examine (Edwards et al. 1982); consequently, their effects can be considerable, yet easily overlooked (Holmes et al. 1979). One such group, shorebirds, has been examined in a variety of intertidal habitats (Schneider 1978, Pitelka 1979, Frank 1982, Goss-Custard et al. 1982). They have been linked to changes in the distribution, abundance, and size structure of prey populations (O'Connor & Brown 1977; Schneider 1978; Frank 1982; Sutherland 1982a, b; Marsh 1983; Quammen 1984). Oystercatchers (Haematopus spp.) and other shorebirds that forage on rocky shores have been studied thoroughly in temperate locales (Feare 1971; Heppleston 1971a, b; Hartwick 1974, 1976, 1978, 1981; O'Connor & Brown 1977; Hartwick & Blaylock 1979; Frank 1982; Goss-Custard et al. 1982; Hulscher 1982; Sutherland 1982a, b), but not at lower latitudes. We have recently studied the feeding activity of the American oystercatcher, Haematopus palliatus (Temminck), along rocky intertidal shores of the Pacific coast of the Republic of Panama. The rocky shores of this region have been described (Lubchenco et azl. 1984 and included references). Tides are semidiurnal with a range of approximately 6 m. The beds of sessile bivalves or of erect algae typical of most temperate areas are absent here (Lewis 1964, Dayton 1971, Paine 1971, Stephenson & Stephenson 1972), but there is a diverse and abundant assemblage of snails, limpets, pulmonates, and chitons (Lubchenco et al. 1984). The distribution of oystercatchers along Panama's Pacific shore was estimated in the course of other studies (1976-1983). Their presence is easily noted since these birds are noisy and conspicuous in flight, nest or roost in open areas just above the high tide line, and forage on rocky intertidal shores. Our observations were reinforced by reports from other scientists and birdwatchers, and covered virtually the entire coast. Birds have been reported in Los Santos (Ridgely 1976), and occasional sightings have been made elsewhere in Panama (P. W. Glynn & D. C. Schneider, pers. comm.), but oystercatchers appear commonly only in the Pearl Archipelago (ca 825 'N, 79?W). We roughly estimate a resident population of 250-350 birds, based on two surveys of islands within the archipelago (October 1982, April 1983). This study examines feeding rates and foraging behavior of oystercatchers within the Pearl Archipelago, describes species of prey, and suggests some potential effects on intertidal molluscs and upon the intertidal community.

Tythegston quarry landfill site

Tythegston quarry lies near Bridgend in South Wales Fig. 1 The quarry covers an area of about 9 hectares and has been used as a waste disposal site since 1968. The maximum depth of the quarry was 38 m, with rock benches along the northwest and south sides about 20 m below ground level. Solid wastes, 50% being crude domestic refuse, have been tipped at the rate of about 86 000 tonnes per annum since December 1968. Initial disposals were into the central, seasonally flooded, depression located just to the right of the area photographed (fig.) In 1971, drums of chemical wastes and motor tyres were codeposited but since 1972 only untreated domestic and paper mill wastes have been tipped. The quarry is excavated into the Main Limestone Series of the Carboniferous Limestone Table 1, at the C 1 and C 2 /S 1 zonal boundary. The limestone is massively bedded (Fig.2) with a northerly dip of 5° (B) but with some cross folding and associated small faults, for example, the zone of intense fissuring visible on the rock face (F), locally increasing dips to between 35° and 55°N. The photograph is of part of the northern wall of the quarry defined by conjugate joints trending 010–190° (J1) and 115–295° (J2). The northerly joints (J2) show evidence of solution widening (karstification) but with Triassic sediment infilling to depths of over 40 m. The widened joints in zone F have been infilled with more recent, possibly Pleistocene, material. The

Processes of colluvial slope development at MC Mechen, west Virginia

The Applachian Plateau Physiographic Province of the United States has severe landslide problems that are attributed primarily to steep topography and thick colluvial soils. Two basic modes of colluvial soil accumulation on hillsides are identified: 1) on rock benches; and 2) in swales. The large colluvial masses, unless disturbed by construction activity, were assumed to be stable and to have developed under periglacial conditions. However, processes of colluvial slope development and a recent landslide, activated by abnormally high precipitation at McMechen, indicate that such colluvial masses form continuously and do not require periglacial conditions to explain their existence.

Walrus ethology. II. Herd structure and activity budgets of summering males

Male walruses, Odobenus rosmarus (L.), summering (outside the breeding season) at an Alaskan hauling ground, use all available kinds of beach habitat: cobble and boulder beaches, rock benches, and large boulders. Formation and dissolution of large herds on land can occur rapidly. Walruses are very gregarious and positively thigmotactic. In cool weather about 98% of walruses on land lie in passive body contact with other walruses. Dominant walruses (large, with long unbroken tusks) are most successful in entering herds on land, and in keeping positions in them. This results in overrepresentation of subordinates in the periphery of herds. Agonistic interactions occupy 5–10% of the time of walruses in large herds on land, and cause local disturbances that lead to agonistic involvement of up to 20 animals. Fewer kinds of social activity occur on land than in water. On land and in shallow water, small walruses are generally more active than large ones. Extensive body contact while they are hauled out is chiefly an adaptation for heat conservation and may also facilitate molting. The extreme gregariousness of walruses may have evolved because individuals joining large herds have a greater probability of achieving extensive body contact than have those joining small herds.

Coastal morphology; Southwest Great Abaco Island, Bahamas

An examination of a portion of Great Abaco Island. Bahamas, reveals that the coastal zone may be subdivided into two morphological units: barrier and lagoon complexes, and rock benches with boulder ramparts. The sand barriers, colonized by multistoried vegetation, are stratigraphically thin and are characterized by narrow coralline beaches and beach rock. The imbricate boulder ramparts adjacent to deeper water, reveal that, on occasion, high wave energy conditions occur. It is apparent that hurricanes and tropical storms are significant in modifying coastal southwest Great Abaco.

State of stress in a bench undercut by a slot in its base

The widespread adoption of the method of working soils and rocks by caving is hindered by our inadequate knowledge of the laws of this process. The main factors inflnencing the process of caving of soft or rock benches are the characteristics of the undercuts, the height of the caved layer, the physicochemical properties of file ground, etc. Detsrmination of the conditions of caving of the ground must be preceded by a consideration of the stats of stress arising in the bench when it is undercut by slots. One of the most promising scberrms for working a bench by caving is shown in Fig. 1; we shall use this scheme as a basis for our calculatiom on the state of stress of the bench when undercut by a slot in its base. The bench will be regarded as an elastic parallalepiped, prevented from moving on two adjacent faces (aoc and boc) and loaded only by its own weight. The components of displacements of points in the body along the coordinate axes U(x, y, z), V (x, y, z), and Wfx, y, z) must satisfy Lamd's equation [I]:

Types of Pediments in the San Pedro Valley Region, Arizona

1954of Pacific Coast Geographers17 TYPES OF PEDIMENTS IN THE SAN PEDRO VALLEY REGION, ARIZONA Tuan Yi-fu University of California, Berkeley The word pediment, as first defined by McGee (1) and as used in this paper, refers to an exposed bedrock surface at the foot of the mountain front, or a rock surface covered by a layer of alluvium no thicker than the depth of the stream scour. In igneous rocks of coarse texture, the pediment may be covered only by its own regolith — i.e, the material weathered in place rather than transported alluvium. Under this definition, which excludes all postulates of an extension of the surface beneath the alluvium, the pediment is only a fragmentai feature of the arid landscape in the American Southwest in contrast to the ubiquity of alluvial fans, bahadas, and bevelled piedmont surfaces or "peripediments". There are at least two major types of pediments in the San Pedro Valley region of Arizona: 1. The narrow rock benches or embayments which fringe a broad mountain mass, and are separated from the alluvial deposits of the basin by a high-angle fault; 2. The extensively bevelled bedrock surfaces out of which the mountains appear to rise as mere residuals. The¦Santa Catalina and Huachuca pediments belong to the first type, whereas the south Dragoon and Johnny Lyon Hills pediment belong to the second. The Santa Catalina Pediments The northwest and south fronts of the Santa Catalina Mountains show narrow pediments because they have had little time to develop since the latest major uplift along the normal faults in late Tertiary or early Quaternary time (?). The morphology of these pediments can be described best in the context of their adjoining mountain fronts. (Figure 1) The south front of the Santa Catalina Mountains is composed of gneiss. It is much ravaged by erosion, and has a series of parallel canyons separated by bare, sharp-crested ridges. The northwest front is divisible into two morphologic units along the Romero and Montrose canyons. The belt occupied by these canyons marks the transitional zone between gneiss of the southern forerange and an intrusive igneous rock of distinctly granitic texture in the northern portion of the northwest front, where it is known as the Samaniego Ridge. Canyons and sharp crests are absent in the Samaniego Ridge which is rounded at the top and has long, smooth, concave slopes leading down to the foot of the mountain. It is along this granite ridge that amphitheatre-like, incipient pediments have formed. (2) Wide pediments generally do not occur along a deeply dissected mountain front, and the southern forerange off the Santa Catalina Mountains is no exception. Here, large alluvial fans appear to emerge directly from the mouths 18 Yearbook of The Association Vol. 16 Figure 1. Diagram of the northwest corner of the Santa Catalina mountains. SW SANTA CATALINJA PEDIMENT BENCH NE Pediment Bench Fan A (Mesa) Fan B Fan C Fan B Gneiss . Conglomerate beds (Tilted and deformed) Figure 2. Diagrammatic cross-section of the south front of the Santa Catalina mountains showing the relationship between the pediment benches and the alluvial fans. (Modified from Blissenbach ) 1954of Pacific Coast Geographers19 of the canyons, and the steep mountain wall seems to be in sharp contact with the aggradational slope of the fans. But this simple arrangement is deceptive and fades upon closer scrutiny. The fans have been much eroded. Blissenbach recognizes three fan levels, the highest and oldest remnants of which are isolated knolls or mesas. (3) The highest fan mesa, when projected toward the mountain front and across the fault that separates the gneiss from the alluvium, correspond in elevation to the narrow pediment benches developed on gneiss, whereas a more recent fan (fan B) still covers much of the lower gneissic surface. It is outside the scope and purpose of this paper to trace the history of the uplift of the mountain front along the fault, and the relation of the succesive fans to the pediment benches. Figure 2 summarizes a possible interpretation in diagrammatic form. The rock benches are at most only a few hundred feet wide. They have suffered much from erosion, and are best preserved in the intercanyon areas...

Pediment Characteristics and Terminology

p LANATE bedrock surfaces, rock pediments and rock benches, form a large proportion of the comparatively flat relief elements of arid and semiarid landscapes. The origin of these landforms has been controversial since Gilbert first recognized and described them'. More than seventy years of subsequent research has produced a variety of terms, descriptions, and theories of development. An objective appraisal of this body of material is warranted at the present time. Analyses of the published descriptions reveal the apparent universal nature of some of the physical characteristics of the pediment. Herein attention is focused on such diagnostic criteria. The terminology generally used to designate the erosion surface is examined with purpose to select the most appropriate terms. In this respect terms suggesting genesis are avoided, emphasis being placed rather on geomorphic location. It is further suggested that the condition of alluviation of the bedrock surface, a criterion of significance as to mechanics of truncation, may be employed to qualify the feature further. Two major theories of origin have proven acceptable to students of this subject. One proposes that rock planation is a result of lateral corrasion by steepgradient, ephemeral streams; the other contends that weathering of escarpment faces causes their retreat, leaving as a consequence a relatively planate bedrock surface. Some recent workers have combined certain tenets of these two concepts. The present writing is not concerned with an evaluation of hypotheses. The intent is rather to offer a summarization of physical characteristics and terms. A summmary tabulation of 43 works pertinent to the subject is arranged in an appendix

GLACIAL EROSION AND THE BURIED WYOMING VALLEY OF PENNSYLVANIA

Engineering investigations on the Susquehanna River in Pennsylvania, in New England, and Quebec have thrown new light on the factors controlling glacial erosion, especially glacial overdeepening, and show the importance of geologic structure and preglacial topography. Near Bingham, Maine, the ice sheet widened a northwest-southwest valley in a hard, massive, crystalline schist with rectangular jointing, but had little effect where a belt of soft slate crosses the valley. Other instances of the failure of ice to remove rock benches and deepen valleys, due to the influence of geologic structure or of preglacial topography, are described at Passamaquoddy Bay, Maine, on the Connecticut River, New Hampshire, on the Deerfield River, and at Cambridge, in Massachusetts, and on the St. Maurice River in Quebec. The buried Wyoming Valley on the Susquehanna River strikes parallel to the direction of ice movement and contains overdeepened rock basins separated by rock ridges. The maximum over-deepening is about 150 feet. Engineering investigations show that immediately upstream from the Wyoming Valley is another buried valley which has been overdeepened continuously for 8 miles, from West Pittston to Falls, although it is generally transverse to the direction of ice movement. The influence of geologic structure and of preglacial topography upon glacial erosion are indicated.