Individual Boulders Research Articles

Interest and design of magnetic properties measurements on planetary and asteroidal landers

Intrinsic magnetic properties, like susceptibility, provide a precise determination of the iron phases with a penetration depth not available with other chemical and mineralogical sensing tools, thus allowing to unravel space weathering effects. Systematic measurements of meteorites demonstrate that susceptibility measurements on asteroid surface could be a very efficient way to assign a meteorite class to a given asteroid. Another application could be the characterization of the highly magnetic Martian regolith. On the other hand, natural remanent magnetization (NRM) measurements are crucial to interpret magnetic field anomalies such as those found on Mars and Moon, and likely to be found on Mercury. NRM gives also access to past magnetic fields and extinct planetary dynamo. Rugged, light and low consumption systems already exist for such measurements on Earth and we present a scheme to integrate both magnetic susceptibility (using a LC oscillator) and NRM (using a 3 axis fluxgate or a gradiometer) to offer a versatile instrument package for any mission involving a lander. For the LC oscillator calibration of the geometric factor is presented. The fluxgate can be used both for making local magnetic anomaly maps, thus investigating subsurface structures, and for evaluating NRM of individual boulders.

Calibration ofin situmagnetic susceptibility measurements

SUMMARY The aim of this work is to calibrate a magnetic susceptibility field probe (SM30) in order to allow measurement on natural boulders of any shape and size. This calibration was performed through measurements on pebbles of different shapes, sizes and lithologies. We model a correction factor (geometric factor) that has to be applied to the SM30 measurement to obtain the real volume magnetic susceptibility of a sample of known volume. This geometric factor depends mostly on the volume of the sample, and to a lesser extent on its shape. We also present an original magnetic measurement scheme that provides, with three SM30 measurements at different distances, an estimation of the volume of a sample. By combining the two calibration models, it is possible to obtain the volume magnetic susceptibility of a natural sample with only three SM30 measurements, without requiring additional information such as sample volume. On the other hand, the calibration performed on a semi-infinite homogeneous body with variable measuring distance leads to a 2-D model of the SM30 response over its integration volume and allows the determination, with a sufficient number of discrete measurement at variable distance, of the thickness and susceptibility of the different layers of a composite body. Both types of calibration were successfully validated on natural or synthetic samples. Therefore the SM30 portable susceptometer appears to be a suitable instrument to perform in situ magnetic susceptibility measurement of individual boulders or to establish magnetic susceptibility profiles. In addition, the modest dimensions of the probe and the relative simplicity of the proposed measurement schemes should enable automation of the measurements that could find applications for robotic exploration of solid bodies of the solar system, for example.

Contribution of real‐time kinematic GPS in the study of creeping mountain permafrost: examples from the Western Swiss Alps

AbstractThe use of real‐time kinematic (RTK) GPS for studying the dynamics of creeping mountain permafrost is evaluated on the basis of measurements carried out on three test sites in the Western Swiss Alps. The high precision of the method and its ease of use make the RTK GPS appropriate for this kind of study. A wide range of information can be obtained by comparing the position of a set of surveyed points after two campaigns of measurement: determination of both horizontal and vertical components of surface movements, identification of compression and extension zones, estimation of thickness variation and, finally, determination of three‐dimensional (3D) rotational movements of individual boulders. Temporal variations of all these parameters can then be identified after more than two repetitions of the measurement series. The results obtained on the test sites are presented and discussed from a geomorphological point of view. They were sometimes surprising, for instance the strong difference in surface velocities of two morphologically similar rock glaciers situated in the same area or the small 3D rotation of blocks located on the front of a rapidly moving rock glacier. Copyright © 2004 John Wiley & Sons, Ltd.

Remotely sensed geology from lander‐based to orbital perspectives: Results of FIDO rover May 2000 field tests

Blind field tests of the Field Integration Design and Operations (FIDO) prototype Mars rover were carried out 7–16 May 2000. A Core Operations Team (COT), sequestered at the Jet Propulsion Laboratory without knowledge of test site location, prepared command sequences and interpreted data acquired by the rover. Instrument sensors included a stereo panoramic camera, navigational and hazard‐avoidance cameras, a color microscopic imager, an infrared point spectrometer, and a rock coring drill. The COT designed command sequences, which were relayed by satellite uplink to the rover, and evaluated instrument data. Using aerial photos and Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data, and information from the rover sensors, the COT inferred the geology of the landing site during the 18 sol mission, including lithologic diversity, stratigraphic relationships, environments of deposition, and weathering characteristics. Prominent lithologic units were interpreted to be dolomite‐bearing rocks, kaolinite‐bearing altered felsic volcanic materials, and basalt. The color panoramic camera revealed sedimentary layering and rock textures, and geologic relationships seen in rock exposures. The infrared point spectrometer permitted identification of prominent carbonate and kaolinite spectral features and permitted correlations to outcrops that could not be reached by the rover. The color microscopic imager revealed fine‐scale rock textures, soil components, and results of coring experiments. Test results show that close‐up interrogation of rocks is essential to investigations of geologic environments and that observations must include scales ranging from individual boulders and outcrops (microscopic, macroscopic) to orbital remote sensing, with sufficient intermediate steps (descent images) to connect in situ and remote observations.

High-resolution geophysical investigations of estuarine and nearshore sediment dynamics, River Bann estuary, Northern Ireland coast.

The River Bann, Northern Ireland, discharges onto the high-wave energy, swell-dominated North Atlantic coast. The funnel-shaped estuary, buttressed by Tertiary basalt outcrops, is partially barred by dunes developed on a mid-Holocene gravel barrier. The meso-scale evolution of the estuary and nearshore zone therefore demonstrates the interaction of fluvial, marine and coastal processes. Echo sounder, side-scan sonar and Chirp sub-bottom profiler were used to map bathymetry, surficial sediments and sediment stratigraphy respectively. Side-scan sonar data, ground-truthed by surficial sediment sampling, yielded 3 distinct acoustic facies, interpreted as sand (90% of study area) bedrock (9%) and gravel (1%). Oblique nearshore sandwaves, (wavelength 20 - 60m, height 0.8 - 2.4m) are replaced 1.5km offshore by planar sands (20m water depth). Adjacent to the bedrock headland of Portstewart Point lies a relict shore platform (at −30m OD Belfast), likely formed during a late-Pleistocene lowstand. Individual boulders up to 1m diameter are imaged on the bedrock surface. The surface is partially overlain by active, shore parallel gravel wedges surfaced by ripple beds (wavelength 1.5m, height 1.0m) which are partially overlapped by planar sand. Repeat seasonal surveys show general onshore surface bedform migration during winter and offshore re-distribution of sediment in summer. These changes suggest that west to east tidal currents re-circulating sediment between the offshore and nearshore zones are modified seasonally by changes in wave regime and river discharge. Chirp surveys revealed two main seismic stratigraphic units, termed inshore and offshore, overlying the acoustic basement, the inshore unit being more internally complex. Four major controls on Holocene nearshore sedimentation on the north coast of Northern Ireland are suggested: (1) antecedent geological constraints, (2) sediment supply, (3) wave climate and (4) tidal current regime. Apart from antecedent geology, these controls have been, and continue to be, modified by RSL fluctuations and storm events.

Bryophyte Species Richness on Insular Boulder Habitats: The Effect of Area, Isolation, and Microsite Diversity

Abstract The bryophyte communities on glacial erratic boulders in the Adirondack Mountains of New York are used as an experimental system to test the predictions of island biogeographic theory. Moss species composition, abundance, richness, and diversity were determined for 39 boulders. Bryophyte species richness on these insular habitats ranges from 3 to 14. Competitive dominance of the component moss species was determined. Boulders were characterized as to size, degree of isolation, and microhabitat heterogeneity. Patterns of disturbance and gap formation in the moss cover, and patterns of recolonization were quantified. The distribution of bryophyte species among individual boulders does not conform to the predictions of island biogeographic theory. Neither the size of the boulder nor its degree of isolation are significantly related to species richness. Richness was also found to be independent of the abundance of the competitive dominants. Overall measures of microhabitat heterogeneity were not corr...

Field Measurement of Boulder Flow Drag

A field method for measuring the flow drag force on individual boulders on a riverbed is identified and tested. Drag is determined from the momentum deficit in the boulder wake, obtained by measuring the flow velocity in a traverse across the wake. A field test yielded a mean drag coefficient of 0.36, which is low compared with flume measurements of 1.1–1.5 for cubes. This may be the result of higher velocity water near the riverbed being sucked into and reenergizing the wake. Further tests are needed to define the conditions in which the method can be used and to indicate how well the assumptions behind the method can be accommodated in a natural flow environment.

An Alluvial Surface Chronology Based on Cosmogenic36Cl Dating, Ajo Mountains (Organ Pipe Cactus National Monument), Southern Arizona

A chronology of alluvial surfaces on piedmont slopes below the western Ajo Mountains, southern Arizona, has been obtained using cosmogenic36Cl accumulation and AMS radiocarbon dating. The apparent36Cl ages of individual boulders range from 520,000 to 13,000 yr, and the14C ages of organic material in the two young terraces are 2750–2350 and 17,800 cal yr B.P. The sequence of36Cl ages is consistent with the apparent stratigraphic order, but groupings of similar ages for different surfaces appear to result from repeated reworking of older surfaces associated with the deposition of younger ones. The youngest surface gave a distribution of36Cl ages about 30,000 yr older than the14C and soil ages; however, this distribution had36Cl ages that overlapped with36Cl ages from active channels and hillslopes. We attribute the older-than-expected exposure ages of sampled boulders to inheritance of36Cl while residing near the surface during very slow erosion on the mountain front. Our results show that although cosmogenic nuclide accumulation can help establish chronologies for surfaces in piedmont settings, care must be used in evaluating the effects of complex exposure histories.

Extensive Boulder Erosion Resulting from a Range Fire on the Type-Pinedale Moraines, Fremont Lake, Wyoming

In July 1988, the FlatFish Range Fire burned over the type-Pinedale moraines at Fremont Lake, Wyoming, and caused extensive exfoliation of exposed boulder surfaces. The mass of exfoliated material from 130 of 1030 boulders investigated was measured and recorded with information concerning factors that could influence the extent of fire-induced exfoliation. The range in thickness of material removed from 98 randomly selected boulders within the burn area (averaged over the entire exposed boulder surface area) is 6.1 to < 0.1 mm. The mean thickness loss for all 98 boulders is 0.9 mm/fire and the expected loss from individual boulders (median) is 0.4 mm/fire. At the 95% confidence level there is no significant relationship between the degree of exfoliation and boulder size, lithology, grain size, proximity to vegetation, or vegetation density. The expected fire-induced boulder surface erosion rates range from 5.9 to 0.3 x 10 -3 mm/yr on boulders in sagebrush rangeland where fire recurrence intervals are typically every 20 to 400 yr. Fire-induced exfoliation may account for differences in boulder size and abundance on Pinedale and Bull Lake moraines. Surface dating methods using varnish or cosmogenic nuclides may yield exposure ages that are too young if the consequences of range fires are not considered when sampling boulder surfaces that are within about 2 m above ground level.

The Geological Sources and Transport of the Bluestones of Stonehenge, Wiltshire, UK

Stonehenge on Salisbury Plain is one of the most impressive British prehistoric(c.3000–1500 BC) monuments. It is dominated by large upright sarsen stones, some of which are joined by lintels. While these stones are of relatively local derivation, some of the stone settings, termed bluestones, are composed of igneous and minor sedimentary rocks which are foreign to the solid geology of Salisbury Plain and must have been transported to their present location. Following the proposal of an origin in south-west Wales, debate has focused on hypotheses of natural transport by glacial processes, or transport by human agency. This paper reports the results of a programme of sampling and chemical analysis of Stonehenge bluestones and proposed source outcrops in Wales.Analysis by X-ray-fluorescence of fifteen monolith samples and twenty-two excavated fragments from Stonehenge indicate that the dolerites originated at three sources in a small area in the eastern Preseli Hills, and that the rhyolite monoliths derive from four sources including northern Preseli and other (unidentified) locations in Pembrokeshire, perhaps on the north Pembrokeshire coast. Rhyolite fragments derive from four outcrops (including only one of the monolith sources) over a distance of at least 10 km within Preseli. The Altar Stone and a sandstone fragment (excavated at Stonehenge) are from two sources within the Palaeozoic of south-west Wales. This variety of source suggests that the monoliths were taken from a glacially-mixed deposit, not carefully selected from anin situsource. We then consider whether prehistoric man collected the bluestones from such a deposit in south Wales or whether glacial action could have transported bluestone boulders onto Salisbury Plain. Glacial erratics deposited in south Dyfed (dolerites chemically identical to Stonehenge dolerite monoliths), near Cardiff, on Flatholm and near Bristol indicate glacial action at least as far as the Avon area. There is an apparent absence of erratics east of here, with the possible exception of the Boles Barrow boulder, which may predate the Stonehenge bluestones by as much as 1000 years, and which derived from the same Preseli source as two of the Stonehenge monoliths. However, 18th-century geological accounts describe intensive agricultural clearance of glacial boulders, including igneous rocks, on Salisbury Plain, and contemporary practice was of burial of such boulders in pits. Such erratics could have been transported as ‘free boulders’ from ‘nunataks’ on the top of an extensive, perhaps Anglian or earlier, glacier some 400,000 years ago or more, leaving no trace of fine glacial material in present river gravels. Erratics may be deposited at the margins of ice-sheets in small groups at irregular intervals and with gaps of several kilometres between individual boulders.‘Bluestone’ fragments are frequently reported on and near Salisbury Plain in archaeological literature, and include a wide range of rock types from monuments of widely differing types and dates, and pieces not directly associated with archaeological structures. Examination of prehistoric stone monuments in south Wales shows no preference for bluestones in this area. The monoliths at Stonehenge include some structurally poor rock types, now completely eroded above ground. We conclude that the builders of the bluestone structures at Stonehenge utilized a heterogeneous deposit of glacial boulders readily available on Salisbury Plain. Remaining erratics are now seen as small fragments sometimes incorporated in a variety of archaeological sites, while others were destroyed and removed in the 18th century. The bluestones were transported to Salisbury Plain from varied sources in south Wales by a glacier rather than human activity.

Adding teeth to wave action: the destructive effects of wave-borne rocks on intertidal organisms.

Observations in rocky intertidal areas demonstrate that breaking waves 'throw' rocks and cobbles and that these missiles can damage and kill organisms. Targets in the intertidal were dented by impacts from wave-borne rocks. New dents/day in these targets was positively correlated with the daily maximum significant wave height and with new patches/day in aggregations of the barnacle Chthamalus fissus. Impact frequency was highest in the upper intertidal and varied dramatically between microhabitats on individual boulders (edges, tops and faces). These patterns were reflected in the microhabitat abundances of 'old' and 'young' barnacles. Comparisons were made of the survivorship and the frequency of shell damage in two populations of the limpet Lottia gigantea living in habitats which differed primarily in the number of moveable rocks (i.e. potential projectiles). The mortality rate and frequency of shell damage were significantly higher in the projectilerich habitat. In addition only in this habitat did the frequency of shell damage covary significantly with seasonal periods of high surf. Investigation of the response of limpet shells to impacts suggests that shell strength varies between species and increases with shell size. Species-specific patterns of non-fatal shell breakage may have evolved to absorb the energy of impacts. In two of the intertidal habitats studied, wave-borne rock damage was chronic and, at least in part, may have governed the faunal makeup of the community by contributing to the physical 'boundaries" of the environment within which the inhabitants must survive.

Some Observations on Boulder-Cored Frost Boils

resulting from vertical frost heaving of individual boulders. They were observed in various stages of development on two glacier forelands in Jotunheimen, southern Norway. In their final stage the features comprise a boulder core partially exposed at the surface but retaining a sediment/vegetation capping. Surrounding the boulder is a shallow trough or moat beneath which is a distinct concentration of stones and gravel. Separating this aureole of clastic material from the sides of the boulder is a gap up to 5 cm wide. It is concluded that the 'frost puir mechanism of frost heave is most likely to bring large boulders to the surface. During winter freezing a boulder acts as a heat sink, leading to radial advance of the freezing plane into the surrounding till. Once the boulder has stabilized, the resulting lateral frost sorting concentrates coarser elements of the till diamicton around the embedded portion of the boulder. The formation of boulder-cored frost boils on till surfaces exposed by post-AD 1750 glacier retreat indicates the rapidity of their development and demonstrates the advantages of glacier forelands for the study of periglacial processes and landforms. BOULDER-CORED MOUNDS up to about 1.5m in diameter, and in various stages of development, were observed at sites near the glaciers Boverbreen and Storbreen, Jotunheimen mountains, southern Norway (Fig. 1). These small-scale landforms were found in poorly-drained tills deposited within the 'Little Ice Age' limits of the glaciers. Frost heaving of individual boulders towards the surface is considered to be the main process responsible for their formation. Although up-heaved boulders that have disturbed or partly emerged from the ground surface have been reported in the literature (Dionne, 1966; Washburn, 1969; Price, 1970), little attention has been given to developmental sequences. We describe here a complete sequence, including previously unrecorded forms characteristic of the later stages. Inferences are made concerning the vertical and horizontal frost sorting processes involved in their formation, the significance of which are discussed in the broader context of frost heave, with particular reference to the so-called 'frost push' and 'frost pulL mechanisms. Nature of the sites At Boverbreen, boulder-cored frost boils are associated with a complex 'Little Ice Age' end moraine system (Fig. 1, inset A). The site is immediately adjacent to a lake, dammed by the moraines, which maintains a high moisture content and a high water table in the till. The lake level lies within 0.5m of the ground surface at this site. At Storbreen the boils occur between the two outermost 'Little Ice Age' end moraines, at the foot of the proximal slope of the outermost moraine (Fig. 1, inset B). This site, crossed by ephemeral streams, is characterized by pools of standing water in wet weather. The site at Storbreen was deglaciated after ice retreat from the glacier limit of about AD 1750 (Matthews, 1974, 1977). It is probable that the Boverbreen site

Design of Habitat Structures for Open Channels

In many cases, channel modifications have extremely adverse impacts on aquatic communities. Rudimentary design guidelines are presented for simple structures (sills, current deflectors, and individual boulders) used to accelerate biological recovery of modified stream channels. These structures increase the physical habitat diversity of a modified channel. Data from nine case studies are presented in tabular form, including stream channel characteristics, structure dimensions, and an analysis of biological effectiveness and structural durability. Available literature on habitat structures focuses on applications to unenlarged coldwater streams with coarse bed material. Documented experience with enlarged warmwater streams that have poorly armored channels is extremely limited.

On the depth of the lunar regolith

The aim of this paper is to point out that if the sinuous rilles on the Moon represent trenches in the mare ground in which they meander, the existence of a great number of individual boulders on their slopes - as discovered on the high-resolution photographs taken by US Lunar Orbiters 4 and 5 in 1967 - suggests that the solid substrate of the lunar globe is covered by broken-up debris produced by cosmic abrasion - and hereafter referred to as lunar regolith - of thickness comparable with the depth of the respective rilles - at least of those lacking flat floors; which is generally in the order of 200–300 m. This depth is much greater than that indicated previously by other methods possessing more limited depth in range; and need not apply uniformly all over the Moon. In point of fact, marial regions abounding in sinuous rilles may represent loci where the lunar regolith has developed its maximum depth.

Phenomena Due to Weathering and Erosion of Granite in Northern Part of the Nová Bystřice Hills

The northern part of the Nová Bystřice Hills - situated in the Javořice system - is composed of two types of granite. The Čímeř type (called "Einsgarn type" by austrian geologist) is two-mica; middle to finely-grained granite. The Landštejn type is two-mica, coarse-grained granite, porpháric in places. In the area under investigation two types of phenomena due to the weathering of granite may be distinguished. Phenomena dating from the warm Tertiary climate display rounded forms, in some inistances also surface sculpturing (rocky dishes). There are loaf-shaped boulders of considerable size (ranging from 2 m to 12,5 m), the size of individual boulders being closely dependent from the jointing of granite (sheet to rectangular jointing). All these boulders are in fact more resistant "cores" of granite which have not been chemically affected and as such have survived in the weathered out mantle. At a later date, the weathered material was removed by denudation and consequently the "cores" got to the surface. Some boulders were torn by mechanical activity of fros in Pleistocene. Phenomena datinf from the Quarternary were sculptured by mechanical weathering in Pleistocene and only slightly altered in Holocene. Phenomena which originated in the Periglacial era of the Pleistocene are represented here by psudocirques and rock currents. This paper describes three pseudocirques. The height of their hinder wall makes 8,5 m to 15 m. The diameter of the dish-shaped depressions ranges from 17 m to 130 m. Two of them are orientated to the North and one to the West. All of them occur at an altitude of 630 m. Rock currents streaming from them reach the length of 120-700 m, their width ranging from 40 to 150 m. More often occur smaller phenomena - the socalled frost scarps and stone fields - which are due to the frost-weathering. Frost scarps occur only at altitudes over 600 m. Their height ranges from 1,80 to 11,5 m. The dip angle of the frost scarps makes 75° to 90°: Sometimes part of the wall hangs over. In the Čímeř type of granite, scarps are higher and more regular than in the Landštejn type. Three types of frost scarps may be distinguished. The first forms a "step", the second a morphologically characteristic wall, and the third type is irregular. The orientation of the scarp depends before all upon the prevailing direction of jointing. Tone fields cover almost all areas situated above 600 m. In the Čímeř granite type boulders are smaller, sharp-edged, mostly covered by a continuous mantle of vegetation. On the other hand, in the Landštejn type, boulders are larger (up to 4,5 m) and partly rounded. Blocks of stone currents and stone fields have been transported by solifluction. In Holocene slight sculpturing of past phenomena took place. A characteristic feature of this area are "rocky dishes" with their outlets. Four stages may be distinguished in their development according to general sculpturing. In the youngest stage a primary depression originated. In the second stage the process of deepening and formation of vertical to overhanging walls took place. The third stage is characterized by the origin of a run-off rill and its gradual deepeing down to the level of the floor of the depression. The later passes into the last stage of extinction since the water cannot stagnate any more. When determining the age of the dishes, we must consider their size (diameter and depth) and the general sculpturing of the very boulder in which they are situated. In the area of question, there are dishes dating from the end of the Tertiary (mostly torn by frost in Pleistocene) of depth of 85 cm, and dishes dating from the olecene and reaching the depth of as much as 40 cm. In the Holocene, slight sculpturing took place characterized by the origin of "rill lapies" and by further widening of all former fissures and cracks.