Unimodal Grain Size Distributions Research Articles

The grainsize of volcanic fall deposits: Spatial trends and physical controls

Volcanic tephra fall deposits, which form during explosive eruptions, are commonly characterized in terms of their thickness and grainsize. While significant efforts have been undertaken to relate spatial trends in thickness to plume dispersion processes, comparably few studies have focused on understanding variations in grainsize. Yet, grainsize is a key parameter providing insight into eruption dynamics, from magma fragmentation to plume transport processes, and modulates the impacts of tephra. Here, we present a set of grainsize data extracted from the published record for 56 deposits that represent a range of eruption intensities and magnitudes. We systematically analyze the deposits in terms of modality (bimodal or unimodal grainsize distributions) and provide the median particle diameter with distance from source for component distribution modes. We found that bimodal fall deposits are formed by eruptions with large amounts of fine particles (<100 µm) and that all tephra-fall deposits show characteristic patterns of grainsize decay with distance from source that can be related to eruption plume height and thus intensity. The grainsize decay trends are also related to ash dispersion and deposition processes such as individual particle settling versus collective settling mechanisms. The maximum distance from source reached by particles of different sizes is controlled by a combination of source and transport processes. This data set provides insight into the preservation potential of deposits of different grainsizes at varying distances from their sources. Finally, we emphasize the importance of using grainsize trends in combination with thickness trends to interpret tephra-fall deposit records.

Estimation of unimodal water characteristic curve for gap-graded soil

Soils with a bimodal grain-size distribution (gap-graded soils) can be associated with unimodal or bimodal soil-water characteristic curves (SWCCs). Many equations have been developed to estimate SWCCs using grain-size distribution curves to overcome the high cost and long duration of SWCC experiments. Most of the equations are limited to the estimation of the SWCCs of soils with a unimodal grain-size distribution. Few studies have been conducted on the estimation of unimodal SWCCs for gap-graded soils. In this paper, procedures, equations and computer codes are proposed for estimating the unimodal SWCCs of gap-graded soils. The proposed equations are found to perform well in estimating the unimodal SWCCs of gap-graded soils.

The opening subplinian phase of the Hekla 1991 eruption: properties of the tephra fall deposit

The 1991 Hekla eruption started on 17th of January with an intense 50-min-long explosive phase that transitioned into fire fountain activity lasting for 2 days. The eruptive plume rose to maximum height in about 10 min and the total mass of tephra deposited from the opening phase was 8.6 × 109 kg (VEI 3 event). The principal axis of tephra fall is to the NNE of Hekla and grain-size analysis reveals a systematic decrease in grain-size away from source. Majority of sample sites show typically unimodal grain-size distributions, although a few have bimodal distributions where the secondary mode is a subtle finer peak. The calculated total grain-size distribution (TGSD) is bimodal, with a coarse primary peak (−3.5 to −2.5 φ) and a subordinate fine peak (2.5 to 3.5 φ). The coarse peak is lapilli-dominated and was deposited within the first 25 km of transport, whereas the fine peak is coarse-ash-dominated and fits well with the modal grain-size of samples deposited >65 km from Hekla. Ascent rate of the magma and conditions for vesiculation in the shallow conduit became increasingly uniform with time through the 1991 opening phase.

An R-Based Function for Modeling of End Member Compositions

Clastic sediment grains have been exposed to numerous physical and chemical processes during erosion, transport and accumulation. It is widely accepted that weathering and sorting processes are strongly controlled by climate, making grain-size distributions a potential recorder for paleoenvironmental conditions. Polymodal grain-size distributions of sediments are in general a result of mixing of unimodal grain-size distributions (end members) from different sources. Thus, unmixing of polymodal grain-size distributions into their unimodal end member compositions (end member modeling) provides a powerful method for revealing sediment sources and environmental conditions of the past. Since the early 1960s, several algorithms for unmixing of such compositional data were published. However no source code is available in a modern open access programming language. This study provides an algorithm and some additional functions for the decomposition of compositional data into end-member compositions based on a free available statistical programming language. The source code is provided as supplementary materials. To prove the functionality of the algorithm, end member models of sandstone compositions and grain-size distributions were calculated.

The grain-size depositional process in wetlands of the Sanjiang Plain and its links with the East Asian monsoon variations during the Holocene

Compared to the well-studied the grain-size depositional process in hydraulic and aeolian sediments, the process in wetlands is still poorly understood. In this study, the grain-size components of a well-dated mud/peat profile (SJ2) in the Sanjiang Plain were partitioned using a lognormal distribution function. The results suggest that the lacustrine deposits contain four distinct unimodal grain-size distributions, C-1 through C-4, with their modal sizes ranging 0.7–1.6, 2.4–8.7, 15.7–27.8, and 380.2–529.6 μm respectively. Only the C-1 and C-3 were identified in the peat layers. A grain-size depositional modal was introduced to explain the environmental meanings of the four components. C-3, as a nearshore suspension component, was the most sensitive component to the variation of local hydraulic conditions. Based on the variation of C-3 percentages of the profile SJ2, the local hydraulic conditions were tentatively reconstructed, and the results showed a close relation with the monsoon variations during the Holocene. Accordingly, we suggest the relative coarse component of the polymodal peat deposits in monsoonal regions can be used as a sensitive indicator for the hydraulic conditions and the monsoon circulations in the geological past.

Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms of the 6 August 2012 Mt. Tongariro eruption, New Zealand

The weak geophysical precursors of the 6 August 2012 Te Maari eruption of Mt. Tongariro and a lack of obvious juvenile components in its proximal ballistic deposits imply that the eruption was caused by the sudden decompression of a sealed, hot hydrothermal system. Strong magmatic signals in pre- and post-eruption gas emissions indicate that fresh magma had intruded to shallow levels shortly before this eruption. Here we examine the volcanic ash produced during the August eruption with the aim of determining whether juvenile magma was erupted or not. The widely applied criteria for identifying fresh juvenile pyroclasts provided inconclusive results. The Te Maari ash sorting and trend towards a unimodal grain-size distribution increase with distance along the dispersal axis. Proximal to intermediate sites showing polymodal grain-size distributions can be related to the re-fragmentation of different pre-existing lithologies, overlapped erupted pulses and transport mechanisms, and to particle aggregation. Between 69 and 100vol.% of particles coarser than 3 ϕ and 45–75vol.% of grains finer than 3 ϕ were sourced from the pre-existing, commonly hydrothermally altered, vent-area lavas and pyroclasts. Free crystals (pyroxene>plagioclase>magnetite>pyrite) make up 0–23vol.% of particles coarser than 3 ϕ, and 22–41vol.% of grains finer than 3 ϕ. Brown to black fragments of fresh glass are a small (1–15vol.%), but notable, component. Under SEM, these blocky, glassy particles are poorly vesicular, and irregularly shaped, some with fluidal or bubble-wall surfaces, and others with fragmented stepped surfaces and fine adhering ash. In thin section, they contain variable amounts of microlites within an isotropic groundmass. The range in silica content of the microprobe-analysed glass is very wide (56–77wt.%) and cannot be correlated to any specific particle textural type. These chemically and texturally diverse glassy fragments are identical to mechanically broken pieces of country rock lavas and pyroclasts; both their diversity, and their match with vent country rocks, argue strongly against a “juvenile” origin for the glassy fragments. We conclude that rising magma provided only heat and gas into the overlying, sealed vapour-dominated hydrothermal system. A landslide from this area led to a rapid decompression and ash was produced by top-down hydrothermal explosions. Careful attention must be paid to the combination of compositions and textures of fine ash particles in such situations, as well as to the context of their source vent, in order to be confident that new magma has reached the surface.

A model for linking grain-size component to lake level status of a modern clastic lake

Grain-size distributions of fluvial, eolian and marine sediments were explicated decades ago. For lake sediments, however, there is still great uncertainty in explaining the genesis of grain-size components due to the inherent complexity of their polymodal distributions. In this study, the grain-size components of the surface sediments of Daihai Lake, Inner Mongolia, were partitioned using a lognormal distribution function and the relationship between the identity of each component and the specific sedimentary environment was investigated. The data indicate that the modern clastic sediments of Daihai Lake contain five distinct unimodal grain-size distributions representing five grain-size components. Each of the components retains its identity including modal size, manner of transportation and environment of deposition although the relative percentage varies with the hydraulic condition throughout the lake. These components are specified from fine to coarse modes as long-term suspension clay, offshore-suspension fine silt and medium-to-coarse silt, and nearshore-suspension fine sand and saltation medium sand. The percentage of the components interpreted as an indication of nearshore environments displays a negative correlation with water depth across the modern lakebed, suggesting a model for linking the nearshore components in sediment cores to the lake level status in the geological past. The model was applied to a sediment core from the lake where high percentages of the nearshore components in the core sediments were correlated with low regional precipitations reconstructed on the pollen profile of the same core. The coincidences between two independent proxies do not only demonstrate the validity of lognormal distribution function in partitioning polymodal sediments but also reveals the potential of the grain-size component–lake level status model for lake’s paleohydrological reconstruction.

High-Resolution Particle Size Analysis of Naturally Occurring Very Fine-Grained Sediment Through Laser Diffractometry

ABSTRACT In this paper, we present results from a large number of experiments aimed at quantifying method and instrument uncertainty associated with laser diffraction analysis. We analyzed the size distribution of fine-grained sediment ( 24 hours prior to analysis and using 60 seconds of ultrasonication during analysis. (2) Obscuration--a measure of the concentration of the suspension during analysis--produced the most reproducible results at about 20%. (3) Variations in refractive-index settings can significantly alter estimated grain-size distributions. (4) Assumed values for absorption (the degree to which sediment grains absorb the light) can have a profound effect on grain-size results. Absorption settings near 0 resulted in unexpected bimodal grain size distributions for sediments in the < 10 µm size fraction and significantly skewed the fine-grained tail of coarser samples, probably because of sub-optimal diffraction by particles with a diameter similar in size to the laser wavelength. Absorption settings closer to 1 produced very reproducible results and unimodal grain-size distributions over a wide range of refractive indexes. Our study has shown that laser diffraction can measure very fine-grained sediments (< 10 µm) quickly, with high precision ( 5% at 2 standard deviations), and without the need for extensive mineralogical determinations. These results make possible a new generation of studies in which high-resolution time-series data sets of sediment grain size can be used to infer subtle changes in paleohydrology.

An equation to represent grain-size distribution: Reply

The authors wish to thank the discusser for putting together his case for the possibility of using a log-normal type representation to characterize particle size distributions. Log-normal type plots have found a degree of acceptability in the area of hydrology; however, similar attempts to use log-normal distributions have not been met with the same enthusiasm in the soil mechanics discipline. Geotechnical engineers do not view particle size distribution as statistical or probability-based information related to the number of particles associated with each particle size range. In the original paper, the authors point out that “Gardner (1956) proposed a two-parameter, log-normal distribution to provide representation of grain-size distribution data.” The idea of using log-normal distributions to characterize grainsize distribution curves has been around for a long time. It does not appear to have gained significant acceptability in geotechnical engineering nor engendered further research interest. It was against this background that the present research study was formulated. It was decided to further investigate the use of an empirical, mathematically based equation where the fitting parameters had some physical meaning. The equations were focused on the generally accepted “percent passing” versus the logarithm of the particle size plots. There has been a recent proliferation of studies related to the estimation of the soil-water characteristic curve from a grain-size distribution curve in the soil science and related disciplines. This also encouraged further attempts to mathematically characterize the grain-size distribution curve. The use of a continuous mathematical equation that can be differentiated and integrated has appeal because it can be used for further analysis in soil mechanics. There appears to be a fundamental difficulty in viewing a grain-size distribution curve as a purely statistical phenomenon since the grain-size distribution is commonly plotted as a “percent passing” by mass (as opposed to a cumulative distribution based on the number of particles) versus the logarithm of the particle size. The “percent passing” by mass does not lend itself to a statistical evaluation in the same way as if the number of particles was being assessed (Ang and Tang 1975). The largest particles have the dominance with respect to mass but in general, have the least influence on soil behavior. For example, only 30% clay size particles render the behavior of the soil like that of a clay soil. Hazen (1930) suggested that the D10 size of particles best represented the particle size for the assessment of the coefficient of permeability of a soil. In other words, conventional statistical measures based strictly on frequency do not provide the geotechnical engineer with the most meaningful parameters required when attempting to foresee the mechanical and hydraulic behavior of a soil. As a result, D50 does not have special meaning over other “percent passing” values in soil mechanics. Part of the natural bias towards the coarse end of the particle size spectrum (i.e., because of the mass basis for presentation of the data) is overcome in soil mechanics by plotting particle size on a logarithm scale. The differentiation of the percent passing grain size curve gives a log-normal type distribution, but this is not often used by geotechnical engineers. The particle size scale is equally spaced on a logarithm scale, and while this presents some difficulties in interpretation, it still appears to be a meaningful way to visualize the particle sizes in the distribution. The discusser states that he feels “that this paper represents a continued divergence from available approaches.” The authors maintain that it might represent a divergence from approaches quite common to hydrology; however, the approach proposed is consistent with the historical and present interpretive procedures used in soil mechanics. The authors used a four-parameter equation in the case of a unimodal grain-size distribution curve and a sevenparameter equation in the case of a bimodal grain-size distribution curve. This may appear to be a large number of parameters, but the parameters come out of a regression analysis and allow a consistently accurate representation of the entire grain-size curve. When used in conjunction with a

Volume strain, strain type and flow path in a narrow shear zone

This study explores the state of finite strain and changes in the mean kinematic vorticity number, grain size, whole-rock chemistry and mineralogy across an upper amphibolite-facies shear zone in a metadiorite, northern Malawi, east-central Africa. P–T conditions during shear-zone formation and deformation were approximately 700–750 °C and 5–7 kbar and are slightly less than P–T conditions for the regional peak of metamorphism. The major rock-forming minerals, plagioclase, hornblende, biotite, and quartz, were deformed by crystal-plastic processes accompanied by, except for hornblende, dynamic recrystallization. The modal abundance of all four major rock-forming minerals shows no systematic change from the country rock into and across the shear zone, indicating that shear-zone development was not associated with retrograde mineral reactions. The grain size of the major rock-forming minerals decreases within the shear zone. Plagioclase and hornblende, which occur as porphyroblasts outside the shear zone, exhibit a bimodal grain-size distribution within the shear zone. Quartz has a unimodal grain-size distribution in the shear zone. Major and trace element chemistry does not change systematically across the shear zone, implying no volume change in the mylonite. Matrix strain data for plagioclase and hornblende by the Fry method and fabric strain as deduced from Rf/φ analysis of plagioclase and quartz grains demonstrate a slightly constrictional strain type (K≈1.5) across the shear zone. The quantitative finite-strain data for the different residual minerals as obtained by unlike methods show no systematic variation, but recrystallized plagioclase grains record higher strain than the residual grains. The mean kinematic vorticity number changes from approximately 0.3 outside to approximately 0.8 within the shear zone, indicating that the bulk deformation path deviated from progressive simple shear. The estimates for finite strain and the degree of noncoaxiality account for approximately 50% of thinning normal to the shear zone.

DEPOSITION AL AND DIAGENETIC CONTROLS ON THE QUALITY OF BARROW GROUP SANDSTONE RESERVOIRS, BARROW SUB-BASIN, NORTH WEST SHELF, WESTERN AUSTRALIA

The main controls on Barrow Group reservoir quality are depositional (lithofacies) and post depositional (di- agenetic). Barrow Group sediments were deposited in environments ranging from non-marine to deep marine. The upper sandstones (Flacourt Formation) in the centra] and southern parts of the sub-basin are dominantly of good reservoir quality, consisting mainly of marine- reworked strandplain and shoreface deposits. The lower sandstones (Malouet Formation), however, in the Barrow Sub-basin depocentre, are composed predominantly of slope apron deposits, consisting of tight, matrix-rich, non-productive sandstones. The Flag Sandstone, distributed along the northern part of the sub-basin, is made up mainly of mounded turbidite deposits consisting of excellent reservoir quality, coarse-grained sheet turbid-ites.Petrographic observations of theliarrow Group show that sands are, in general, extremely well sorted and display a unimodal grain-size distribution. Grain size varies slightly with the depositional environment; the mounded turbidite sandstones average 197 f.im (fine sand) and the slope apron sandstones average 185 pm (fine sand). The grain size in the marine-reworked strandplain sandstones averages 300 j.im (medium sandstones). Detrital depositional matrix, mechanical compaction and cementation are the main causes of poor reservoir quality. Mechanical compaction is responsible for destroying up to 60 per cent of the original porosity. Chemical compaction has had much less effect. Precipitation of authigenic minerals has reduced, on average, the primary intergranular porosity to 12 per cent of the whole rock-volume.In some cases, secondary porosity development has greatly enhanced the reservoir quality of the Barrow Sub-basin sandstones. It results from dissolution of calcite, dolomite and potassium feldspar. Reservoir quality is most improved along the margins of the sub-basin as this is where carbonate and feldspar were most abundant.

Grain Growth in Si3 N4-Based Materials

The growth of Si3N4 grains in the usual sintering process occurs in an oxynitride liquid formed by reactions between sintering additives, impurity SiO2, and Si3N4. The shape of an Si3N4 grain in the liquid matrix is a hexagonal rod, although the aspect ratio (c/a) varies considerably, depending on the processing conditions and final crystalline forms of α or β. Figure 1 shows two types of microstructures observed in sintered Si3N4-based materials. The microstructure shown in Figure la is normal with unimodal grain-size distribution and that of Figure 1b is abnormal with a microstructure of exceptionally large elongated grains within fine matrix grains. When normal grain growth occurs, the microstructure varies little with sintering time, and the development may be described by a simple law. But, when abnormal grain growth occurs, a duplex grain structure with a bimodal grainsize distribution results; then no simple kinetic law can describe the microstructure development.The grain growth in the microstructures shown in Figure 1 exemplifies the growth of faceted grains in a liquid matrix. The grain growth in a matrix occurs via growth of larger grains and dissolution of smaller ones by material transport through the liquid phase. The driving force of the material transport for an individual grain is determined by the difference between its size and the critical grain size, which is invariant at the moment of observation. Since the driving force is usually low, the volume change of each grain is relatively slow and analogous to the crystal growth or dissolution in a liquid matrix under low super- or undersaturation. Therefore, knowledge of the growth behavior of faceted crystals under low supersaturation in a liquid may provide fundamental understanding of the grain growth in Si3N4-based materials.

An artificial recharge experiment in the San Jacinto basin, Riverside, southern California

During a 3 month recharge experiment related to conjunctive use of water resources, 1.5 × 10 6 T of imported water were percolated through a pond of 128 m by 128 m in the San Jacinto basin. The infiltration rates, which declined with time, averaged 1.9 m day −1, equivalent to four times the lowest laboratory-measured hydraulic conductivity of the fluvial deposits. Ponding altered the unimodal grain-size distribution at the ground surface to types without a dominant mode, but this redistribution did not always lead to reduction in conductivities, which varied over at least three orders of magnitude. The water table 80 m downstream from the ponding edge began to rise slowly 1 month after the start of ponding; it leveled off at 8 m above the pre-recharge water table depth of 75 m and did not recede 2 months after termination of ponding. Water levels in wells bottomed in the original vadose zone suggested that an inverted water table migrated downward to meet the rising water table. Minor, local perching occurred at 14 m depth, as indicated by the presence of moist ground near one monitoring well and by hydraulic responses during a 20 day intermission in percolation. As it percolated through the sediments, the imported northern California water gained Ca but lost Mg, so that the Mg/Ca ratio resembled that of local ground water. Such cation exchange has also been demonstrated by leaching experiments in the laboratory. However, the characteristics of the original source waters appear to be retained by D/H isotope ratios and Cl concentrations, as well as cross-plots of SO 4 vs. Cl and B vs. Cl. Such unreactive tracers could serve to monitor transport and mixing of the chemically diverse water used in future recharge programs in the San Jacinto basin.

Determining rheological parameters of debris flow material

A 2.0 m diameter steel 30° inverted cone-and-plate viscometer/rheometer was designed, constructed, and used to test the behaviour of coarse-grained debris flow materials. A 1: 5 scale model machine was also constructed and used to test the internal flow dynamics of the viscometer/rheometer and to obtain results for fluids, grain-fluid mixtures, and debris flow fines. For fluids and grain-fluid mixes, our results were similar to those obtained earlier using standard viscometric systems. Derived rheological parameters for debris flow materials and clay slurries agreed well with those determined from calculation, direct measurement, and field observation. Apparent viscosities were shear-rate dependent, extremely sensitive to water content, and as high as 6000 Pa·s. Instantaneous stresses within shearing debris flow material varied over a wide range. Debris flow materials with bimodal grain-size distributions had a dilatant plastic rheology. Those with a low content of coarse material and unimodal grain-size distribution, or exceptionally high fines content, had a plastic or viscoplastic rheology.

Improved efficiency of CdZnS thin-film solar cells

For low Zn concentrations i.e., x &lt; 0.1, the performance of the Cd1−xZnxS–Cu2S solar cells can be improved by reducing the grain-boundary recombination. This has been achieved by growing well-oriented, homogeneous, ternary compound films.It was found that the Cd1−xZnxS films grown on the polycrystalline CdS films achieved the same larger grain size as that of the base layer. These films had fewer misorientations and had a unimodal grain-size distribution. There is a continuity in the crystallites from the CdS base to the Cd1−xZnxS overlayer, and the bifilms thus grown are less resistive than Cd1−xZnxS single layers.

Reservoir Properties of Conglomerates and Conglomeratic Sandstones: GEOLOGIC NOTES

Bimodal clastic sedimentary rocks can be either packed, where pebbles are dispersed through a continuous sandstone matrix, or pebble packed, where sandstone partially or wholly fills the interstices between a framework of densely packed pebbles. The primary porosity of bimodal reservoir rocks may vary by a factor of 3 to 4, depending on the packing state and the relative proportions of sand and pebble-sized grains in the rock. The relations between permeability and porosity, and water saturation and porosity, in bimodal reservoir rocks may be quite different from those common in clastic reservoirs having unimodal grain-size distributions. The effects of bimodality and diagenesis should be considered separately for a better understanding of the properties of bimod l reservoir rocks.

Relationship of grain-size modes to nearshore sedimentary environments, Lake Superior, Ontario

Longshore-troughs and bars were studied at Batchawana Bay and Pancake Bay, Lake Superior, Ontario. The longshore-trough areas would appear to have been formed by plunging waves that winnowed out the finer sand grains which were then moved lakeward by the undertow to build up the adjacent longshore-bar areas. This process would appear to be confirmed by a grain-size analyses of 242 samples taken from along the axes of the longshore-troughs and bars. The longshore-trough samples had either a unimodal or bimodal grain-size distribution whereas the longshore-bar samples always had a unimodal grain-size distribution. The longshore-trough samples showed a strong tendency towards negative skewness as would be expected from winnowed sediments and would also appear to have a modified grain-size modality of the parent sediment. The longshore-bar samples were finer grained, well sorted and unimodal, and showed a strong tendency towards positive skewness as would be expected if the sediments had been winnowed out of the longshore-trough areas and then re-deposited along the longshore-bars.

Darton Ridge, Pennsylvanian Feature of Wyoming Shelf

Despite the apparent uniform nature of the Tensleep Sandstone and its genetically related time equivalents, petrologic analysis of sections measured in the Big Horn basin of Wyoming yields evidence of statistically significant variability in texture and composition. This variability provides a basis for interpretation of the nature and circulation pattern of shallow-marine currents which transported and deposited the well-sorted, very fine-grained orthoquartzite as a thin blanket sand on the Pennsylvanian cratonic shelf in Wyoming and adjoining areas. Approximately half of the sediment comprising the Tensleep Sandstone was accumulated under the influence of low-velocity longshore currents that moved from northwest to southwest along the shoreline. These currents produced thinly and evenly bedded deposits that display essentially unimodal grain-size distribution, but contain a small silt The other half of the detritus may have been transported by high-energy, storm-related bottom currents whose mean flow direction was southwest. Deposits produced by these currents display planar cross-lamination and are characterized by a polymodal grain-size distribution curve with a coarse tail. Although cross-bedded strata are approximately equal in volume to the evenly laminated beds with which they are intercalated, bottom-current trans ort probably dominated the depositional regime only 10 per cent of the time, and longshore movement was most important during the remaining 90 per cent. An elongate bathymetric prominence, here named the Darton Ridge, significantly affected the distribution pattern of Tensleep thickness, texture, and composition in the Big Horn basin-Big Horn Mountains area by its interaction with current flow. Oriented essentially northeast-southwest across north-central Wyoming, the approximately 45-by-90+-mile Darton Ridge created throughout the Pennsylvanian Period a shallow-water area of reverse by-passing, swept by bottom currents and characterized by a thin accumulation of evenly bedded, unimodal deposits. Strata surrounding the Darton Ridge are thicker and contain a much higher percentage of coarsely bimodal, cross-laminated beds. Feldspar and metamorphic grains, derived from a northeastern source, show characteristic concentration along the north-facing slope of the ridge.