Boulder Deposits Research Articles

Geological Oceanography of the Pliocene Warm Period: A Review with Predictions on the Future of Global Warming

Atmospheric carbon dioxide reached a record concentration of 419 parts per million in May 2021, 50% higher than preindustrial levels at 280 parts per million. The rise of CO2 as a heat-trapping gas is the principal barometer tracking global warming attributed to a global average increase of 1.2 °C over the last 250 years. Ongoing global warming is expected to perturb extreme weather events such as tropical cyclones (hurricanes/typhoons), strengthened by elevated sea-surface temperatures. The melting of polar ice caps in Antarctica and Greenland also is expected to result in rising sea levels through the rest of this century. Various proxies for the estimate of long-term change in sea-surface temperatures (SSTs) are available through geological oceanography, which relies on the recovery of deep-sea cores for the study of sediments enriched in temperature-sensitive planktonic foraminifera and other algal residues. The Pliocene Warm Period occurred between ~4.5 and 3.0 million years ago, when sea level and average global temperatures were higher than today, and it is widely regarded as a predictive analog to the future impact of climate change. This work reviews some of the extensive literature on the geological oceanography of the Pliocene Warm Period together with a summary of land-based studies in paleotempestology focused on coastal boulder deposits (CBDs) and coastal outwash deposits (CODs) from the margin of the Pacific basin and parts of the North Atlantic basin. Ranging in age from the Pliocene through the Holocene, the values of such deposits serve as fixed geophysical markers, against which the micro-fossil record for the Pliocene Warm Period may be compared, as a registry of storm events from Pliocene and post-Pliocene times.

Interplay Between Coastal Elevation and Wave Height Controls the Occurrence of Coastal Boulder Deposits in the Aran Islands, Ireland

Coastal boulder deposits (CBD) are wave-emplaced supratidal accumulations that record extreme inundation on rocky coasts. They are poorly understood but are of growing importance as we seek to better understand the extremes of wave power on coastlines. The Aran Islands, Ireland, host CBD in varying settings ranging from sheer cliff tops to wide shore platforms, and at elevations to about 40 m above sea level. Deposits are known to be active during strong storm events and provide a unique opportunity to examine relationships between wave energy, setting, and CBD occurrence. We use topographic elevation (Z) and offshore 100-years significant wave height (Hs,100) to calculate a dimensionless elevation Z* = Z/Hs,100 at 25 m intervals all along the Atlantic-facing coasts of the Aran Islands, and record whether CBD were present or absent at each location. The data reveal universal CBD presence at locations with low dimensionless elevations and near-monotonic decreasing frequency of CBD occurrence as Z* increases. On the Aran Islands, CBD are restricted to locations with Z*<3.13. For high elevation deposits it appears that unresolved local factors may be the major determinants in whether CBD will form. This approach can be applied at any CBD-bearing coastline and has the potential to change the way that we think about these deposits. Evaluation of dimensionless elevations at CBD locations around the world will help build broader understanding of the impact local shoreline conditions have on CBD formation. Determining these relationships contributes to the ongoing need to better understand interactions between extreme waves and rocky coasts.

Sedimentary record of Late Paleozoic tectonism in the Monitor Range, central Nevada: Implications for convergence along the western Laurentian margin

A growing body of work from upper Paleozoic records along the western margin of Laurentia has shown that the margin was subjected to ongoing deformation from at least Late Devonian through early Permian (Cisuralian) time. However, the geographic extent of this deformation and its effect on basin subsidence along the western Laurentian margin remains poorly constrained. Here, we present geologic mapping, detailed sedimentologic analysis, detrital zircon U-Pb data, and fusulinid and conodont biostratigraphy from a section of virtually unstudied lower Permian to Permian-Triassic sedimentary rocks currently exposed in the Monitor Range of central Nevada. These data suggest (1) coarse-grained sedimentation adjacent to uplifted topography from earliest Permian through earliest Triassic time, (2) early-middle Permian sedimentation that started as deposition of proximal limestone boulders and cobbles and transitioned to a system dominated by far-traveled aeolian sand, and (3) that strata preserved in the Monitor Range record possible wedgetop sedimentation associated with the Permian-Triassic Sonoma Orogeny.

Storm Waves May Be the Source of Some "Tsunami" Coastal Boulder Deposits.

Coastal boulder deposits (CBD) provide what are sometimes the only remaining signatures of wave inundation on rocky coastlines; in recent decades, CBD combined with initiation of motion (IoM) analyses have repeatedly been used as primary evidence to infer the existence of ancient tsunamis. However, IoM storm wave heights inferred by these studies have been shown to be highly inaccurate, bringing some inferences into question. This work develops a dimensionless framework to relate CBD properties with storm‐wave hindcasts and measurements, producing data‐driven relations between wave climate and boulder properties. We present an elevation‐density‐size‐inland distance‐wave height analysis for individual storm‐transported boulders which delineates the dynamic space where storm‐wave CBD occur. Testing these new relations against presumed tsunami CBD demonstrates that some fall well within the capabilities of storm events, suggesting that some previous studies might be fruitfully reexamined within the context of this new framework.

Recalculation of minimum wave heights from coastal boulder deposits in the Bristol Channel and Severn Estuary, UK: implications for understanding the high-magnitude flood event of AD 1607

A high-magnitude coastal flood event catastrophically affected the macrotidal Bristol Channel and Severn Estuary in southwest Great Britain, United Kingdom, on 30th January 1607 causing an estimated 2000 fatalities. Historical and physical evidence has provided a basis for the development of a theory that the flood may have been due to a tsunami rather than a storm. Previous studies have collected field data to test this hypothesis including a dataset of 136 wave-transported boulder clasts that was utilised to estimate minimum wave heights through hydrodynamic equations in 2007, but the dataset has hitherto remained unpublished in full. Since 2007 these equations have undergone refinement and for this paper minimum wave heights were recalculated from boulder measurements using revised hydrodynamic equations and presents the complete dataset for the first time. A recent study claiming that such equations are flawed is considered premature, given ongoing refinements to the equations. The results of the present study indicate that a tsunami 4.2 m high can explain the dislodgement of all boulders measured, equivalent to a storm wave height of 16.9 m, which is considerably greater than observed storm wave heights in the region. An up-channel increase in minimum wave height is also suggested by these data, generally corroborating the 2007 study, which may be due to wave amplification caused by the overall funnel-shape of the embayment. The areas worst affected by the 1607 flood are located in the coastal lowlands of the inner Bristol Channel and Severn Estuary, coinciding with the highest minimum estimated wave heights.

Evaluating optically stimulated luminescence rock surface exposure dating as a novel approach for reconstructing coastal boulder movement on decadal to centennial timescales

Abstract. Wave-transported boulders represent important records of storm and tsunami impact over geological timescales. Their use for hazard assessment requires chronological information on their displacement that in many cases cannot be achieved by established dating approaches. To fill this gap, this study investigated, for the first time, the potential of optically stimulated luminescence rock surface exposure dating (OSL-RSED) for estimating cliff-detachment ages of wave-transported coastal boulders. The approach was tested on calcarenite clasts at the Rabat coast, Morocco. Calibration of the OSL-RSED model was based on samples with rock surfaces exposed to sunlight for ∼ 2 years, and OSL exposure ages were evaluated against age control deduced from satellite images. Our results show that the dating precision is limited for all targeted boulders due to the local source rock lithology which has low amounts of quartz and feldspar. The dating accuracy may be affected by erosion rates on boulder surfaces of 0.02–0.18 mm yr−1. Nevertheless, we propose a robust relative chronology for boulders that are not affected by significant post-depositional erosion and that share surface angles of inclination with the calibration samples. The relative chronology indicates that (i) most boulders were detached from the cliff by storm waves; (ii) these storms lifted boulders with masses of up to ∼ 24 t; and (iii) the role of storms in the formation of boulder deposits along the Rabat coast is more significant than previously assumed. Although OSL-RSED cannot provide reliable absolute exposure ages for the coastal boulders in this study, the approach has large potential for boulder deposits composed of rocks with larger amounts of quartz or feldspar and less susceptibility to erosion.

The geology and glacial history of the Walls of Jerusalem, Central Tasmania – a preliminary study

The Walls of Jerusalem, in Tasmania’s Central Plateau, is marked by several dolerite escarpments, or “walls”, enclosing sheltered grassy valleys with Pencil Pine forests. Geologically, the area is a structurally raised block between two faults, with the peaks and plateaus being 100 m or so higher than the surrounding plateau. The valleys and escarpments may be mainly due to intense faulting and shattering of the dolerite by cross-faults, and removal of this material by glaciers. The higher peaks and plateaus have all been abraded by ice, except for an extraordinary patch at Solomons Throne, which retains irregular scree piles coloured red with lichen. Virtually all the valley floors are covered by glacial till, which probably relates to the younger glacial episodes. It consists mostly of dolerite boulders and clay, and shows subdued morainal forms in most places, with a few matrix-free boulder deposits probably representing lag after meltwater washing. Bedded clay deposits formed in lakes and meltwater streams as the ice retreated are extensive within the drift deposits, reflecting the confined nature of the two glacier lobes which entered the valleys from north and south. Dolerite screes along the scarps appear to post-date most of the morainal deposits, but some older screes may also be present. The Temple eminence within the centre of the Walls consists of fragmented dolerite overlying Triassic sandstone, almost completely covered by periglacial and glacial deposits, including superb examples of solifluction flows. There are many unanswered questions concerning the glacial deposits, but the lack of any dates means it is not yet possible to establish a comprehensive glacial chronology.

Holocene Boulder Beach Eroded from Chromite and Dunite Sea Cliffs at Støypet on Leka Island (Northern Norway)

This project examines the role of high-latitude storms degrading a Holocene coast formed by igneous rocks composed of low-grade chromite ore and dunite that originated within the Earth’s crust near the upper mantle. Such rocks are dense and rarely exposed at the surface by tectonic events in the reconfiguration of old ocean basins. An unconsolidated boulder beach occupies Støypet valley on Leka Island in northern Norway, formerly an open channel 10,000 years ago when glacial ice was in retreat and rebound of the land surface was about to commence. Sea cliffs exposing a stratiform ore body dissected by fractures was subject to wave erosion that shed large cobbles and small boulders into the channel. Competing mathematical equations are applied to estimate the height of storm waves impacting the channel floor and cliffs, and the results are compared with observations on wave heights generated by recent storms striking the Norwegian coast with the intensity of an orkan (Norwegian for hurricane). Lateral size variations in beach clasts suggest that Holocene storms struck Leka Island from the southwest with wave heights between 5 and 7.5 m based on the largest beach boulders. This result compares favorably with recent high-latitude storm tracks in the Norwegian Sea and their recorded wave heights. The density of low-grade chromite ore (3.32 g/cm3) sampled from the beach deposit exceeds that of rocks like limestone or other igneous rocks such as rhyolite, andesite, and basalt taken into consideration regarding coastal boulder deposits associated with classic hurricanes in more tropical settings.

Holocene alluvial fan evolution, Schmidt‐hammer exposure‐age dating and paraglacial debris floods in the SE Jostedalsbreen region, southern Norway

The evolution of several sub‐alpine alluvial fans SE of the Jostedalsbreen ice cap was investigated based on their geomorphology and Schmidt‐hammer exposure‐age dating (SHD) applied to 47 boulder deposits on the fan surfaces. A debris‐flood rather than debrisflow or water‐flow origin for the deposits was inferred from their morphology, consisting of low ridges with terminal splays up to 100 m wide without lateral levees. This was supported by fan, catchment, and boulder characteristics. SHD ages ranged from 9480±765 to 1955±810 years. The greatest number of boulder deposits, peak debris‐flood activity and maximum fan aggradation occurred between ̃9.0 and 8.0 ka, following regional deglaciation at ̃9.7 ka. The high debris concentrations necessary for debris floods were attributed to paraglacial processes enhanced by unstable till deposits on steep slopes within the catchments. From ̃8.0 ka, fan aggradation became progressively less as the catchment sediment sources tended towards exhaustion, precipitation decreased during the Holocene Thermal Maximum, and tree cover increased. After ̃4.0 ka, some areas of fan surfaces stabilized, while Late‐Holocene climatic deterioration led to renewed fan aggradation in response to the neoglacial growth of glaciers, culminating in the Little Ice Age. These changes are generalized within a conceptual model of alluvial fan evolution in this recently‐deglaciated mountain region and in glacierized catchments. This study highlights the potential importance of debris floods, of which relatively little is known, especially in the context of alluvial fan evolution.

Multiphase Storm Deposits Eroded from Andesite Sea Cliffs on Isla San Luis Gonzaga (Northern Gulf of California, Mexico)

The 450-m long spit that extends westward from the northwest corner of Isla San Luis Gonzaga is one of the largest and most complex constructions of unconsolidated cobbles and boulders found anywhere in Mexico’s Gulf of California. The material source derives from episodic but intense storm erosion along the island’s andesitic cliff face with steep northern exposures. A well-defined marine terrace from the late Pleistocene cuts across the same corner of the island and provides a marker for the subsequent development of the spit that post-dates tectonic-eustatic adjustments. A total of 660 individual andesite clasts from seven transects across the spit were measured for analyses of change in shape and size. These data are pertinent to the application of mathematical formulas elaborated after Nott (2003) and subsequent refinements to estimate individual wave heights necessary for lift from parent sea cliffs and subsequent traction. Although the ratio of boulders to clasts diminishes from the proximal to distal end of the structure, relatively large boulders populate all transects and the average wave height required for the release of joint-bound blocks at the rocky shore amounts to 5 m. Based on the region’s historical record of hurricanes, such storms tend to decrease in intensity as they migrate northward through the Gulf of California’s 1100-km length. However, the size and complexity of the San Luis Gonzaga spit suggests that a multitude of extreme storm events impacted the island in the upper gulf area through the Holocene time, yielding a possible average growth rate between 7 and 8 m/century over the last 10,000 years. In anticipation of future storms, a system to track the movement of sample boulders should be emplaced on the San Luis Gonzaga spit and similar localities with major coastal boulder deposits.

Estimating the age and mechanism of boulder transport related with extreme waves using lichenometry

Tsunamis and storms cause considerable coastal flooding, numerous fatalities, destruction of structures, and erosion. The characterization of energy and frequency associated with each wave contribute to the risk assessment in coastal regions. Coastal boulder deposits represent a physical proof of extreme inundation and allow us to study the effects of marine floods further back in time than instrumental and historical records. Age estimation of these deposits is challenging due to lack of materials (such as sand, shells, corals, or organic matter) that retain information about the passage of time. Lichenometry, a simple age estimation method, which is cost-effective, quick to apply, and non-destructive, is here proposed as a solution. A lichen growth model for a calcium-tolerant lichen species was developed and used to estimate the age of a boulder deposit related to extreme marine inundation(s) in Portugal. Estimated ages indicate several very recent events (<700 years) for most of the boulders’ stabilization and agree with results obtained with optically stimulated luminescence of marine sands found beneath boulders. Frequent and recent boulder transport implies a storm-origin for this deposit. These conclusions contrast with other works describing identical deposits that are attributed to paleotsunamis. This study presents a methodology using lichenometry as a successful alternative for age estimation in rocky coastal settings. These results offer an alternative explanation for coastal boulder deposits found on the west coast of Portugal.

Glacial ripping: geomorphological evidence from Sweden for a new process of glacial erosion

ABSTRACT In low relief Precambrian gneiss terrain in eastern Sweden, abraded bedrock surfaces were ripped apart by the Fennoscandian Ice Sheet. The resultant boulder spreads are covers of large, angular boulders, many with glacial transport distances of 1–100 m. Boulder spreads occur alongside partly disintegrated roches moutonnées and associated fracture caves, and are associated with disrupted bedrock, which shows extensive fracture dilation in the near surface. These features are distributed in ice-flow parallel belts up to 10 km wide and extend over distances of >500 km. Our hypothesis is that the assemblage results from (1) hydraulic jacking and bedrock disruption, (2) subglacial ripping and (3) displacement, transport and final deposition of boulders. Soft sediment fills indicate jacking and dilation of pre-existing bedrock fractures by groundwater overpressure below the ice sheet. Overpressure reduces frictional resistance along fractures. Where ice traction overcomes this resistance, the rock mass strength is exceeded, resulting in disintegration of rock surfaces and ripping apart into separate blocks. Further movement and deposition create boulder spreads and moraines. Short boulder transport distances and high angularity indicate that glacial ripping operated late in the last deglaciation. The depths of rock mobilized in boulder spreads are estimated as 1–4 m. This compares with 0.6–1.6 m depths of erosion during the last glaciation derived from cosmogenic nuclide inventories of samples from bedrock surfaces without evidence of disruption. Glacially disrupted and ripped bedrock is also made ready for removal by future ice sheets. Hence glacial ripping is a highly effective process of glacial erosion.

Maximal Heights of Nearshore Storm Waves and Resultant Onshore Flow Velocities

Storm waves, after breaking or overtopping, generate strong onshore flows that do significant mechanical work, including eroding and transporting large boulders. The waves can be amplified on approach, and the flows themselves may be further intensified by local topographic effects. These processes are currently poorly parameterised, but are of great importance for understanding the interactions between waves and coasts. We present a highly generalised equation for estimating maximal coastal wave heights and consequent onshore flow velocities. Although very approximate, this method contains no embedded assumptions, and thus provides a more realistic first-order check of storm wave capabilities than previous approaches. Initial analysis suggests that amplified waves may generate maximal onshore flow velocities up to six times the phase speed of the offshore waves. Although the probability of occurrence in any given storm is very low, the possibility of such extreme values cannot be ignored, especially when interpreting ancient deposits of large boulders. The equations presented here can be used as a first-order test for coastal boulder deposits currently interpreted as tsunami deposits, to evaluate whether a storm-wave origin should be reconsidered. This approach could also be employed at coasts in general, to evaluate long-term probabilities of damaging flows, as a component of coastal risk analysis.

Comparison of Modern and Pleistocene (MIS 5e) Coastal Boulder Deposits from Santa Maria Island (Azores Archipelago, NE Atlantic Ocean)

Modern and palaeo-shores from Pleistocene Marine Isotope Substage 5e (MIS 5e) featuring prominent cobble/boulder deposits from three locations, on the southern and eastern coast of Santa Maria Island in the Azores Archipelago, were compared, in order to test the idea of higher storminess during the Last Interglacial. A total of 175 basalt clasts from seven transects were measured manually in three dimensions perpendicular to one another. Boulders that exceeded the minimum definitional diameter of 25 cm contributed to 45% of the clasts, with the remainder falling into the category of large cobbles. These were sorted for variations in shape, size, and weight pertinent to the application of two mathematical formulas to estimate wave heights necessary for traction. Both equations were based on the “Nott-Approach”, one of them being sensitive to the longest axis, the other to the shortest axis. The preponderance of data derived from the Pleistocene deposits, which included an intertidal invertebrate fauna for accurate dating. The island’s east coast at Ponta do Cedro lacked a modern boulder beach due to steep rocky shores, whereas raised Pleistocene palaeo-shores along the same coast reflect surged from an average wave height of 5.6 m and 6.5 m. Direct comparison between modern and Pleistocene deposits at Ponta do Castelo to the southeast and Prainha on the island’s south shore produced contrasting results, with higher wave heights during MIS 5e at Ponta do Castelo and higher wave heights for the modern boulder beach at Prainha. Thus, our results did not yield a clear conclusion about higher storminess during the Last Interglacial compared to the present day. Historical meteorological records pit the seasonal activity of winter storms arriving from the WNW-NW against the scant record of hurricanes arriving from the ESE-SE. The disparity in the width of the marine shelf around Santa Maria Island with broad shelves to the north and narrow shelves to the south and east suggested that periodic winter storms had a more regular role in coastal erosion, whereas the rare episodic recurrence of hurricanes had a greater impact on southern and southeastern rocky shores, where the studied coastal boulder deposits were located.

Morphological controls and statistical modelling of boulder transport by extreme storms

The study of coastal boulder accumulations generated by extreme marine events, and of the energy and frequency involved in boulder transport, is of paramount importance in understanding the risk associated with extreme marine inundations. One of the frequently asked questions is whether the deposits are storm or tsunami-related, both events being characterized by different return periods. Boulder transport by storms was monitored on the west coast of Portugal. Significant changes were detected in boulders' position as a result of extreme inundation by the 2013/2014 winter storms. Results presented in this work indicate that the wave power associated with the “Christina” and “Nadja” storms occur once every three years. However, this interval is not supported by field observations of boulder displacement, which suggests that wave power over-predicts boulder movement in the study area. Furthermore, wave parameters from the “Christina” and “Nadja” storms were very similar, but have generated different impacts in the boulder accumulation described herein. Differences include the magnitude and direction of boulder movement, and are most likely associated with distinct tidal levels during the events. Higher tide levels generated an increase in the sea surface level and thus in the reach of waves, which generated displacement of larger boulders and consequent cross-shore contribution in boulder transport. Regardless, the combination of monitoring campaigns, wave data, and statistical modelling of extreme values indicate that boulder transport by storms is more frequent than initially expected. Based on recorded boulder movements, we present a conceptual model for boulder ridge formation and development and identify significant control of incoming flow by local geomorphological/topographical features. Storm events, not less frequent tsunamis, are identified as the events responsible for modulating this rocky coastline. These results question a direct attribution of coastal boulder deposits to tsunamis in coastal regions with a high risk of tsunami inundation.

Megagravel deposits on the west coast of Ireland show the impacts of severe storms

Graphical abstractCoastal boulder deposits on Inishmore, one of the Aran Islands off Ireland's west coast, showcase many of the classic features. Note the adult human for scale (in the yellow circle). The cliff-collapse block in the foreground is the largest ever documented to be moved by storm waves. Weighing 620t, it was shifted several metres during the storms of 2013–2014. The platform on which it sits is 5m above high water, and the block is 30m inland. The cliff at this site is 9m high. The boulder ridges behind the figure on the cliff top are 11–15m above high water, and the front of the pile ranges from 45–64m inland. A number of the biggest blocks on the upper platform, which weigh several tens of tonnes, were also moved sideways during the 2013–2014 storms; and within the ridge, the pale grey colours show smaller (1–15t) boulders that were overturned. Drone photograph taken by Dr Eugene Farrell (NUI Galway). Used with permission.

Systematic Review Shows That Work Done by Storm Waves Can Be Misinterpreted as Tsunami-Related Because Commonly Used Hydrodynamic Equations Are Flawed

Coastal boulder deposits (CBD), transported by waves at elevations above sea level and substantial distances inland, are markers for marine incursions. Whether they are tsunami or storm deposits can be difficult to determine, but this is of critical importance because of the role that CBD play in coastal hazard analysis. Equations from seminal work by Nott (1997), here referred to as the Nott Approach, are commonly employed to calculate nominal wave heights from boulder masses as a means to discriminate between emplacement mechanisms. Systematic review shows that this approach is based on assumptions that are not securely founded and that direct relationships cannot be established between boulder measurements and wave heights. A test using an unprecedented dataset of boulders moved by storm waves (with associated sea-state data) shows a lack of agreement between calculations and actual wave heights. The equations return unrealistically large heights, many of which greatly exceed sea states occurring during the boulder-moving storms. This underscores the finding that Nott-Approach wave-height calculations are unreliable. The result is general, because although the field data come from one region (the Aran Islands, Ireland), they represent a wide range of boulder masses and topographic settings and present a valid test of hydrodynamic equations. This analysis demonstrates that Nott Approach equations are incapable of distinguishing storm waves from tsunami transport and that wave heights hindcast from boulder masses are not meaningful. Current hydrodynamic understanding does not permit reliable computation of wave height from boulder measurements. A combination of field, numerical, and experimental approaches is required to quantify relationships between wave power and mass transport onshore. Many CBD interpreted as tsunami deposits based on Nott-Approach analysis may in fact have been emplaced during storms and should therefore be re-evaluated. This is especially important for CBD that have been incorporated into long-term coastal risk assessments, which are compromised if the CBD are misinterpreted. CBD dynamics can be better determined from a combination of detailed field measurements, modeling, and experiments. A clearer understanding of emplacement mechanisms will result in more reliable hazard analysis.

Holocene Hurricane Deposits Eroded as Coastal Barriers from Andesite Sea Cliffs at Puerto Escondido (Baja California Sur, Mexico)

Previous studies on the role of hurricanes in Mexico’s Gulf of California examined coastal boulder deposits (CBDs) eroded from limestone and rhyolite sea cliffs. Sedimentary and volcanic in origin, these lithotypes are less extensively expressed as rocky shores than others in the overall distribution of gulf shores. Andesite that accumulated as serial volcanic flows during the Miocene constitutes by far the region’s most pervasive rocky shores. Here, we define a subgroup of structures called barrier boulder deposits (BBDs) that close off lagoons as a result of lateral transport from adjacent rocky shores subject to recurrent storm erosion. Hidden Harbor (Puerto Escondido) is the most famous natural harbor in all of Baja California. Accessed from a single narrow entrance, it is commodious in size (2.3 km2) and fully sheltered by outer andesite hills linked by two natural barriers. The average weight of embedded boulders in a succession of six samples tallied over a combined distance of 710 m ranges between 74 and 197 kg calculated on the basis of boulder volume and the specific gravity of andesite. A mathematical formula is utilized to estimate the wave height necessary to transport large boulders from their source. Average wave height interpreted by this method varies between 4.1 and 4.6 m. Input from fossil deposits and physical geology related to fault trends is applied to reconstruct coastal evolution from a more open coastal scenario during the Late Pleistocene 125,000 years ago to lagoon closure in Holocene time.

Measuring Change Using Quantitative Differencing of Repeat Structure-From-Motion Photogrammetry: The Effect of Storms on Coastal Boulder Deposits

Repeat photogrammetry is increasingly the go-too tool for long-term geomorphic monitoring, but quantifying the differences between structure-from-motion (SfM) models is a developing field. Volumetric differencing software (such as the open-source package CloudCompare) provides an efficient mechanism for quantifying change in landscapes. In this case study, we apply this methodology to coastal boulder deposits on Inishmore, Ireland. Storm waves are known to move these rocks, but boulder transportation and evolution of the deposits are not well documented. We used two disparate SfM data sets for this analysis. The first model was built from imagery captured in 2015 using a GoPro Hero 3+ camera (fisheye lens) and the second used 2017 imagery from a DJI FC300X camera (standard digital single-lens reflex (DSLR) camera); and we used CloudCompare to measure the differences between them. This study produced two noteworthy findings: First, volumetric differencing reveals that short-term changes in boulder deposits can be larger than expected, and that frequent monitoring can reveal not only the scale but the complexities of boulder transport in this setting. This is a valuable addition to our growing understanding of coastal boulder deposits. Second, SfM models generated by different imaging hardware can be successfully compared at sub-decimeter resolution, even when one of the camera systems has substantial lens distortion. This means that older image sets, which might not otherwise be considered of appropriate quality for co-analysis with more recent data, should not be ignored as data sources in long-term monitoring studies.

Coastal Boulder Deposits of the Neogene World: A Synopsis

Modern geoscience research pays significant attention to Quaternary coastal boulder deposits, although the evidence from the earlier geologic periods can be of great importance. The undertaken compilation of the literature permits to indicate 21 articles devoted to such deposits of Neogene age. These are chiefly case studies. Such an insufficiency of investigations may be linked to poor preservation potential of coastal boulder deposits and methodological difficulties. Equal attention has been paid by geoscientists to Miocene and Pliocene deposits. Taking into account the much shorter duration of the Pliocene, an overemphasis of boulders of this age becomes evident. Hypothetically, this can be explained by more favorable conditions for boulder formation, including a larger number of hurricanes due to the Pliocene warming. Geographically, the studies of the Neogene coastal boulder deposits have been undertaken in different parts of the world, but generally in those locations where rocky shores occur nowadays. The relevance of these deposits to storms and tsunamis, rocky shores and deltas, gravity processes, and volcanism has been discussed; however, some other mechanisms of boulder production, transportation, and accumulation (e.g., linked to seismicity and weathering) have been missed.