Megaclasts: Term Use and Relevant Biases

The term “megaclast” started circulation near the beginning of the 21st century. The present review is aimed at examination of the use of this term in the modern geoscience literature. The main method is bibliographical survey of the articles published during 2000–2017 with the on-line bibliographical database “Scopus”. The main findings are as follows. The term “megaclast” has not been used extensively, but the number of the articles employing this term increased in the mid-2000s and in the early 2010s. The majority of the papers deal with megaclasts of Quaternary age. The megaclast research focuses on five regions, namely West Europe, Australia and New Zealand, Western North America, Southern South America, and the Pacific and circum-Pacific. The most studied are megaclasts occurring on coasts influenced by tsunamis and storms; significant attention has been paid also to those clasts transported by volcanism-triggered debris flows and slope failures, both continental and submarine. There are three serious biases relevant to the use of the term “megaclast” in the geoscience literature, namely stratigraphical, geographical, and genetic biases. Due to this incompleteness in the knowledge of megaclasts, this term should be either used more actively, which is preferable, or abandoned.


Introduction
During the two past decades, there have been some important advances to refine nomenclature of sedimentary rocks and particles. Blair and McPherson [1], Farrell et al. [2], and Lokier and Al Junaibi [3] contributed to this issue. It is equally interesting to realize how well the international research community has perceived the novelties and focused on the study of newly-recognized categories of rocks and particles.
Udden [4] and Wentworth [5] developed the approach for classifying clastic grains by size that is still in wide use. However, this approach is less appropriate for particles that reach several meters and dozens of meters in diameter because these should be recognized as a particular category. An interest in such clasts has increased, particularly because of significant intensification of investigations on modern and ancient tsunami deposits [6]. Moreover, erratic "boulders" and olistoliths have remained interesting features for more than a century. Presently, two additional factors stimulate this interest. The importance of the largest stones on the Earth as geological heritage and geotourist attractions has been realized by Wimbledon and Smith-Meyer [7] and Lubova et al. [8]. Large clasts have been found on various cosmic bodies, and these clasts are described, particularly, by Pajola et al. [9], Bruno and Ruban [10], and Dhingra [11]. The studies of the both kinds require detailed description of large clasts. Blair and McPherson [1] proposed a new, detailed classification of sedimentary particles
The understanding of megaclasts remains poorly defined. There is no agreement of how to limit the category of megaclasts in a scheme of grain-size classification. Blair and McPherson [1] and Terry and Goff [14] proposed a size of 4.096 m as the lower limit. This matches the general principle of the Udden-Wentworth scheme. However, Blott and Pye [13] recommended a size of 2.048 m as the lower limit. Finally, Bruno and Ruban [10] suggested a size of 1 m as the lower limit of megaclasts, particularly to make the classification suitable for application to distant cosmic bodies. Very different terms have been proposed for naming subdivisions of megaclasts. Each following team of specialists expressed concerns about the names suggested by earlier workers. Moreover, the clear separation between boulders, i.e., much smaller clasts, and megaclasts is not fixed in the
The understanding of megaclasts remains poorly defined. There is no agreement of how to limit the category of megaclasts in a scheme of grain-size classification. Blair and McPherson [1] and Terry and Goff [14] proposed a size of 4.096 m as the lower limit. This matches the general principle of the Udden-Wentworth scheme. However, Blott and Pye [13] recommended a size of 2.048 m as the lower limit. Finally, Bruno and Ruban [10] suggested a size of 1 m as the lower limit of megaclasts, particularly to make the classification suitable for application to distant cosmic bodies. Very different terms have been proposed for naming subdivisions of megaclasts. Each following team of specialists expressed concerns about the names suggested by earlier workers. Moreover, the clear separation between boulders, i.e., much smaller clasts, and megaclasts is not fixed in the terminology. For instance, Terry and [14] proposed to identify some megaclasts as mesoboulders and macroboulders, although the latter are not true boulders because boulders are smaller than megaclasts by definition.
Despite the above-mentioned uncertainties, three statements seem to be indisputable. The term "megaclast" is well-suited for description of a separate category or class of grains. The size of megaclasts is measured by meters, dozens of meters, and even hundreds of meters. It is sensible to subdivide this category into several grades. Moreover, it appears that single megaclasts and their accumulations, i.e., clusters or "fields", constitute a specific geological phenomenon, which is aesthetically impressive [21].

Material and Method
The bibliometric approach is becoming an efficient tool for conceptualization in the modern science, as shown recently by Qiu and Liu [22]. It employs diverse methods and modes of data presentation, some of which seem to be suitable for terminological studies. The present review is based on analysis of literature sources that directly use the term "megaclast". A targeted bibliographical survey, which is a kind of bibliometric approach, was conducted for this purpose. The on-line bibliographical database "Scopus" was searched. This database has an excellent coverage of geoscience journals where papers on megaclasts can be published. This database includes many regional-to-local journals, which fact minimizes missing the necessary publications.
All articles published in professional journals and special publication series during the time period of 2000-2017 and containing the term "megaclast(s)" in their titles, abstracts, and keywords were identified. Fifty four sources were found this way (Table 1), and these seem to be the only published works that employ the term actively. Apparently, these sources reflect the status of megaclast research after the publication of the article by Blair and McPherson [1]. Of course, there are other sources that mention megaclasts occasionally. However, consideration of the incorrect use of the term "megaclast" may result in significant "noise" in the collected bibliographical records that may affect the clarity of the subsequent analysis. In contrast, occurrence of the term "megaclast" in the title, abstract, or keywords means that the article more-or-less focuses on megaclasts. There may be other sources not included into "Scopus", but these are chiefly limited to very local journals or proceedings/abstract volumes, which cannot be judged as full representatives of the international geoscience media. In any case, only a part of them can be found with on-line tools and, thus, their consideration will make the analyzed bibliographical data less representative. Indeed, articles about large clasts were published before 2000, but these do not much anyway the time frame of the present review that is limited to the appears of the paper by Blair and McPherson [1]. It should be added that a few very important articles by Paris et al. [23,24], Ramalho et al. [25], Hearty and Tormey [26], and Cox et al. [27] that are devoted to megaclasts and employ this term, although do not retain it in their titles, are added to the collected sources used for the purposes of the present analysis. Three of these articles are published in 2018, but they became available already in 2017 and, thus, these match the analyzed time frame. As a result, the total number of analyzed articles is 59 (Table 1). Table 1. Megaclast-related literature sources for the purposes of examination of the term use.

Author(s), Source Age Location Facies/Origin
Barbano et al. [28] Q Sicily Tsunami Bruno and Ruban [10] extraterrestrial bodies Extraterrestial Canto et al. [29] Pg Philippines Tectonic Carpentier et al. [30] J France Storm Choe et al. [31] K Chile Deep-marine channel Coira and Perez [32] O Argentina Magma-water interaction Dewey and Ryan [33] N, Q New Zealand Tsunami, storm Engel and May [34] Q Caribbean Tsunami Furlanetto et al. [35] PPR northwestern Canada Tectonic Gaylord and Neall [36] Q New Zealand Volcanic Gaylord et al. [37] Q New Zealand Volcanic Goff and Terry [38] Q Pacific Tsunami Hall et al. [39] Q British Isles Coast Hoffmann et al. [40] Q Arabia Tsunami Horak and Evans [41] NPR British Isles -Jackson [42] CZ offshore Brazil Submarine mass wasting Kalnina et al. [43] Q Baltic Region Glacial erosion Keigler et al. [44] Q New Zealand Volcanic and mass wasting Laird et al. [45] Pg New Zealand Channel Laughton et al. [46] PPR northwestern Canada Tectonic + volcanic Le Heron et al. [47] NPR western United States Tectonic and ice-rafting Le Heron et al. [48] NPR western United States Tectonic Le Roux [49] CZ Chile Mass wasting and tsunami Le Roux and Vargas [50] N Chile Mass wasting and tsunami Lecointre et al. [51] Q New Zealand Volcanic and mass wasting Lorang [52] --Tsunami and storm Lubova et al. [8] Q Caucasus Mass wasting Madon [53] T Malaysia Submarine mass wasting Martin-Merino et al. [54] PZ2 Spain Submarine mass wasting McKee et al. [55] K western United States Mass wasting Medina et al. [56] Q Morocco Tsunami and storm Noormets et al. [57] Q Pacific Tsunami Noormets et al. [58] Q Pacific Tsunami Oliveira et al. [59] Q Portugal Storm Ortiz-Karpf et al. [60] Q Caribbean Submarine mass wasting Paris et al. [61] Q Mauritius Tsunami Perez-Alberti et al. [62] Q Spain Storm Pierre [63] Q France Coastal processes Pope and Grotzinger [64] PPR northwestern Canada Evaporite dissolution and mass wasting Preiss et al. [65] NPR Australia Glaciation Roverato et al. [66] Q New Zealand Volcanic and mass wasting Salisbury et al. [67] Cm Australia Extraterrestrial impact Scheffers et al. [68] Q Australia Tsunami Scheffers et al. [69] Q British Isles Storm Scheffers et al. [70] Q British Isles Storm Shane et al. [71] N New Zealand Submarine mass wasting Suttner and Kido [72] D Alps Coastal processes Terry and Goff [14] ---Thorkelson and Laughton [73] PPR northwestern Canada Tectonic Trenhaile [74] --Coastal processes Weckwerth and Pisarska-Jamrozy [75] Q Poland Fluvial-periglacial Williams [76] Q British Isles Storm Williams and Hall [77] Q British Isles Storm Yagishita and Komori [78] N Japan Mass wasting Hearty and Tormey [26] Q Bahamas Storm Cox et al. [27] Q Ireland Storm Paris et al. [23] --Storm Paris et al. [24] Q Cape Verde, Mauritius, Reunion Tsunami Ramalho et al. [25] Q Cape Verde Tsunami, coastal processes Undoubtedly, some megaclasts were the focus of research, but these were not recognized as megaclasts-a typical example is the work of Rovere et al. [79]. The other terms such as "boulders" and "blocks" were used. Such articles are not covered by the quantitative analysis of the present Geosciences 2019, 9, 14 5 of 16 paper because of two reasons. Their consideration would lead to the unbalanced and unjustified bibliographical dataset. It is of special importance to focus on the use of the term "megaclast" in the modern geoscience literature. However, the above-mentioned articles are discussed in this paper separately, which avoids the problem of missing some important sources.
The content of each identified source was examined and, in particular, checked using certain criteria. The years of publication are considered in order to reconstruct research dynamics. Attention is paid to the geological age and geographical location of megaclasts. Palaeoenvironmental context and/or origin of megaclasts mentioned in the chosen sources are specified. The results of the analysis allow the main parameters of megaclast research to be characterized and to summarize its main outcomes with regard to the spatio-temporal distribution of megaclasts and their relevance to facies and specific geological processes.

Basic Trends of Megaclast Research
The total number of the megaclast-related articles, i.e., the articles employing the term "megaclast", published during 2000-2017 is relatively low ( Table 1). On average, only three articles were published each year during this time frame. However, the intensity of term use was unstable ( Figure 2). During the 2000s, the intensity was very low, although the number of the articles increased in the mid-2000s, most probably, as result of interest to tsunami deposits as a consequence of the Indian Ocean Tsunami of 2004 [6]. The situation changed in the 2010s, when the number of articles increased significantly ( Figure 2). Although term use tended to decline later, it appears that megaclasts attracted relatively more attention during this decade. Interestingly, this tendency established before the highly-important contributions of Blott and Pye [13] and Terry and Goff [14] were published and, thus, these contributions themselves did not catalyze the noted intensification in research. What is also important is that megaclast-related articles were published each year after 2001, which implies a kind of continuity of interest on this term.
Undoubtedly, some megaclasts were the focus of research, but these were not recognized as megaclasts-a typical example is the work of Rovere et al. [79]. The other terms such as "boulders" and "blocks" were used. Such articles are not covered by the quantitative analysis of the present paper because of two reasons. Their consideration would lead to the unbalanced and unjustified bibliographical dataset. It is of special importance to focus on the use of the term "megaclast" in the modern geoscience literature. However, the above-mentioned articles are discussed in this paper separately, which avoids the problem of missing some important sources.
The content of each identified source was examined and, in particular, checked using certain criteria. The years of publication are considered in order to reconstruct research dynamics. Attention is paid to the geological age and geographical location of megaclasts. Palaeoenvironmental context and/or origin of megaclasts mentioned in the chosen sources are specified. The results of the analysis allow the main parameters of megaclast research to be characterized and to summarize its main outcomes with regard to the spatio-temporal distribution of megaclasts and their relevance to facies and specific geological processes.

Basic Trends of Megaclast Research
The total number of the megaclast-related articles, i.e., the articles employing the term "megaclast", published during 2000-2017 is relatively low ( Table 1). On average, only three articles were published each year during this time frame. However, the intensity of term use was unstable ( Figure 2). During the 2000s, the intensity was very low, although the number of the articles increased in the mid-2000s, most probably, as result of interest to tsunami deposits as a consequence of the Indian Ocean Tsunami of 2004 [6]. The situation changed in the 2010s, when the number of articles increased significantly ( Figure 2). Although term use tended to decline later, it appears that megaclasts attracted relatively more attention during this decade. Interestingly, this tendency established before the highly-important contributions of Blott and Pye [13] and Terry and Goff [14] were published and, thus, these contributions themselves did not catalyze the noted intensification in research. What is also important is that megaclast-related articles were published each year after 2001, which implies a kind of continuity of interest on this term.  The citation rate of megaclast-related articles is low-to-moderate. As the most representative example, it is possible to say that the work of Blair and McPherson [1] was cited only 165 times during 18 years, and not always in relevance to large clasts. The articles of Barbano et al. [28], Noormets et al. [57,58], and Williams and Hall [77] are among the most successful articles in regard to the number of citations. Many others can boast not more than 10 citations. This evidence also implies relative weakness in the areas of megaclast research.
Those articles analyzed appear to be diverse, thematically. Although many focus on megaclasts as indicators of coastal susceptibility to tsunamis and storms, others deal with a wide range of topics. The latter include continental and submarine slope processes, evolution of volcano edifices, Neoproterozoic glaciations, terrane accretion, extraterrestrial environments, etc. Thematic diversity is related chiefly to discussion of megaclasts of different ages and in connection with different geological processes (Table 1). Of special importance is the appearance of a series of conceptual papers. Some of them are directly concerned with megaclasts. For instance, Terry and Goff [14] focused on megaclast-related nomenclatures and Bruno and Ruban [10] critically reviewed megaclast studies on various cosmic bodies. Articles by Noormets et al. [57,58] provide almost classical discussions of megaclast transport relevant to tsunamis. These are complemented by the work of Le Roux and Vargas [49]. Another conceptually important topic draws a distinction between storm and tsunami effects on megaclasts, which is treated comprehensively in the article by Lorang [52]. Lubova et al. [8] explained the importance of large sedimentary particles with regard to geological heritage conservation and geotourism. There are also conceptual papers that deal with the other subjects, but also treat megaclasts as an important issue. A typical example is review of rocky coasts [74] where megaclasts are common sedimentary particles.

Geological Spatio-Temporal Dimension of the Term Application
Bibliographical information collected for the purposes of the present article permits consideration of spatio-temporal distribution of megaclasts. Indeed, the latter include only those large sedimentary particles termed as "megaclasts".
Megaclast-related articles fall into different intervals of geological history ( Figure 3). Most often, these studies are drawn from very young formations of Quaternary age. For instance, there are megaclast accumulations in the coastal areas of Morocco that were produced by the mid-18th-century tsunami [56]. Paleozoic and Mesozoic megaclasts are rarely studied. Surprisingly, significant attention has been paid to Proterozoic megaclasts ( Figure 3). These were examined in northwestern Canada [35,46,64,73], the Western United States [47,48], and the British Isles [41]. The Paleoproterozoic Wernecke Supergroup in Yukon and the Neoproterozoic Kingston Peak Formation in California provide representative examples of Precambrian megaclasts. example, it is possible to say that the work of Blair and McPherson [1] was cited only 165 times during 18 years, and not always in relevance to large clasts. The articles of Barbano et al. [28], Noormets et al. [57,58], and Williams and Hall [77] are among the most successful articles in regard to the number of citations. Many others can boast not more than 10 citations. This evidence also implies relative weakness in the areas of megaclast research.
Those articles analyzed appear to be diverse, thematically. Although many focus on megaclasts as indicators of coastal susceptibility to tsunamis and storms, others deal with a wide range of topics. The latter include continental and submarine slope processes, evolution of volcano edifices, Neoproterozoic glaciations, terrane accretion, extraterrestrial environments, etc. Thematic diversity is related chiefly to discussion of megaclasts of different ages and in connection with different geological processes (Table 1). Of special importance is the appearance of a series of conceptual papers. Some of them are directly concerned with megaclasts. For instance, Terry and Goff [14] focused on megaclast-related nomenclatures and Bruno and Ruban [10] critically reviewed megaclast studies on various cosmic bodies. Articles by Noormets et al. [57,58] provide almost classical discussions of megaclast transport relevant to tsunamis. These are complemented by the work of Le Roux and Vargas [49]. Another conceptually important topic draws a distinction between storm and tsunami effects on megaclasts, which is treated comprehensively in the article by Lorang [52]. Lubova et al. [8] explained the importance of large sedimentary particles with regard to geological heritage conservation and geotourism. There are also conceptual papers that deal with the other subjects, but also treat megaclasts as an important issue. A typical example is review of rocky coasts [74] where megaclasts are common sedimentary particles.

Geological Spatio-Temporal Dimension of the Term Application
Bibliographical information collected for the purposes of the present article permits consideration of spatio-temporal distribution of megaclasts. Indeed, the latter include only those large sedimentary particles termed as "megaclasts".
Megaclast-related articles fall into different intervals of geological history ( Figure 3). Most often, these studies are drawn from very young formations of Quaternary age. For instance, there are megaclast accumulations in the coastal areas of Morocco that were produced by the mid-18th-century tsunami [56]. Paleozoic and Mesozoic megaclasts are rarely studied. Surprisingly, significant attention has been paid to Proterozoic megaclasts ( Figure 3). These were examined in northwestern Canada [35,46,64,73], the Western United States [47,48], and the British Isles [41].   Megaclasts are reported from almost all continents and oceans (Figure 4). Most intensively, these were studied in five major regions of the world, namely West Europe, Australia and New Zealand, Western North America, Southern South America, and the Pacific and circum-Pacific. Best known are Quaternary megaclasts of the British Isles and New Zealand. The former were studied, particularly, by Scheffers et al. [69] and Williams and Hall [77], whereas Keigler et al. [44], Gaylor and Neall [36], and Roverato et al. [66] made a significant contribution to the knowledge of the latter. Interestingly, not only Quaternary, but also Paleogene and Neogene megaclasts were examined in New Zealand and, thus, the latter seems to have been the most important place for modern megaclast research because of successful locally-developed projects resulting in a series of publications. these were studied in five major regions of the world, namely West Europe, Australia and New Zealand, Western North America, Southern South America, and the Pacific and circum-Pacific. Best known are Quaternary megaclasts of the British Isles and New Zealand. The former were studied, particularly, by Scheffers et al. [69] and Williams and Hall [77], whereas Keigler et al. [44], Gaylor and Neall [36], and Roverato et al. [66] made a significant contribution to the knowledge of the latter. Interestingly, not only Quaternary, but also Paleogene and Neogene megaclasts were examined in New Zealand and, thus, the latter seems to have been the most important place for modern megaclast research because of successful locally-developed projects resulting in a series of publications. Megaclasts demonstrate an affinity to very different facies (Table 1). Most often, these occur near the shoreline, i.e., on beaches and inner shelves. In such cases, megaclasts are formed by rockfalls and rockslides on retreated cliffs and then transported by tsunami and storm waves. As such, rocky coasts open to oceans are the principal depositional environment for megaclast development. Typical examples are the coasts of Oahu [57,58], Oman [40], and Galicia [62]. Although megaclasts are large and heavy, these are often formed on destructed slopes and easily involved in the slope processes in continental and submarine environments. As a result, they are typical for colluvial deposits and also linked to volcanism-triggered debris flows and turbidites. Among other interesting phenomena, it is important to note links of some megaclasts with past glaciations-e.g., erratic megaclasts of Latvia described by Kalnina et al. [43] and gigantic Sturtian megaclasts South Australia mentioned by Preiss et al. [65]-and magma-water interaction in ancient basins-e.g., Ordovician megaclasts from the Argentina's Puna Highland [32]. Megaclasts can be produced by extraterrestrial impacts. On the Earth, this is reported by Salisbury et al. [67] in connection with the Lawn Hill circular structure in northwest Queensland (Australia). However, such an origin of megaclasts is most typical on various cosmic bodies [10]. Generally, megaclasts are produced by and involved in different geological processes, the best documented, but actually not all of which are summarized in Figure 5. Megaclasts demonstrate an affinity to very different facies (Table 1). Most often, these occur near the shoreline, i.e., on beaches and inner shelves. In such cases, megaclasts are formed by rockfalls and rockslides on retreated cliffs and then transported by tsunami and storm waves. As such, rocky coasts open to oceans are the principal depositional environment for megaclast development. Typical examples are the coasts of Oahu [57,58], Oman [40], and Galicia [62]. Although megaclasts are large and heavy, these are often formed on destructed slopes and easily involved in the slope processes in continental and submarine environments. As a result, they are typical for colluvial deposits and also linked to volcanism-triggered debris flows and turbidites. Among other interesting phenomena, it is important to note links of some megaclasts with past glaciations-e.g., erratic megaclasts of Latvia described by Kalnina et al. [43] and gigantic Sturtian megaclasts South Australia mentioned by Preiss et al. [65]-and magma-water interaction in ancient basins-e.g., Ordovician megaclasts from the Argentina's Puna Highland [32]. Megaclasts can be produced by extraterrestrial impacts. On the Earth, this is reported by Salisbury et al. [67] in connection with the Lawn Hill circular structure in northwest Queensland (Australia). However, such an origin of megaclasts is most typical on various cosmic bodies [10]. Generally, megaclasts are produced by and involved in different geological processes, the best documented, but actually not all of which are summarized in Figure 5.

Discussion
The present analysis implies that megaclast research has progressed since the beginning of the 21st century. However, the intensity of use of the term "megaclast" remains questionable. On one hand, the similarly-constrained search in the same bibliographical database demonstrates that the number of articles published annually on cobbles and pebbles was in 45 and 121 times greater, respectively than on megaclasts. On the other hand, cobbles and pebbles are significantly more common sedimentary particles, and these terms have been employed actively during the past century. In contrast, megaclasts are rare, and the term itself is new. In this case, it would be incorrect to say that modern megaclast research is low in intensity. However, this kind of research is likely to become more active in recognition of the largest stones on the Earth as megaclasts and to better understand their origin.
It should be stressed that the distribution of megaclasts in geological time and space considered above is deduced from only the article employing the term "megaclast". In such a case, further comparison of this documented distribution with the expected distribution of megaclasts termed so or not indicates on biases in use of the term "megaclast". Three kinds of biases, namely stratigraphical, geographical, and genetic biases can be detected, and these are discussed below.
The young age of the majority of studied megaclasts (Figure 3) implies their poor preservation in the geological record. Most probably, they are subject to weathering and erosion by water and wind, and so they disintegrate quickly into smaller particles over longer geological time scales. Megaclasts often occur on rocky shores, and the relevant facies are also uncommon in the geological record [81,82]. However, the recognition of Precambrian and early Paleozoic megaclasts implies that some of such large particles can be preserved under specific conditions, among which rapid sedimentation seems to be the most important. The distribution of megaclasts through the geological time as reflected by the published articles ( Figure 3) may also be biased. If the relevant research is facilitated by an interest in modern and historical tsunamis, it is not strange that large clasts of chiefly Quaternary age are the main research focus in this trend.
The currently available knowledge of megaclast distribution (Figure 4) appears to be seriously biased. Studies in Russia [8], Iran [83], and Egypt [84,85] point out the existence of numerous megaclasts and their "fields" that are yet to attract any special investigation. Some impressive megaclasts are known as geotourist attractions in different parts of the world-e.g., the erratic block on Letipea Peninsula in Estonia [7], but these were not studied from a sedimentological point of view. Numerous large clasts on various cosmic bodies such as planets and satellites, asteroids, and

Discussion
The present analysis implies that megaclast research has progressed since the beginning of the 21st century. However, the intensity of use of the term "megaclast" remains questionable. On one hand, the similarly-constrained search in the same bibliographical database demonstrates that the number of articles published annually on cobbles and pebbles was in 45 and 121 times greater, respectively than on megaclasts. On the other hand, cobbles and pebbles are significantly more common sedimentary particles, and these terms have been employed actively during the past century. In contrast, megaclasts are rare, and the term itself is new. In this case, it would be incorrect to say that modern megaclast research is low in intensity. However, this kind of research is likely to become more active in recognition of the largest stones on the Earth as megaclasts and to better understand their origin.
It should be stressed that the distribution of megaclasts in geological time and space considered above is deduced from only the article employing the term "megaclast". In such a case, further comparison of this documented distribution with the expected distribution of megaclasts termed so or not indicates on biases in use of the term "megaclast". Three kinds of biases, namely stratigraphical, geographical, and genetic biases can be detected, and these are discussed below.
The young age of the majority of studied megaclasts ( Figure 3) implies their poor preservation in the geological record. Most probably, they are subject to weathering and erosion by water and wind, and so they disintegrate quickly into smaller particles over longer geological time scales. Megaclasts often occur on rocky shores, and the relevant facies are also uncommon in the geological record [81,82]. However, the recognition of Precambrian and early Paleozoic megaclasts implies that some of such large particles can be preserved under specific conditions, among which rapid sedimentation seems to be the most important. The distribution of megaclasts through the geological time as reflected by the published articles (Figure 3) may also be biased. If the relevant research is facilitated by an interest in modern and historical tsunamis, it is not strange that large clasts of chiefly Quaternary age are the main research focus in this trend.
The currently available knowledge of megaclast distribution (Figure 4) appears to be seriously biased. Studies in Russia [8], Iran [83], and Egypt [84,85] point out the existence of numerous megaclasts and their "fields" that are yet to attract any special investigation. Some impressive megaclasts are known as geotourist attractions in different parts of the world-e.g., the erratic block on Letipea Peninsula in Estonia [7], but these were not studied from a sedimentological point of view. Numerous large clasts on various cosmic bodies such as planets and satellites, asteroids, and comets have been recognized as megaclasts only very recently [10]. Undoubtedly, megaclast research should expand to the Arctic and Antarctica, Southern and Southeastern Asia, Africa, Central and Eastern Americas, etc., to fill existing geographical gaps. In other words, megaclasts known in these parts of the world should be termed properly, i.e., as megaclasts.
An analysis of the available literature sources (Table 1) demonstrates that the majority of studies focus more on the transport of megaclasts in a particular depositional environment than on their origin. This is a very significant research bias. Of course, tsunami-versus-storm debates relevant to transport of megaclasts on shores [23,26] are important, but it often remains unclear how these megaclasts were formed initially. For instance, mechanisms revealed by Panek et al. [86] imply that megaclast formation may be not so simple and fast as one may expect. If so, megaclasts may remain "attached" to the place of their origin for some time. The other bias relevant to facies is linked to an evident over-emphasis on large clasts occurring on ocean coasts. Quaternary megaclasts linked to active volcanism in New Zealand [36,37] and the Paleoproterozoic megaclasts of Northwestern Canada [35,46,73] appear to be highly-specific in regard to their origin, transport, and mode of preservation. Much attention has been paid to them "occasionally", i.e., only because of the long-term research projects focused on unusual features of the host sedimentary complexes. In contrast, some other environments, in which megaclasts are thought to be common, have been investigated less intensively. The proposed scheme of the main megaclast-related geological processes that is based on the present literature review ( Figure 5) fails to consider several important phenomena. Growth and retreat of continental ice sheets should produce significant number of erratic megaclasts that can be found, for instance, in northern Europe and northern North America. Large particles may be formed on some lake coasts with steep slopes formed of hard rocks such as granites. For instance, these can be studied on the shore of the Lake Malawi in Africa. Weathering can lead to appearance of megaclasts via gradual "sculpturing" from the parent rock, as this is described in Egypt by Sallam et al. [85]. Similarly, epikarst development may result in separation of large blocks because of joint-controlled grike growth, as this is known in the Lagonaki Highland of Russia ( Figure 6). Road construction and other kinds of engineering works may result in the appearance of artificial megaclasts. Particularly, these were reported by Lubova et al. [8] from the Western Caucasus in Russia. Newly-formed and transported megaclasts should be differentiated. For instance, many megaclasts associated with storm and tsunami deposits were created via slope failure resulted from "normal" wave abrasion, not necessarily by big waves. Generally, the available knowledge of megaclast origin ( Figure 5) appears to be strongly biased when compared to the more general genetic classification resulting from consideration of possible situations of megaclast formation and transport (Table 2). Additionally, it may be sensible to distinguish types of megaclasts depending on their lithological composition, i.e., siciciclastic, carbonate, mixed, etc. For instance, a typical carbonate megaclast is shown on Figure 1.
The wide application of the term "megaclast" is a relatively new trend, and many geologists, unfortunately, still avoid it. On the one hand, this narrows megaclasts research. On the other hand, such megaclasts that are not known as megaclasts should be re-considered. For the purposes of this brief review, it appears to be important to give several examples of important works that contribute potentially to megaclast research, although do not use the term "megaclast". Dott and Byers [87] in their description of the Cambrian strata of Wisconsin, the United States note the existence of some "boulders" that reach meters in diameter. Their origin is linked to palaeostorms. Johnson et al. [88] report large "boulders" in Pliocene massive delta outwash deposits on an island in the Gulf of California in Mexico, the formation of which was related to heavy, hurricane-related rain fall on land that cleared mountain canyons of rock debris. Rovere et al. [79] focus on Quaternary storm-related giant "boulders" of the Bahamas. Panek et al. [86] describe Quaternary formation of gigantic limestone blocks on the Crimean Peninsula linked to processes of karstification and the Black Sea transgression. The studies of Soukopova [89,90], Erdmann et al. [91], Hongo et al. [92], Johnson et al. [93], and Lau et al. [94] should also be noted. In all these cases, megaclasts are considered in fact. Some regions like Sicily in Italy [28,[95][96][97][98][99] are well-known for large clast occurrence. Special attention should be paid to them in order to document these particles and to distinguish true megaclasts from boulders. Undoubtedly, the number of large clasts that are yet to be identified as megaclasts is significant.    Surprisingly, there is also important evidence of megaclasts from tourism. Due to their physical and aesthetic parameters, megaclasts are often employed as tourist attractions [8]. A tentative analysis of Internet resources points to several impressive megaclasts occurrences in different countries; importantly, these include erratic clasts and clasts formed by erosion and weathering (Table 3). Moreover, some megaclasts serve as important elements of the local historical and cultural heritage. Typical examples are the gigantic erratic Thunder Stone used as the basis of the famous monument in the Russian city of Saint Petersburg ( Figure 7A) and the stone erased to commemorate the historical foundation of the other Russian city of Cherepovets ( Figure 7B). The both are essentially erratic megaclasts that experienced slight modification by artists. Apparently, these tourist attractions are almost totally missed from the field of the term use.

Conclusions
The present review allows five general conclusions to be made. The megaclast research has progressed significantly after the publication of a detailed classification for large sedimentary

Conclusions
The present review allows five general conclusions to be made. The megaclast research has progressed significantly after the publication of a detailed classification for large sedimentary particles by Blair and McPherson [1], and the use of the term "megaclast" accelerated in the mid-2000s and the early 2010s. Although the age of megaclasts ranges from the Paleoproterozoic to the Present, the majority are of Quaternary in age, which reflects partly on the low preservation potential of very large sedimentary particles and partly the stratigraphical bias of the term use. The known geographical distribution of megaclasts with over-emphasis on West Europe, Australia, and New Zealand, Western North America, Southern South America, and the Pacific and circum-Pacific is strongly biased geographically. Megaclasts are studied in different facies and geological processes, among which rocky coasts, volcanism-triggered debris flows, and continental and submarine mass wasting are the best known; this is the other, genetic bias of the use of the term "megaclast". The proposed genetic classification of megaclasts implies a wider spectrum of depositional environments than emphasized in the literature. Erratic, karst-related, artificial, and some other megaclasts are mainly not termed as megaclasts, which is a kind of failure in the term use.
Generally, the present review shows that use of the term "megaclast" in the modern research is not only significantly restricted, but also biased. This makes the very knowledge of megaclasts incomplete despite two decades of investigations. Active application of the term is necessary to avoid current situation where we know about megaclasts defined as such and megaclasts defined somehow else or not defined specifically. Otherwise, the term should be abandoned, although this is not desirable because of its evident suitability. An important task for further research is understanding why researchers accept the term so slowly. This depends on finding more efficient ways for introduction of new terms in today's geoscience.