**1. Introduction**

Modern marine sedimentology grows rapidly, and new research directions have strengthened in the past two decades. One of such directions embraces studies of coastal boulders. On the one hand, these studies aim at development of nomenclature of large clasts. Concerning nomenclature, some advances have been made in the works by Blair and McPherson [1], Blott and Pye [2], Bruno and Ruban [3], and Terry and Goff [4]. On the other hand, boulders are regarded as precious evidence of present and ancient rocky shore facies and extreme events (such as storms, tsunamis, hurricanes, typhoons, and cyclones). This evidence has been examined by Autret et al. [5], Bhatt et al. [6], Biolchi et al. [7], Cox et al. [8–10], Dawson [11], Engel et al. [12], Erdmann et al. [13], Hearty and Tormey [14], Herterich et al. [15], Hongo et al. [16], Johnson et al. [17,18], Kennedy et al. [19], Kortekaas and Dawson [20], Lau et al. [21], Olsen et al. [22], Paris et al. [23], Pepe et al. [24], Scheffers et al. [25], Schneider et al. [26], Shah-Hosseini et al. [27], Suanez et al. [28], Terry and Goff [29], Terry et al. [30], Trenhaile [31], Watanabe et al. [32], and Weiss and Sheremet [33]. Most probably, the devastating catastrophes like the 2004 Indian Ocean tsunami [34] and the 2011 Tohoku tsunami [35] have fueled the interest of researchers in coastal sedimentology and, particularly, large clasts [36]. Evidently, investigations of the two noted issues often interconnect.

A significant amount of information about coastal boulders has accumulated, and it appears to be highly important to systematize it for further critical analysis and conceptualization. Such an approach is very common in social sciences [37–39], although geoscientists, unfortunately, often underestimate its potential. Marine sedimentologists need a simple guide permitting orientation in the growing research direction. The objective of the present paper is to offer an overview of the literature on Neogene coastal boulder deposits with some inferences on the current state of research. The importance of such a bibliographical analysis in megaclast research was demonstrated earlier [40]. The peculiarities of this paper are triplicate. First, it presents a synopsis summarizing the already-published data. Second, it focuses on the principal literature sources on the noted subjects, which means articles in international journals accessible via major bibliographical databases and considering large clasts in their title, abstract, and/or keywords. Third, this paper deals with the only Neogene Period, the sedimentary record of which is significantly more representative than that of the earlier periods, but differs from the Quaternary coastal deposits.

#### **2. Conceptual Basis**

#### *2.1. Terminology*

The focus of the present paper requires clear definition of several terms, from which two principal terms are "boulder" and "coastal boulder deposit". Evidently, the former indicates a sedimentary particle (clast, grain), and the latter indicates a specific sediment type consisting of (dominated by) such particles.

In the "classical" geological literature, the term "boulder" refers to particles larger than 256 mm (e.g., according to the widely used Udden–Wentworth classification scheme) [3]. However, intensification of studies of large clasts occurring in storm- and tsunami-related deposits raised the question of a more detailed nomenclature of such sedimentary particles. In 1999, Blair and McPherson [1] proposed a nomenclature of large clasts and limited the upper size of boulders to ~4 m (larger clasts are blocks). Different approaches were proposed later [2–4]. Of special interest is the distinction between boulders, mesoboulders, and macroboulders attributing to different categories [4]. Boulders are also opposed to megaclasts (Figure 1). Up to now, there is not broad, international agreement of how large clasts should be termed. As a result of this, it is not a mistake to use the very general term "boulder" for all sedimentary particles larger than 256 mm, except for only some specific studies focusing on the nomenclature development or devoted to a very particular size category of large clasts. In the present paper, this term is considered in such a broad way, and its partial substitutes (like megaclasts) are also taken into account.

**Figure 1.** Different definition of boulders (see text for references).

The term "coastal boulder deposit" has been used in the works of several authors, although it still requires proper definition. Consideration of the context of its usage in the journal articles permits outlining some characteristics of such a sedimentary formation. These include, particularly, accumulation of boulders distinguished by their large size and/or huge weight [9,10,17,18,25–27], angularity with certain roundness [8], high-topography and inland occurrence [8,9,25,27], high-energy coastlines [5,8,15,17], and relevance to storm and tsunami activity [9,10,18,26,27]. It is notable that the previous works focused more on boulders individually rather than on entire deposits. Even the superficial analysis of the available literature implies that one should distinguish coastal boulder-dominated deposits, i.e., deposits consisting chiefly of boulders (say with their amount of >50%), from coastal boulder-bearing deposits, i.e., deposits dominated by sedimentary particles of lesser size (sand, gravel, cobble, etc.) and bearing a small number or individual boulders. Interestingly, such individual boulders, if too large in size, may look sediment-dominating. The problem seems to be even more complicated in the case of ancient deposits. Large clasts themselves are subject to erosion in the coastal zone with active hydrodynamics, and, thus, large clasts tend to disappear quickly from the geological record. As a result, an ancient coastal boulder-bearing deposit may be legacy of the really existed boulder-dominated deposit. Until these problems are resolved and the nomenclature of large-clast deposits is fixed, it is possible to apply the general term "coastal boulder deposit" broadly, but preferably in those cases when boulders tend to concentrate.

Dewey and Ryan [41] introduced the term "boulderite". Evidently, this can be applied to boulder-dominated deposits. Importantly, the both modern and ancient deposits of this type are called as boulderites [41]. It is the right of the noted authors to use it so, although one may question whether the term "boulderite" can be used for only ancient boulder-dominated deposits, i.e., sedimentary rocks, not recent sediments.

### *2.2. Stratigraphical Framework*

The Neogene Period lasted ~20.5 Ma, and it is subdivided into the Miocene and Pliocene Epochs. After strong disputes in the 2000s when the Neogene was extended to the Holocene, a "classical" (almost) scheme has been fixed by the International Commission on Stratigraphy (Table 1), although it is not excluded that the formal definition of the Anthropocene would result in reorganization of the Late Cenozoic stratigraphical nomenclature with subsequent changes in the extent of the Neogene.


**Table 1.** Current version of the Neogene time scale (after International Commission on Stratigraphy [42]).

What is necessary to note is the significant disproportion of the Neogene subdivision: the Miocene constitutes ~87% of the period length, and, thus, the Pliocene seems to be too short. This fact should be taken into account when the temporal distribution of any class of geological objects like coastal boulder deposits is analyzed by epochs.

#### **3. Bibliographical Synopsis**

#### *3.1. Research Foci*

Although coastal boulder deposits are mentioned in the modern geoscience literature not so rarely, the majority of works deal with the recent and Quaternary boulders. The knowledge of the Neogene sediments of this type remains very restricted. The number of the principal sources does not exceed two dozen. Most probably, this reflects the both low preservation potential of large clasts that themselves are subject of erosion and destruction starting immediately after their deposition and the absence of well-known and broadly accepted techniques for their investigations in the geological record. Nonetheless, the research in the Neogene coastal boulder deposits intensified in the 2010s when up to a half of these principal works were published (Table 2).

**Table 2.** General information about the localities considered in the main articles on Neogene coastal boulder deposits (see also Tables 3–5 for terms, ages, and depositional environments).


Interestingly, different researchers use different terminology (Table 3). The majority informs about boulders. In only one case megaclasts are mentioned. Coastal boulder deposits are indicated in five works, although in none of them the term "coastal boulder deposit" is used. These deposits are recognized as boulder beach, boulder conglomerate, or boulderite. Boulder-bearing conglomerate and breccia are also considered, but these should be distinguished from boulder-dominated deposits (see terminological notes above). Finally, a few works employ two or even three terms simultaneously.


**Table 3.** Coastal boulder-related terminology in the main articles on Neogene coastal boulder deposits.

The majority of the works are case studies focusing on a given location and given stratigraphical intervals. Only two papers of general kind (conceptual) are found (Table 4). The first is the synthetic work of Johnson [52] who overviewed the knowledge of rocky shorelines where boulders often accumulate and the relevant palaeoecosystems. Particularly, he noted that the Neogene deposits of this facies are often linked to ramps, in contrast to the dominance of terrace deposits in the Pleistocene. The second paper of this kind can be judged conceptual only provisionally because this is dealing with the comparison of the examples of the modern and Neogene coastal boulder deposits with a discussion of their storm versus tsunami origin [41]. Importantly, this paper [41] employs the term "boulderite" as equivalent to "boulder-dominated deposit". The other works explore some particular aspects of Neogene coastal boulder deposits, including their relevance to extreme events such as storms and tsunamis, as well as palaeoecological issues.


**Table 4.** Stratigraphical and geographical foci of the main articles on Neogene coastal boulder deposits.

It is possible to classify all principal sources on the basis of their stratigraphical, geographical, and genetic foci (Tables 4 and 5). The main observations are as follows. First, Miocene and Pliocene coastal boulder deposits have been generally considered with attention (Table 4). Second, the relevant studies tend to represent different parts of the world (Table 4). Third, the diversity of the discussed mechanisms leading to boulder production, transportation, and accumulation in coastal zone is moderate if not low (Table 5).


**Table 5.** Genetic focus of the main articles on Neogene coastal boulder deposits.

#### *3.2. Further Inferences*

The Miocene coastal boulder deposits are considered in 57% of the analyzed works, and those Pliocene are considered in 52% of the works (two articles deal with the both epochs). Apparently, this means equal attention to the both epochs. However, it is necessary to take into account that the Miocene is by ~6.5 times longer than the Pliocene (Table 1). In regard to this fact, it is possible to conclude about significant overemphasis on the Pliocene coastal boulders. Although it cannot be excluded that such a disproportion results from occasional bias in the international research, it can be also hypothesized that the Pliocene environment was more favorable for production and accumulation of boulders in coastal zones of seas and oceans. The evidence of a potentially greater number of hurricanes under the conditions of the Pliocene warming [63–65] makes this hypothesis meaningful. For coastal zone dynamics, sea-level fluctuations seem to be important control of boulder production. Rising sea level accelerates abrasion (especially of sea cliffs) and also leads to growth of shoreline length. For instance, boulders are reported from some areas that were embraced by the sea in the Neogene, but are located inland nowadays, as in the case of the Sorbas Basin in Spain [57]. The global sea level was rather high in the Miocene, but it experienced significant fluctuations that intensified in the Pliocene [66–71]. On the one hand, the relevant instability of the coastal zones could contribute to more boulder formation. On the other hand, the same instability could trigger boulder motion and destruction by waves.

The geographical distribution of the reported Neogene coastal boulder deposits is broad (Figure 2). Despite the rarity of the described locations, the latter occur in all parts of the world (except for Antarctica). It is notable that these deposits have been described chiefly in the same regions where the modern rocky coasts with boulders exist. This is not surprising because of the absence of too striking differences in the position of continents and oceans between the Neogene and the Recent. However, another, complex explanation can be proposed. Sedimentologists and geomorphologists specialized in the studies of coastal boulder deposits often deal with the modern objects. If so, it is evident that they are able to detect ancient deposits of this kind in the same geographical loci. Nonetheless, it is evident that the knowledge of Quaternary coastal boulders is much wider. For instance, these have been reported from many localities of the Mediterranean, including (but not limited to) Istria [7], Sicily [24], northern Egypt [27], Malta [72], Ibiza [73], Crete [74], Lesvos [75], southern France [76], and Apulia [77]. Better to say, boulders and their accumulations are found on the majority of coasts of the Mediterranean Sea. In contrast, Neogene large clasts are reported from very few localities of the same basin. Most probably, this reflects the both sedimentological research bias and low preservation potential of boulders.

An interesting inference is linked to the origin of the Neogene coastal boulder deposits. Many previous studies focused on their relevance to storms and tsunamis as the leading boulder production, transportation, and accumulation forces, as well as on gravity processes linked to downslope movement with consequent cliff retreat (Figure 3). The main depositional environments analyzed in the course of the coastal boulder research are rocky shores, deltas, and areas of volcanism (Figure 3). On the one hand, it is clear that chiefly extreme events like storms, tsunamis, and volcanic eruptions are able to provide the energy necessary to produce and to move large clasts. On the other hand, it seems to be questionable if some other forces were responsible. For instance, seismicity would cause giant cliff collapse or heterogeneity of exposed substrate would lead to its differential erosion. Finally, what about the possible role of wind erosion in coastal zones? Undoubtedly, identification and correct interpretation of such phenomena even in geological records as young as that of the Neogene is highly challenging and requires very creative analysis. Examples of the latter can be found in the works deciphering the origin of boulders from the Miocene upper bathyal deposits of the Chita Peninsula (Japan) [58,59] ensures the possibility of such state-of-the-art investigations. Anyway, coastal boulders, especially those measured by meters are highly specific and uncommon geological objects, and their analysis should be undertaken in regard to individual peculiarities of each given locality.

**Figure 2.** Geographical focus of the studies of Neogene coastal boulder deposits (based on Table 4). See Table 2 for locality IDs.

**Figure 3.** Genetic focus of the studies of Neogene coastal boulder deposits (based on Table 5).

#### **4. Conclusions**

The bibliographical synopsis of the knowledge of Neogene coastal boulder deposits implies that the relevant research has been weak. Nonetheless, this research has generated significant evidence of these deposits. The main findings of the present analysis are as follows.

(1) Case studies of the Neogene coastal boulder deposits prevail over conceptual works.

(2) Attention has been paid to the both epochs of the Neogene (although with overemphasis on the Pliocene), to many parts of the world, and to the really principal mechanisms of boulder production, transportation, and accumulation (first of all, to extreme events).

(3) The stratigraphical, geographical, and genetic foci of the research demonstrate certain biases that can be explained, particularly, by peculiarities of the geological record.

Generally, this means that although the Neogene coastal boulder deposits are highly specific and rather uncommon geological objects, the latter have been studied more or less adequately to make further interpretations of their relevance to the dynamics of the Neogene world.

**Funding:** This research received no external funding.

**Acknowledgments:** The author gratefully thanks M.E. Johnson (USE) for his kind invitation to contribute to this special issue and various support, as well as the reviewers for their helpful suggestions.

**Conflicts of Interest:** The author declares no conflict of interest.
