**1. Introduction**

Lithodynamic processes at the land–sea interface result in different morphologies of the coastal zone, dominated by elevated cliff sections or coastal dunes. These forms may reflect the history of development of the coastal zone, where swash hydrodynamics steer the intensity of erosion and accumulation processes [1,2]. The spatial and temporal changes with the coastal sediment budget are usually related to increased cliff and beach erosion and redeposition of sedimentary material [3]. Undercut dunes or cliffs, with visible signs of mass movement, point to domination of erosion and

marine ingression, whereas the development of dune ridges on sandy spits, as well as dunes at the forefront of temporarily inactive cliff shores, is indicative of equilibrium or even accumulation occurring in the coastal zone [4–6]. However, the swash-dominated gravel beaches are scarcely mentioned in sediment transport studies, or related to morpholithodynamics [2,7–11].

Gravel beaches are accumulations of shore material formed into distinctive shapes by waves and currents, and contain lithic particles in the gravel size range (2–64 mm). The beach form is a generally seaward-sloping boundary between a water body and mobile sediment, and a flat or landward-sloping surface at the upper limit of the beach. One or more gravel ridges may exist in the subaerial profile. Natural beaches composed of coarse material are common especially in the high latitudes, where coastal sediments indicate glacial transport history and reworking processes. Gravelly beaches also occur in other geographical latitudes, being associated with eroded cliff sections in the backshore and formation of a nearshore sedimentary bench of glacial or postglacial origin [12].

Mixed sand and gravel beaches are often partially composed of coarse material (>2 mm), with supplementary fine fractions, which are the main products of fluvial, glacial, or erosive processes. Gravel beaches are especially important for coastal dynamics, with protection provided by a natural system of shoreline stabilization where its response to sea-level forcing is distinctive and can be preserved, even in the long-term geological record [13].

Distinctive sedimentary features of mixed sand and gravel beaches include (i) a greater amount of coarse sand fraction and relatively high content of fine gravel fraction; (ii) the existence of multimodality within the gravel fraction; (iii) very limited shape sorting; and (iv) high spatial and temporal variability. The beach profile responses occur over semidiurnal, spring-neap, and seasonal time scales [14,15]. The coarse–fine grain sorting processes are usually related to longshore movement of sedimentary material in the swash zone, especially during storms [2,16–19].

Saltation, traction bedload, and sheet flow dominate the nearshore of gravel beaches. The individual clast motion of coarse grains is dictated by a number of micromechanical factors attributable to size and shape variation occurring over a heterogeneous bed. Transport mode affects sediment sorting and morphodynamic feedbacks. Gravels are relatively large, compared to the sandy fraction, so they account for a greater proportion of the sediment volume in the swash zone. Therefore, sheet flow is likely to be most important in gravel beach dynamics, especially when developed as a fluid-thin backwash [2,20].

The different cross-shore size–shape zonation of gravel pebbles, compared to sand, exists on gravel beaches and was postulated by numerous authors, e.g., [21–23]. The discoidal and spheroidal particles usually show a tendency to be transported upslope, acting like a hydrodynamic "wing". The spherical and spindle gravels are usually transported downslope [22,24–26]. Additionally, the flattened particles are prone to longshore sediment transport. However, in some cases it is not clear whether sorting by size and shape, are achieved by these two mechanisms, or what aspects of anisotropy are important (e.g., "shape" or sphericity, aspect ratio, and elongation, and the axially less dominant third dimension, or *c*-axis, which may produce different responses to flow, either individually or as part of mixed beds) [27,28]. The longshore transport of particles cause colliding and rubbing, which may produce different shapes of pebbles that may be used as an indicator of erosion processes [29]. Irregular particles generally indicate a high potential for mechanical reworking, whereas rounded particles are a direct manifestation of the erosion that has occurred. These processes are partially related to lithological variability and textural features of different types of rocks. Soft rocks (limestones, marbles, shales or sandstones) are definitely less resistant, compared to magmatic or metamorphic components of gravel beaches [30]. Coarse sediments may be directly related to changes in lithodynamics, sediment redistribution pathways, or intense material transport. Heavy gravels are less sensitive to wave and wind dynamics, and are applicable for estimating relative changes in sediment transport rates.

Lithological responses to erosion are established in particle size, roundness, sphericity, flatness, and asymmetry. Pebble roundness, flatness, and rate of mechanical destruction depend on wave intensity, initial rock types, and nature of the coast (sheltered or exposed). The intensity of erosion is

established by particle morphology, mainly their roundness and flatness. Thus, a greater intensity of erosion is indicated by larger roundness and flatness indices [29,30]. The variability of gravel pebbles may show a different transport pattern under low energy conditions. Spherical grains, resulting from their capability to roll over in the swash zone, usually cover longer distances and indicate more dynamic conditions, when compared to discoidal grains [19]. Compared to sandy beaches, gravel beach sediments are usually spatially differentiated in terms of size and shape to a greater degree, affecting more obvious textural zonation, mainly in the form of mixtures of relatively fine and coarse fractions [21,24]. The step, cusp horns, strands, and berms are composed of larger grains compared with the foreshore, although a number of levels of textural zonation may be discernible as sediments are continuously redistributed [2,24].

Studies on geodynamic processes and lithodynamics of the coastal zone are essential in different branches of the economy (e.g., fisheries, tourism) and in maritime spatial planning [24,31–34]. Increased coastal erosion resulting from a sediment deficit, related to a reduction of river input, dredging processes, and coastal embankments, affects the formation of narrow beaches composed of relatively coarse sedimentary components. Gravel beaches usually form due to erosion of indurated cliff coasts, built mainly of moraine tills with small amounts of loose sediments. Increased volumes of beach rocks exposed due to the coastal erosion can effectively protect sensitive parts of the seacoast; however, during high sea level stands caused by wave storms, water can flow through gaps in coarse material, thereby increasing water saturation and slope instability [3,35].

Recently, erosion has been dominant along nearly the full length of southern Baltic coast. Erosion rates are highly variable and accompanied by simultaneous accumulation processes, either generating or rebuilding accumulative forms. The spatial and temporal distribution of these forms has varied significantly during the last 100 years, related mainly to increases in mean sea level and anthropogenic activity, which affected the intensity and varied the redistribution patterns of sedimentary material. The present rapid rate of dune and cliff erosion is an indirect proof of insufficiency of these sediment resources [36].

Frequent wave storms are the main cause of coast erosion in the southern Baltic. The foreshore in the study area is shallow, with the 10 m isobath located more than one kilometer from the shoreline. The source of redeposited sedimentary material is related to eroded sections of cliff and dune coast; however, the geologic framework of the shallow foreshore also contributes to the total balance of pebble load. During the elevated sea level stands and wave storms, thin layers of sands and gravels developed on moraine tills are mixed, crushed, and redistributed towards the shoreline, and incorporated with the sands and gravels eroded from the cliff sections [1,37].

The Baltic nearshore in the study area is frequently covered by Holocene sands constituting the in-shore bedload [38–40]. The Pleistocene series is lithologically more variable compared to the Holocene sands, and composed of moraine tills, glaciofluvial sands, muds, clays, and organic sediments (mainly peats). Sediments are often laminated and lie in the form of beds or lenses. In some cases, water-bearing horizons are present and promote mass displacements or cliff degradation. Residuals of beach pebbles reflect regional differentiation of glacial tills from which they originate [38]. For example, a greater content of limestone among the other petrographic components of beach gravels located east of Pogorzelica indicates erosion of the bench and morainic material redeposition towards the shore.

The article characterizes the changes in petrographic composition and grain shape variability of beach gravels sampled in the storm berm (or beach face) along the almost 30 km-long shore of the Baltic Sea, extending from Pogorzelica, Poland, (coastline kilometer363.0) and ending at the River Dziwna mouth (391.4 km; Figure 1). Grain size parameters of beach gravels were estimated by application of the traditional Zingg method [41].

**Figure 1.** The Western Pomerania coastline of the Baltic Sea, Poland, which is characterized by presence of Pleistocene–Holocene glacial and fluvial deposits, shallow nearshore sediments, and divergence of sedimentary transport [37,42,43].

The major aims of the study are to determine the petrographic composition of beach gravels, compare these with similar initial data for gravels collected from cliff sections, map coast sections affected by erosion and accumulation processes, and then link the results to morphodynamics estimated by applying nongeological methods, accounting for the presence of coastal protection structures. This is also the first detailed study of petrographic composition and measurements of Zingg grain shape parameters of beach gravels conducted along the coastline of Western Pomerania, Baltic Sea, Poland. By assessing of developmental trends of the coast, the petrographic composition and morphometric characteristics of gravels may indicate changes in erosion intensity, and contribute complementary information indicating longshore bedload transport [42,44].
