*3.1. Example 1: Hondarribia (Spain; Biscay Bay)*

The locality is situated in the vicinities of the town of Hondarribia in northern Spain (Figure 1), where the rocky shore of the Biscay Bay hosts a lot of megaclasts that form significant accumulations. Their origin is linked to the destruction of the Paleogene marine rocks exposed in cliffs and directly along the coastline [44]. Apparently, the main factors of megaclast accumulation are gravity processes and wave abrasion.

On the eastern plot, the coastal megaclast deposits were represented by the dense accumulation of angular blocks with the size of 1–5 m; the maximum size reached 7–8 m (Figure 2). Importantly, the deposits were spatially heterogeneous—the mean size of blocks increased locally (for instance, near the shoreline and close to parent rock exposures). Although the deposits covered almost the entire area, some 'islands' of parent rock exposures and semi-detached megaclasts were also visible. On the western plot, angular megaclasts were larger in size: these include blocks and megablocks; the size of the largest particle exceeded 25 m (Figure 3). Megablocks were more numerous in the right half of the plot. Parent rock exposures and semi-detached megaclasts occurred in the left half. On both plots, clasts >1 m in size were distinguished unequivocally, and the evidence was enough to confirm the presence of true coastal megaclast deposits. The resolution of the satellite view permitted us to distinguish blocks from megablocks; the presence of boulders was evident, although these could not be registered individually. Dense package of the deposits was well visible. Size and shape of individual blocks can be registered when their size exceeds 1–2 m. The spatial position and orientation of stones did not leave an impression of significant re-working, i.e., primary, undisturbed coastal megaclast deposits could be hypothesized.

**Figure 2.** The eastern plot of the Hondarribia locality (the view provided by the Google Earth Engine). CMD labels coastal megaclast deposits (the same is on the following figures).

**Figure 3.** The western plot of the Hondarribia locality (the view provided by the Google Earth Engine).

The information from this locality fills an important gap in the knowledge of the geographical distribution of coastal megaclast deposits along the Atlantic coast of Europe. These deposits were reported previously from the British Isles [21,27–35,45–51], northern France (first of all, Brittany) [52–56], and the western and southwestern coasts of the Iberian Peninsula [57–61]. The Hondarribia locality represents the other domain, i.e., the Biscay Bay, the coasts of which are prone to severe storms creating and transporting megaclasts, similarly to the above-mentioned regions [62]. This may be the first megaclast locality reported from the Biscay Bay coast.

#### *3.2. Example 2: Ponza Island (Italy; Tyrrhenian Sea)*

The locality corresponds to the Ponza Island (Pontine Archipelago) that is situated in the Tyrrhenian Sea, near the western coast of central Italy (Figure 1). The Pliocene volcanic and volcaniclastic rocks are exposed in tall cliffs [63–65], the collapse of which leads to the formation and accumulation of numerous megaclasts. As rocky shores with cliffs dominate the island, the distribution of coastal megaclast deposits is significant there. Gravity processes and abrasion contribute to local megaclast formation.

On the southern plot, the coastal megaclast deposits were represented by the accumulation of angular blocks with the common size of 1–5 m (Figure 4). Blocks of a bigger size and rare small megablocks (up to 12–13 m in size) occurred along the very shoreline, as well as in the northern part of the plot. Density of the deposit differed: it was significant near the shoreline, but megaclasts occurred individually or in small groups in the other places where they were either mixed with boulders or lay directly on the rock surface. On the northwestern plot, a small group of fully- and partially-detached megaclasts was observed (Figure 5). These include several megablocks with the size up to 40 m and numerous blocks in between. These megaclasts were angular, and some smoothed surfaces result from wave and wind 'polishing'. On both plots, clasts >1 m in size were distinguished easily, and the delineation of true coastal megaclast deposits was easy. The resolution of satellite images permitted us to distinguish blocks from megablocks, the presence of boulders was evident, although these could not be registered individually. Different package density was visible. Size and shape of individual particles can be registered when their size exceeded 1–2 m. However, a shadow from the tall cliff on the northwestern plot 'masked' some megaclasts and did not permit us to characterize them on an individual basis (Figure 5). Some redeposition of megaclasts could not be excluded for the southern plot, but the view of the deposits on the northwestern plot leaves impression of a 'fresh' and then undisturbed rockfall.

**Figure 4.** The southern plot of the Ponza Island locality (the view provided by the Google Earth Engine).

**Figure 5.** The northwestern plot of the Ponza Island locality (the view provided by the Google Earth Engine).

The Mediterranean is a 'classical' region for studies of coastal megaclasts and boulder deposits [9–13]. Particularly, megaclast accumulations have been reported from the Western Mediterranean continental and island coasts [66–68]. However, Ponza Island is of special importance because of two reasons. First, it represents lenticular, almost round-island distribution of the coastal megaclast deposits. Second, the latter originates from volcanic and volcaniclastic rocks. Therefore, finding this locality with the Google Earth Engine extends the regional knowledge of coastal megaclast deposits.
