*3.1. Data Collection*

Original data were collected for both modern and MIS 5e coastal boulder deposits at two localities (Prainha and Ponta do Castelo, Figure 1b) and only MIS 5e from one locality (Ponta do Cedro, Figure 1b). Measurements from modern deposits were performed at mean sea level, while Pleistocene deposit heights varied from ~2.5 m and as much as ~9.0 m above mean sea level (amsl) at five localities, all on the southern and eastern shores of Santa Maria Island. The definition for a boulder adapted to this exercise is that of Wentworth [35] for an erosional clast equal or greater than 256 mm and less than 4096 mm in diameter, with cobbles defined as clasts pertaining to the class 64-256 mm in maximum diameter. For each data set, 25 of the largest available clasts were selected along a transect line parallel to the shore spaced no more than one meter apart. Each clast required three measurements along principle axes (long *a*, intermediate *b*, and short *c*). The initial calculation for volume was simply a multiplication of the long, intermediate, and short-axis values. In all cases, this resulted in a cubical to a rectangular shape that did not take into account the rounding of the boulders by erosion; therefore, a final adjustment to 75% was made, regarding the volume estimates for each boulder, in accordance with previous works [18,19]. Triangular plots were employed to demonstrate variations in boulder shape, following the practice of Sneed and Folk for river pebbles [36]. Data regarding the maximum and intermediate lengths perpendicular to one another from individual boulders were plotted in bar

graphs to show potential shifts in size from one transect to the next. A brief description of the fossil fauna in each site was also provided. This was important information, as, together with the geological data, the presence of the characteristic MIS 5e thermophilic taxa [4] attested to the age of the deposit, which was not possible to date due to the absence of suitable biogenic material (e.g., corals).

#### *3.2. Hydraulic Model*

With the determination of specific gravity based on the standard density value for oceanic basalt at 3.0 g/cm3, a hydraulic model might be applied to predict the energy needed to transfer larger basalt blocks from rocky shoreline to an adjacent coastal boulder deposit as a function of wave impact. Basalt is a volcanic rock that forms from surface flows with variable thicknesses and a propensity to develop vertical fractures. These factors regulate the size and general shape of blocks loosened in the cliff face [6]. Herein, we used two different formulas to estimate the magnitude of storm waves applied to joint-bounded boulders derived, respectively, from Equation (36) in the work of Nott [37] (Equation (1)) and from a recent formula of Pepe et al. [38] that used the velocity equations of Nandasena et al. [39] to estimate wave heights (Equation (2)):

$$H\_S = \frac{\left(\frac{\rho\_s - \rho\_w}{\rho\_w}\right)a}{\mathcal{C}\_l} \tag{1}$$

$$H\_S = \frac{\left(\frac{\left(\frac{\rho\_s - \rho\_W}{\rho\_l}\right)}{\zeta\_l}\right) \cdot \varepsilon \left(\frac{\cos \theta + \mu\_s \times \sin \theta}{\zeta\_l}\right)}{100} \tag{2}$$

where *Hs* is the height of the storm wave at breaking point; ρ*<sup>s</sup>* is the density of the boulder (3 tons/m<sup>3</sup> or 3 g/cm3); ρ*<sup>w</sup>* is the density of water at 1.02 g/cm3; *a* is the length of the boulder on long axis in cm; *c* is the length of the boulder on short axis in cm; θ is the angle of the bed slope at the pre-transport location (1◦ for joint-bounded boulders); μ*<sup>s</sup>* is the coefficient of static friction (=0.7); *C*<sup>l</sup> is the lift coefficient (=0.178). Equation (1) was more sensitive to the length of a boulder at the long axis, whereas Equation (2) was more sensitive to the length of a boulder on the short axis. Therefore, some differences were expected in the estimates of *HS*.

#### **4. Results**

#### *4.1. Prainha on the South Shore*

Located 3.5 km east of the harbor at Vila do Porto (Figure 1b), Prainha's area presented well-developed modern CBD, at heights of ~0.5 m amsl (Figure 2), as well as exposed Pleistocene marine sequences, deposited on top of volcaniclastic rocks at an elevation of ~3 m amsl [40] (Figure 3). Although CBDs were present all-year-round at Prainha, the location of this site (south coast of the island) combined with the fact that Prainha was found within a bay, and taking into consideration the wave regime, it resulted that, in some cases, the modern CBD was covered by sand. Raw data on clast size in three dimensions collected from the two parallel transects at this locality are present in Tables 1 and 2.

Data points representing individual boulders grouped by transect were plotted on a set of Sneed-Folk triangular diagrams (Figure 4a,b), showing shape variations. Those points clustered nearest to the core of the diagrams were most faithful to an average value with somewhat equidimensional axes in three directions. Only rarely any points fell into the upper-most triangle, which signified a cube-shaped endpoint.

The majority of points from both sets fell within the central part of the two tiers beneath the top triangle. However, the overall trend shared between the two sets traced a similar pattern angled toward the lower right corner of the diagrams. The modern CBD at Prainha (Figures 2 and 4a) demonstrated a greater tendency to elongate shapes represented by the endpoint for bar-shaped clasts. No points appeared in the lower-left tier of either diagram, which represented an endpoint reserved

for plate-shaped clasts. Although the general trend in slope was similar between the modern and Pleistocene CBDs, the plots had no bearing on actual variations in clast size.

Variations in boulder size as a function of maximum and intermediate axis length were plotted using bar graphs (Figure 5), based on raw data drawn from Table 1. The greatest number of boulders in the sample measured from the modern CBD fell within a maximum diameter size range between 26 and 35 cm (Figure 5a), which qualified as small boulders in the Wentworth scheme [35]. The largest boulders encountered at Prainha were few in number but ranged in size between 46 and 55 cm. The tendency towards an elongated shape among these clasts was shown by a marked shift in the dominant bin-size for the intermediate axis, within an interval of 16 to 25 cm (Figure 5b).

**Figure 2.** Modern coastal boulder deposit (CBD) eroded from adjacent basalt sea cliffs at Prainha (site 1). Note that some boulders on the far inland end of the littoral boulder cordon were embedded within the sand.

**Figure 3.** Pleistocene (MIS 5e, Marine Isotope Substage 5e) marine sequence, 2.5 m above the modern CBD at Prainha (site 2). Note that, as modern CBD, Pleistocene boulders were covered by Pleistocene fossiliferous sands.


**Table 1.** Quantification of boulder size, volume, and estimated weight from the modern coastal boulder deposit at site 1 at Prainha, on the southern coast of Santa Maria Island (see Figure 1b). The standard density of basalt at 3.0 gm/cm<sup>3</sup> was applied uniformly in order to calculate wave height for each boulder. EWH: estimated wave height (in meters), calculated according to equations from Nott [37] and Pepe et al. [38]. See the methods Section 3.2. (hydraulic model).

**Table 2.** Quantification of boulder size, volume, and estimated weight from the Pleistocene (MIS 5e, Marine Isotope Substage 5e) coastal conglomerate at site 2 at Prainha, on the southern coast of Santa Maria Island (see Figure 1b). The standard density of basalt at 3.0 gm/cm3 was applied uniformly in order to calculate wave height for each boulder. EWH: estimated wave height (in meters), calculated according to equations from Nott [37] and Pepe et al. [38]. See the methods Section 3.2. (hydraulic model).


**Figure 4.** Set of triangular Sneed-Folk diagrams used to show variations in boulder and cobble shapes compared between modern and Pleistocene (MIS 5e) deposits. (**a**,**b**): Prainha. (**c**,**d**): Ponta do Castelo. (**e**–**g**): Ponta do Cedro. (**a**,**c**): modern CBD at Prainha and Ponta do Castelo, respectively. (**b**,**d**), (**e**–**g**): Pleistocene (MIS 5e) CBD at Prainha, Ponta do Castelo, and Ponta do Cedro, respectively.

In strong contrast, all clasts encountered from the Pleistocene (MIS 5e) conglomerate fell into the category of cobbles as defined in the Wentworth scheme [35]. By far, the largest number of clasts within the sample fell into the size range between 6 and 15 cm in maximum diameter (Figure 5c), also replicated by the same frequency for the intermediate axis (Figure 5d). However, a subsample of

smaller clasts in the size range of pebbles to small cobbles appeared in the sample measured for the intermediate axis. Comparing the modern CBD at Prainha (Figure 2) with the general contents of the Pleistocene conglomerate at the same locality, it was clear that wave conditions on the modern shore eroded significantly larger clasts from the parent basalt rocky shore.

**Figure 5.** Bar graphs used to appraise variations in the long and intermediate axes on basalt clasts from modern and Pleistocene CBDs at Prainha; (**a**) Long-axis length from clasts in the modern CBD; (**b**) Intermediate-axis length from clasts in the modern CBD; (**c**) Long-axis length from clasts in the Pleistocene CBD; (**d**) Intermediate-axis length from clasts in the Pleistocene CBD.

### *4.2. Context of the Pleistocene Fauna at Prainha*

Prainha is one of the six Pleistocene (MIS 5e) fossiliferous outcrops described for Santa Maria Island. MIS 5e CBDs are known from Prainha, Ponta do Castelo, Ponta do Cedro, and Pedra-que-pica, but do not occur at Lagoinhas nor at Vinha Velha.

Prainha is the best-studied outcrop from Santa Maria Island, and its fossils being first reported by Portuguese geologists [41,42]. After these pioneer works, Santa Maria outcrops' and their fossiliferous remains have been systematically described, with a total of 148 fossil marine-specific taxa presently reported from the Santa Maria Last Interglacial deposits [4]. The most biodiverse invertebrate group is, by far, the marine mollusks, with a total of 138 taxa (114 Gastropoda and 24 Bivalvia) [41,43–45] reported for all MIS 5e deposits in the island (of these, 100 gastropods and 20 bivalve taxa are reported from Prainha). It is followed by the Echinodermata (three taxa) and Cnidaria Anthozoa (one coral taxa) [40,46,47]. Four species of coralline red algae have also been reported from the MIS 5e of Prainha [48]. Finally, rare vertebrate remains have been described from the MIS 5e sedimentary sequences in Santa Maria: one bony fish species, *Sparisoma cretense* (Linnaeus, 1758) reported from the MIS 5e of Vinha Velha [49], and one undetermined Balaenopteridae species (a baleen whale) reported

from the MIS 5e of Prainha [50]. As a result of its high palaeobiodiversity, a high number of scientific studies, education and touristic potential, and the presence of extremely rare vertebrate cetacean remains, Prainha is considered as a high-relevance, national geosite [15,16].
