*Article* **Late Pleistocene Boulder Slumps Eroded from a Basalt Shoreline at El Confital Beach on Gran Canaria (Canary Islands, Spain)**

**Inés Galindo 1, Markes E. Johnson 2,\*, Esther Martín-González 3, Carmen Romero 4, Juana Vegas 1, Carlos S. Melo 5,6,7,8, Sérgio P. Ávila 6,8,9 and Nieves Sánchez <sup>1</sup>**


**Abstract:** This study examines the role of North Atlantic storms degrading a Late Pleistocene rocky shoreline formed by basaltic rocks overlying hyaloclastite rocks on a small volcanic peninsula connected to Gran Canaria in the central region of the Canary Archipelago. A conglomerate dominated by large, ellipsoidal to angular boulders eroded from an adjacent basalt flow was canvassed at six stations distributed along 800 m of the modern shore at El Confital, on the outskirts of Las Palmas de Gran Canaria. A total of 166 individual basalt cobbles and boulders were systematically measured in three dimensions, providing the database for analyses of variations in clast shape and size. The goal of this study was to apply mathematical equations elaborated after Nott (2003) and subsequent refinements in order to estimate individual wave heights necessary to lift basalt blocks from the layered and joint-bound sea cliffs at El Confital. On average, wave heights in the order of 4.2 to 4.5 m are calculated as having impacted the Late Pleistocene rocky coastline at El Confital, although the largest boulders in excess of 2 m in diameter would have required larger waves for extraction. A review of the fossil marine biota associated with the boulder beds confirms a littoral to very shallow water setting correlated in time with Marine Isotope Stage 5e (Eemian Stage) approximately 125,000 years ago. The historical record of major storms in the regions of the Canary and Azorean islands indicates that events of hurricane strength were likely to have struck El Confital in earlier times. Due to its high scientific value, the outcrop area featured in this study is included in the Spanish Inventory of Geosites and must be properly protected and managed to ensure conservation against the impact of climate change foreseen in coming years.

**Keywords:** coastal storm deposits; storm surge; hydrodynamic equations; upper pleistocene; marine isotope substage 5e; North Atlantic Ocean

**Citation:** Galindo, I.s; Johnson, M.E; Martín-González, E.; Romero, C.; Vegas, J.; Melo, C.S; Ávila, S.P; Sánchez, N. Late Pleistocene Boulder Slumps Eroded from a Basalt Shoreline at El Confital Beach on Gran Canaria (Canary Islands, Spain). *J. Mar. Sci. Eng.* **2021**, *9*, 138. https:// doi.org/10.3390/jmse9020138

Received: 23 December 2020 Accepted: 19 January 2021 Published: 29 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **1. Introduction**

Evidence for the influence of hurricanes in the northeast Atlantic Ocean during the Late Pleistocene is based on analyses of storm beds preserved on Santa Maria Island in the Azores archipelago [1] and Sal Island in the Cabo Verde archipelago [2]. This line of research follows a growing interest in coastal geomorphology as related to the accumulation of mega-boulders attributed to superstorms or possible tsunami events [3]. Documentation of eroded mega-boulders from rocky shores in Mexico's Gulf of California during the subsequent Holocene [4–6] adds to our knowledge of the physical scope of deposits that can be directly related to historical storm patterns. The Canary Islands in Spain represent seven main islands and numerous islets and seamounts in the North Atlantic located over lithospheric fractures off the northwest coast of Africa, with radiometric dates that vary from 142 Ma to 0.2 Ma along a general east to west axis [7]. At the center of the archipelago, Gran Canaria Island has a volcanic history dominated by subaerial development dating back to 13.7 Ma with successive stages of construction and erosion of the island edifice [8].

The attraction of Gran Canaria for this study is based on a distinctive rocky paleoshore formed by a 3-m thick lava flow exposed laterally along El Confital beach, on the southwest side of La Isleta peninsula, located in the northeastern quarter of Gran Canaria Island. Subaerial flows around several craters preserved atop the Isleta volcanoes exhibit variable Pliocene to Quaternary ages [8], but the basalt shore at El Confital beach is dated to approximately 1 Ma [9,10]. Prior to this contribution, a paleontological study at El Confital beach described an older assemblage of Miocene fossils at one end and a more extensive assemblage of intertidal to shallow subtidal invertebrates confined to the last interglacial epoch attributed to the Eemian Stage, also correlated with Marine Isotope Substage 5e, approximately 125,000 years in age [11]. The goals of this subsequent study include a review of the marine fossil fauna with particular emphasis on species zonation, and with an added emphasis on the extent of encrustations by coralline red algae. Most importantly, this work features extensive analyses of boulders eroded from the paleoshore, among which the Upper Pleistocene biota is preserved. In particular, the physical analyses conducted for clast shape and size are integral to estimates of wave heights based on competing mathematical equations applied to a rocky shoreline composed of joint-bound basalt and hyaloclastite layers.

The range in estimated wave heights extrapolated from average boulder size, contrasted against maximum boulder size at multiple sample sites, suggests that a pattern of repetitious storms is implicated with the coastal erosion of La Isleta and more generally of Gran Canaria Island over an extended period of Late Pleistocene time. The viability of this inference is tested against the historical record of cyclonic disturbances in the eastern North Atlantic Ocean around the Azorean and Canary Islands. Increasing interest in the historical record of storms in this region is available mainly in Spanish-language reports, but also well summarized in the international literature [12]. This approach with reference to historic storms is predicated on a similar analysis of coastal storm beds from the Pleistocene of Santa Maria Island in the Azores [1] as well as coastal boulder beds from the Holocene of Mexico's Baja California [4–6].

#### **2. Geographical and Geological Setting**

Gran Canaria is the third largest and most centrally located island in the Canary Archipelago, situated 150 km off the northwest coast of Africa (Figure 1a,b). With an area of 1560 km2, the island exhibits a circular map outline with an outer perimeter of about 50 km showing a pattern of ravines radiating from the island center at an elevation of 1949 m above present sea level. La Isleta peninsula lies in the northeastern part of Gran Canaria Island, now linked to it by a sandy isthmus about 200 m in width and around 2 km long (Figure 1b,c). Gran Canaria island emerged during the Miocene and reflects a complex geological evolution including the formation and erosion of several stratovolcanoes. Pliocene-Quaternary mafic fissure eruptions were concentrated along the northeastern half of the island. Volcanic activity on La Isleta began during the early Pleistocene with the formation of submarine volcanoes more than 1 Ma ago [9,10]. At least two submarine Surtseyan eruptions separated by marine sedimentary rocks have been identified. The later subaerial phase includes several mafic fissure eruptions (Figure 1c). In El Confital Bay, located off the east-southeast part of La Isleta, erosion has dominated since the middle Pleistocene (>152 ka) and marine, aeolian, and colluvial deposits have been deposited overlying the volcanic sequence (Figure 1d). This area was intensively modified by quarry operation, military activities, and expansion of a shantytown. The clearing of the coastal zone after illegal settlement between 1960 and 1995 and its subsequent elimination by civil authorities, resulted in the area of the deposit encompassed by a larger part of the Confital platform being compromised.

**Figure 1.** Canary Archipelago and features of the centrally located Gran Canaria Island: (**a**) Canary Archipelago with inset showing the position of Gran Canaria; (**b**) central island of Gran Canaria with an arrow pointing to La Isleta on the northern periphery of the island; (**c**) geological map of the volcanic edifice that forms La Isleta with inset box marking El Confital beach (coordinates in UTM m, H28 R); (**d**) map of the kilometer-long beach at El Confital showing seven localities from which sedimentological and paleontological data were drawn for this study. Modified from the 1990 Geological Map by Balcells and Barrera at a scale of 1:25,000 [10].

El Confital bay has a U-shape open to the southwest and is bound on the northeast by an escarpment more than 100 m high. The beach has been developed over a littoral platform (Figure 1c,d) covering approximately 0.1 km<sup>2</sup> that consists of highly fractured hyaloclastite tuffs from the submarine stage and volcanoclastic deposits including marine fossils. Parallel to the beach, a basalt cliff line extends parallel to the coast for a broken distance of about a kilometer. Cliffs crop out at an elevation of 1 m in the central part of the area and rise to more than 10 m at opposite ends in the southeast and northwest. In these parts, tuffs are overlain by subaerial lava flows and the cliff reaches a height of more than 10 m.

#### **3. Materials and Methods**

#### *3.1. Data Collection*

Gran Canaria was visited in April 2018, when the organizing author was invited to appraise the beach and rocky shoreline at El Confital beach on the east side of Las Palmas de Gran Canaria. The original data for this study was collected in October 2020 from deposits dominated by basalt boulders consolidated within a limestone matrix. Individual basalt clasts from six stations were measured manually to the nearest half centimeter in three dimensions perpendicular to one another (long, intermediate, and short axes). Differentiated from cobbles, the base definition for a boulder adapted in this exercise is that of Wentworth [13] for an erosional clast equal to or greater than 256 mm in diameter. No upper limit for this category is defined in the geological literature [14].

Triangular plots are employed to show variations in clast shape, following the design of Sneed and Folk [15] for river pebbles. In the field, all measured clasts were characterized as sub-rounded and a smoothing factor of 20% was applied uniformity to adjust for the estimated volume first calculated by the simple multiplication of the lengths of the three axes. Comparative data on maximum cobble and boulder dimensions were fitted to bar graphs to show size variations in the long and short axes from one sample to the next. Comparative data on maximum cobble and boulder dimensions were fitted to bar graphs to show size variations in the long and intermediate axes from one sample to the next. The rock density of basalt from the Pleistocene sea cliffs on El Confital beach is based on laboratory analyses in an unpublished PhD thesis that yields a value of 2.84 g/cm3 [16].

#### *3.2. Hydraulic Model*

Dependent on the calculation of rock density for basalt, a hydraulic model may be applied to predict the force needed to remove cobbles and boulders from a rocky shoreline with joint-bound blocks as a function of wave impact. Basalt is the typical extrusive volcanic rock characteristic of many oceanic islands. Herein, two formulas are applied to estimate the size of storm waves against joint-bounded blocks derived, respectively from Equation (36) in the work of Nott [17] and from an alternative formula using the velocity equations of Nandasena et al. [18], as applied by Pepe et al. [19].

$$Hs = \frac{\left(\frac{\rho\_s - \rho\_w}{\rho\_w}\right)a}{\mathcal{C}\_1} \tag{1}$$

$$\mu^2 = \frac{2\left(\frac{\rho\_s - \rho\_w}{\rho\_w}\right) \lg \left(\cos \theta + \mu\_s \sin \theta\right)}{\mathcal{C}\_1} \tag{2}$$

where *Hs* = height of the storm wave at breaking point; *ρ<sup>s</sup>* = density of the boulder (tons/m3 or g/cm3); *ρ<sup>w</sup>* = density of water at 1.02 g/cm3; a = length of the boulder on long axis in cm; θ is the angle of the bed slope at the pre-transport location (1◦ for joint-bounded blocks); *μ<sup>s</sup>* is the coefficient of static friction (=0.7); Cl is the lift coefficient (=0.178). Equation (1) is more sensitive to the length of a boulder on the long axis, whereas Equation (2) is more sensitive to the length of a boulder on the short axis. Therefore, some differences are expected in the estimates of *HS*.

#### **4. Results**

#### *4.1. Characteristics of the Conglomerate*

Two principal facies of Pleistocene conglomerates occur at El Confital. The first is linked to the background cliffs at opposite ends of the bay and the second is related to the open paleo-platform 5 to 10 m distal from the cliffs in the more central part of the beach. Along the higher cliffs, a marine conglomerate fills paleo-channels and forms ridges trending NNE–SSW perpendicular to the shore (Figure 2a,b). The channel conglomerate is well cemented and consists of coarse-grained (pebble to boulder size) clasts embedded in a white matrix of bioclastic calcarenite. The matrix incorporates marine mollusks and calcareous algae. Commonly reaching a cubic meter in size, boulders are generally ellipsoidal to angular in shape and well rounded. The largest boulders exhibit a long axis in excess of 2 m. Locally, the matrix is composed of reddish sands derived from eroded hyaloclastite tuffs (Figure 2c). Neptunian dykes related to the emplacement of this deposit cut into the tuffs and volcanoclastic deposits filled with carbonates and zeolites as well as bioclasts and gravels (Figure 2d). The more distal conglomerate is deposited nearly horizontal across the shore platform. Here, boulders are smaller and more rounded, and the matrix consists of coarse-grained bioclastic sand.

**Figure 2.** Conglomerate slumps above El Confital beach south of La Isleta; (**a**) incision approximately 3.5 m deep in cliff face filled with basal boulders loosened from the adjacent basalt formation (backpack for scale); (**b**) boulder-filled cut in the same cliff face with individual boulders exposed in bas relief (the largest boulder at the bottom has a long axis of approximately 75 cm); (**c**) reddish sands derived from eroded hyaloclastite; (**d**) details showing filling of a neptunian dyke and syndepositional fractures in basalt (tape-measure case for scale).

#### *4.2. Comparative Variation in Clast Shapes*

Raw data on clast size in three dimensions collected from each of six sample sites are recorded in Appendix A (Tables A1–A6). Regarding shape, points representing individual cobbles and boulders are fitted to a set of Sneed-Folk triangular diagrams (Figure 3a–f). The spread of points across these plots reflects a consistent pattern in the variation of shapes from one sample to another. As few as one to three points fall within the upper triangle in each diagram, which represents an origin from a perfectly cube-shaped endpoint as a jointbound block of basalt. No more than five points from each sample fall within the lower, right-hand rhomboid in these diagrams. This extreme corner signifies elongated blocks with one super-attenuated axis in relation to two axes that are significantly shorter by 75% or more. The result is a bar-shaped piece that originated as a joint-bound block of basalt. The plot with the greatest number of points in these two extremes is from locality 5 (Figure

3e), from a position closer to the western end of the paleoshore. Numbering between 10 and 15 per sample, the majority of points from each of the six samples fall within the central two rhomboids directly below the top triangle. Such points clustered at the core of a Sneed-Fold diagram are typical of clasts for which two of the three dimensions are closer in value than the length of the third axis. It is notable that no points appear anywhere along the margin of rhomboids on the left side of these diagrams. It is clear that no tendency in shape towards plate-shaped clasts is evident in the data. The general slope of points in agreement from the six plots follows a uniformly diagonal trend from the top to the lower right-hand corner. The trend in distributed points from these plots signifies the rounding of clasts in which two of the dimensions (maximum and intermediate lengths) are more closely matched with the third as an outlier.

**Figure 3.** Set of triangular Sneed-Folk diagrams used to appraise variations in cobble and boulder shapes sampled along the Upper Pleistocene paleoshore east to west at El Confital beach south of La Isleta: (**a**–**f**) sample localities 1 to 6 represented, respectively.

#### *4.3. Comparative Variation in Clast Sizes*

Drawn from original data (Tables A1–A6), clast size is plotted to the best effect on bar graphs as a function of frequency against maximum and intermediate lengths of the two longest axes perpendicular to one another. The dozen graphs plotted (Figure 4a–l) exhibit trends in clast size sorted by intervals of 15-cm, in which the boundary between cobbles and boulders is embedded within the range for clasts between 16 and 30 cm in diameter. The left-hand column (Figure 4a,c,e,g,k) depicts lateral variations in maximum boulder length from the six samples on an east to west transect along the Upper Pleistocene paleoshore. Overall, each of the samples in this dimension is numerically dominated by boulders in contrast to cobbles at ratios from 3:2 and 4:1. Samples from the east end (Figure 4a,c) reflect differences that conform to a normal bell-shaped curve, whereas samples from the west end (Figure 4i,k) are strongly skewed to include a few boulders of extreme size in excess of one meter. In contrast, the right-hand column (Figure 5b,d,f,h,j,l) shows lateral variations in values for intermediate length of clasts. In all but two examples (Figure 4f,j) the number

of measurements falling within the interval of 16 to 30 cm exceeds those registered for the same interval as measured for the long axis. This result reflects a general shift in size to smaller frequencies compared to the left-hand column and confirms the diagonal trend in shapes illustrated by the Sneed-Folk diagrams (Figure 4a–f). Skewness that mirrors the inclusion of extra-large clasts is especially evident in the bar graphs at the extreme ends of the paleoshore (Figure 4b,l). The largest basalt clast identified in the entire project registered a long axis of 214 cm, an intermediate axis of 94 cm, and short axis of 50 cm (Table A6).

**Figure 4.** Set of bar graphs used to contrast variations in maximum and intermediate boulder axes from six samples at El Confital beach south of La Isleta: (**a**,**b**) bar graphs from locality 1; (**c**,**d**) bar graphs from locality 2; (**e**,**f**) bar graphs from locality 3; (**g**,**h**) bar graphs from locality 4; (**i**,**j**) bar graphs from locality 5; (**k**,**l**) bar graphs from locality 6.

**Figure 5.** Examples of abundant marine fossils from the paleoshore exposed at El Confital beach: (**a**) filling among larger boulders that includes the elbow shell (*Patella aspera*) and barnacles (*Balanus trigonus*) with bottlecap for scale; (**b**) bivalve (*Chama gryphoides*) with pen-tip for scale; (**c**) gastropod (*Gemophos viverratus*) with 3-cm scale; (**d**) gastropod (*Cerithium vulgatum)* with scale bar = 1 cm; (**e**) large colony of gastropods (*Dendropoma cristatum*) with rock hammer for scale.

#### *4.4. Paleontological Inferences on Water Depth*

A moderately rich molluscan fauna of 42 marine gastropods and eight bivalves from El Confital (Table 1) that also includes barnacles and rhodoliths formed by coralline red algae is maintained in the permanent collections of the Tenerife Science Museum. Overall, these fossils reflect organisms that lived under inter-tidal to shallow subtidal conditions as confirmed by outcrop relationships, where currently the fossil remains are visible cemented in place along small sections of the bay and in ravines that form after heavy rains. The Pleistocene fauna at El Confital corresponds to a high-energy setting against a rocky shore highlighted by the abundance of mäerl and rhodoliths associated with relatively abundant patelid gastropods (Figure 5a), chamid bivalves (Figure 5b), and other mollusks (Figure 5c,d). Extensive colonies of vermetid gastropods (*Dendropoma cristatum*) are represented as discrete biological clasts incorporated within the conglomerate (Figure 5e). Although extensive shell fragmentation is evident, it is worth noting the high rate of complete shells, which include delicate ornamentation. In the case of pateliform shells, preservation also features evidence for stacking. This taphonomic trait is characteristic of a high-energy regime. At the base of the deposit on the platform at El Confital (Figure 2, locality 7), there is evidence of bioturbation possibly related to the activity of crabs. Also, trace fossils likely related to the activity of polychaets are preserved within the neptunian dykes that are common on the platform.

**Table 1.** Summary species list of epifaunal, infra-intertidal invertebrates from the Upper Pleistocene strata exposed at El Confital beach correlated with Marine Isotope Substage 5e. Extinction is denoted by asterisk.


In addition to preservation of whole but also fragmented rhodolith debris (Figure 6a,d) that signify the remains of a Pleistocene mäerl bed at El Confital, deposits are notable for the conglomerate consisting of mixed basalt cobbles and large boulders that exhibit erosional smoothing. The growth of coralline red algae in thin layers encrusted around and among these basalt clasts is widespread (Figure 6). In some examples, algal crusts are localized

and fail to completely surround individual cobbles leaving some parts free as viewed profile (Figure 6a). This scenario implies that some clasts were only partially coated by crustose algae elsewhere and subsequently were transferred to the conglomerate. In other examples, thick growth of algal crusts completely fills voids between boulders and small cobbles fixed in between (Figure 6b). This suggests an alternative scenario in which algal growth occurred perhaps on the outer margin of the conglomerate after its mass accumulation. Crustose red algae also are found in patches attached to basalt boulders exposed in three-dimensional relief (Figure 6c).

**Figure 6.** Examples of basalt cobbles and boulders heavily encrusted by coralline red algae: (**a**) space between two adjacent boulders filled by algal-encrusted cobbles and pebbles (scale bar = 2.5 cm); (**b**) close-up view of thick algal encrustations in the space among basalt clasts (bottle cap for scale); (**c**) hat-size basalt boulder with remnants of coralline red algae (clasp on hat string = 3 cm).

#### *4.5. Storm Intensity as a Function of Estimated Wave Height*

Clast sizes and maximum boulder volumes drawn from the six field localities are summarized in Table 1, allowing for direct comparison of average values for all clasts, as well as values for the largest clasts in each sample based on Equations (1) and (2) derived from the work of Nott [16] and Pepe et al. [19].

The Nott formula [17] shown in Equation (1) yields an average wave height of 4.5 m for the extraction of joint-bound blocks from basalt sea cliffs exposed at El Confital beach and their subsequent transfer as slump boulders in samples 1 to 6. A much larger value for a wave height of 11 m is derived from the average of the largest single blocks of basalt recorded from the six locations. More sensitive to clast length from the short axis, the more sophisticated Equation (2) from Pepe et al. [19] yields values that are consistently lower for the estimated average wave height and estimated maximum wave height. The difference between the two calculations for estimated average wave height as well as estimated maximum wave height is, however, very small at 30 cm. Notably, the value for average maximum wave height derived from Equation (1) from Nott [17] is 2.5 times as high as the value found for overall average wave height using the same formula. Essentially the same factor applies to the slightly lesser values derived from Equation (2) according to Pepe et al. [19]. Clearly, the hydraulic pressure with extreme wave impact is necessary to loosen and budge the largest fault-bound blocks of basalt in the Pleistocene cliff line, as represented by the enormous block at locality 6 estimated to weigh 2.25 metric tons (Table A6). The essential issues under consideration in the following sections pertain to the singularity of a lone event of extreme magnitude as opposed to the repetition of many, but less energetic events in the shaping of the Pleistocene boulder slumps at El Confital beach.

#### **5. Discussion**

#### *5.1. Integration of Paleontological and Physical Data*

Boulders derived from horizontal layers of joint-bound basalt that originated as a subaerial flow about 1 million years ago at El Confital are estimated to have undergone wear that resulted in a 20% reduction in volume from more cubic or bar-shaped blocks (Figure 4) due to mutual friction under wave shock and subsequent erosion that smoothed sharp corners. Pleistocene fossils incorporated within the resulting conglomerate (Table 1, Figure 5) reflect an age correlated with Marine Isotope Stage 5e during the last interglacial epoch [11,20–23].

The assemblage represents a high-diversity, intertidal to shallow subtidal fauna dominated by mollusks that thrived on a basalt shelf on the southern margin of a small volcanic edifice. Except for those few limited to the MIS5e, most of the species listed in Table 1 continue to inhabit the contemporary coasts of the Canary Islands. These taxa are characterized by a preference for rocky or mixed littoral bottoms (sandy with rocky clasts of different sizes) up to a depth of about 3 m within the intertidal zone. The various species occupy different niches, such as crevices or intertidal pools or beneath rocks, which provide protection from the intense northwesterly waves that dominate the shores of the Canary Islands. In the case of El Confital today, there also occurs a wide rocky intertidal flat with little slope. In these shallows, the organisms are grouped in parallel bands or remain associated with pools at low tide, depending on their ability to adapt to environmental factors such as desiccation, temperature, salinity, and water agitation that condition life in that environment [23]. The intertidal shallows are home to a high number of marine organisms, highlighted by the dominant populations of pateliform and trochid gastropods.

Some of the larger clasts and boulders are encrusted with crustose red algae exhibiting rinds in excess of a centimeter in thickness (Figure 6). In places, surviving patches of red algae retain a rose coloration typical of coralline red algae (Figure 6c), but it may be due to inorganic discoloration. In addition to platy red algae cemented directly onto cobbles and boulders, whole rhodoliths and the debris of broken rhodoliths occur in pockets scattered throughout the conglomerate (Figure 5a). Some faunal elements may have lived within the interstices of adjacent boulders after the conglomerate was formed during slump events. Platy red algae are concentrated unevenly on the sides of cobbles and smaller boulders (Figure 6a), leaving other faces vacant. Some open spaces between adjacent boulders appear to have been filled by the continual growth of crustose red algae (Figure 6b). On the other hand, rhodoliths potentially composed of the same species of coralline red algae in unattached growth forms expressed by spherical shapes would have expired due to a lack of mobility.

A proper survey has yet to be undertaken to identify the genera and possible range of species belonging to coralline red algae that encrust cobbles and boulders in the Upper Pleistocene deposits at El Confital. Such a study necessarily entails the collection of samples for the making of thin sections transversely through crusts in order to identify features diagnostic at least on a genus level. Until such time, the best that can be said is that the present-day distribution of marine algae is widespread throughout the Canary Islands and keyed to habitats around individual islands in the Canary archipelago. Among the Corallinaceae known to be specific to Gran Canaria island, at least five genera are present, including *Hydrolithon*, *Lithophyullum*, *Lithoporella*, *Mesophylum*, and *Neogoniolithon*. Among these, *Lithophyllum* is the most diverse with four species locally attributed to that genus around the island [24]. From a paleoecological point of view, what is most telling about the boulder beds at El Confital at this stage of investigation is that they came to reside in shallow water within the upper photic zone consistent with the associated fossil fauna.

Overall, the combination of paleontological and physical evidence points to an open shelf setting on the margin of a small volcano around which a distinctly shallow-water biota thrived prior to interruption by storm events that eroded a series of parallel channels perpendicular to the strike of the paleoshore. These submarine gullies define the depositional space in which the Upper Pleistocene conglomerate was accommodated and preserved (Figure 3a,b).

Based on the application of two competing mathematical models that consider different dynamics [17,19], the estimated average height of storm waves that broke onto the south shore of La Isleta are remarkably similar in the range between 4.2 and 4.5 m (Table 2). However, the largest half-dozen boulders sampled from six study sites yield a much higher average estimated height of storm waves between 10.8 and 11.1 m (Table 2). Under any circumstances, such results would represent extremely large waves. These numbers represent clear outliers in the data, although based on boulders of extraordinary size and weight up to 2.25 metric tons. It may be that lesser waves were instrumental in gradually loosening the biggest basalt blocks from a joint-bound condition to a point where gravity slid them into nearby channels enlarged by multiple storm events. Rare hurricane events are more likely to have generated storm waves on the order of 6 to 8 m that impacted the Pleistocene rocky shore at El Confital. An entirely different set of equations is used to estimate the landward onrush of water due to tsunami events, but the complete absence of basalt boulders on the slopes of La Isleta (Figure 2) mitigates against this scenario.

**Table 2.** Summary data from Appendix A (Tables A1–A6) showing maximum bolder size and estimated weight compared to the average values for sampled boulders from each of the transects together with calculated values for wave heights estimated as necessary for boulder-beach mobility. Abbreviations: EAWH = estimated average wave height, EMWH = estimated maximum wave height.

