*5.6. Regional Patterns for Coastal Boulder Beds*

Study of rocky-shore attrition around the Gulf of California due to impact by hurricanes through the last 10,000 years has barely commenced with the only previous example based on the limestone CBB on the east coast of Isla del Carmen [2]. The largest up-turned blocks of layered limestone from the Carmen CBB are estimated to weigh between 5.8 and 28 metric tons. The largest megaclast in the Almeja CBB (Table A1, Transect 1) is close to 5 metric tons in weight. Approximately 30% of the megaclasts measured from Transect 1 exceed one metric ton in weight. By comparison, only two of the megaclasts from the next transect (Table A2, Transect 2) exceed one metric ton in weight and only one from the third transect (Table A3) exceeds that amount. None of the boulders in the last transect (Table A4) come close to a metric ton.

To what extent might other examples of Holocene or older Pleistocene CBBs exist throughout the Gulf of California and what source rocks are most typically represented? The Loreto area offers additional possibilities for expanded studies. Located 23 km south of Loreto (Figure 1b), Puerto Escondido is a natural harbor with a single entrance from the southeast leading to a large inner lagoon sheltered by islets linked by boulder barriers eroded from an adjacent headland (El Chino) at one end and the largest island (La Enfermeria) at the other. Overall, the andesite clasts on the barriers are poorly sorted with a wide range of sizes similar to the Almeja CBB. Future research may determine to what extent the barriers were formed by long-shore currents or a combination of factors including storm activity.

A short distance north of Loreto, the south shore of Isla Coronados (Figure 1b) is clad by andesite boulders forming an extensive berm. Given that the south shore is on the leeward side of the island sheltered from the north winds and related sea swell, the berm is more likely to have been activated by hurricane activity. In addition, a Pleistocene lagoon inland from the unconsolidated boulder berm is filled with limestone that dips northward away from a bedrock ridge with the internal carbonate layers interpreted as over-wash deposits derived from rhodolith debris [25]. The most likely mechanism for north-directed over-wash events on Isla Coronados would have resulted from major storms or hurricanes arriving from the south.

A fitting analog for the Almeja CBB is the 400-m long paleoshore near Punta San Antonio (Figure 1b), formed by mixed granodiorite and andesite boulders to which a diverse Pleistocene biota is attached in growth position [26]. In particular, granodiorite boulders derive from the adjacent headland at Punta San Antonio that occupies a flanking position comparable to the rhyolite headland at Ensenada Almeja. The paleogeography of the Punta San Antonio site also features a former embayment comparable in size to Ensenada Almeja. As there is no bedrock exposure of granodiorite north of the former bay, wind-driven currents from that direction could not have been responsible for development of the Pleistocene CBB. The only alternative is an energy source associated with the passage of Pleistocene hurricanes with a counter-clockwise rotation suited to erosion of the Punta San Antonio headland to the east.

Another area with rich potential for future studies on CBBs is located in the upper Gulf of California o ff Bahía Los Angeles (Figure 1a). The southeast end of Isla Angel de la Guarda is known for its closed lagoons with elevated salinities that favor living microbial colonies [27], commonly recognized by paleontologists and geologists as stromatolites. Based on personal exploration (MEJ and JL-V), the smaller lagoon on Isla Angel de la Guarda (Figure 10, number 1) is closed o ff by a CBB formed by large andesite boulders. The principal source for this material is the adjacent rocky shore to the north, which features eroded sea stacks. As long-shore currents from the north are blocked by nearby Isla Estanque, the alternative energy source for erosion of the andesite cli ffs close to the small lagoon is likely to have been the result of episodic storms or hurricanes. Based again on personal experience, andesite clasts on the deposit closing o ff the larger lagoon on Angel de la Guarda (Figure 10, number 2) are mostly the size of cobbles. Long-shore drift may have been more constructive in the development of the enclosing berm. Isla Estanque has ye<sup>t</sup> to be explored with any focus on lagoon development (Figure 10, numbers 3 and 4) but the clock-wise rotation of storm systems o ffers a promising hypothesis for the development of boulder spurs ye<sup>t</sup> to completely isolate related lagoons.

**Figure 10.** Aerial photo from an altitude of 15,000 m showing boulder deposits from the southeast coast of Isla Angel de la Guarda in the upper Gulf of California that define closed lagoons (1 and 2) and distinct spurs formed by bolder deposits on Isla Estanque in the process of closing other lagoons (3 and 4).

### *5.7. Comparison to Selected CBBs Elsewhere in the World*

Ruban et al. (2019) compiled a representative collection of 58 published studies concerning Earth-bound processes capable of producing megaclasts [11]. From this sample, more than half (53%) are limited to boulder deposits that formed during Quaternary time, 20 of which represent coastal depositional settings. The data base distinguishes between Quaternary CBBs attributed to storms as opposed to tsunamis in nearly equal parts. A clear-cut example of a huge tsunami event derives from the study by Ramalho et al. (2015) with regard to basalt megaclasts as much as 8 m in diameter with a maximum estimated weight of as much as 1000 metric tons, left high on the flanks of Santiago in the Cape Verde Islands [21]. An equally clear-cut study by Cox et al. (2018) relates to blocks with an estimated weight as much as 620 metric tons pealed back from Carboniferous limestone layers exposed at the top of high sea cli ffs in western Ireland [28] that are unequivocally linked to major sea storms. Not included in the data base of Ruban et al. (2019) are other studies on massive carbonate megaclasts from the Bahamas and Bermuda interpreted ambiguously as either tsunami or storm-related [29] or unequivocally as storm related [30]. Current literature on Quaternary CBBs appears to be skewed towards studies on carbonate megaclasts, such as the work by Biolchi et al. (2019) from the northern Adriatic Sea [31]. Bedded limestone formations exposed in sea cli ffs are especially vulnerable to erosion by storm-induced waves, as exemplified by our previous study on a Holocene CBB from Isla del Carmen in the Gulf of California [2]. Volcanic flows composed of layered basalt and andesite also form extensive sea cli ffs around the Gulf of California [3] and many other parts of the world. The geomorphology of CBBs stripped from igneous basement rocks is underrepresented in the literature and o ffers a research target worthy of future investigations especially in the context of likely hurricane deposits.
