**1. Introduction**

Based on a coastal survey using satellite imagery [1], volcanic flows of Miocene age that accrued as andesite were found to account for more than 700 km of peninsular and island shores in the western Gulf of California. By far, andesite is the most common rock type, accounting for 24% of all shores including sand beaches. Given the dominant occurrence of these rocks, it is pertinent to ask how it responds to forces of physical erosion. This contribution is the third in a series to examine rocky shores in the context large-scale boulder deposits attributed to storms of hurricane intensity that impacted the peninsular inner shores of Mexico's Baja California. The strength and behavior of recent storms, such as Hurricane Odile in 2014, follow a consistent pattern that allows predictions to be tested as to the specific vulnerability of different rock types. Previous work focused on a coastal boulder deposit (CBD) with metric-ton blocks of Pliocene limestone torn from the outer margin of a 12-m marine terrace on Isla del Carmen in the Gulf of California [2]. A subsequent study examined similar-size boulders pried

from a rhyolite coast not far to the north at Ensenada Almeja [3]. Limestone rocky shores amount to only 7.5% of shores in the western Gulf of California and rhyolite is so uncommon, it was not part of our original satellite reconnaissance [1].

Here, we consider the natural setting at Puerto Escondido (Spanish for Hidden Harbor), which has a restricted entrance but opens to a large lagoon otherwise entirely surrounded by andesite foothills related to the rugged Sierra de la Giganta. The inner lagoon is large enough to accommodate a small armada and the working harbor has been modernized to accommodate anchorage for visiting yachts and sailboats as well as larger vessels that call at the main wharf just inside the entrance. Puerto Escondido is renowned for the description by Steinbeck and Ricketts [4] during their epic voyage to the Gulf of California aboard the *Western Flyer* in 1940. Marine biologist, Ed Ricketts, who in 1939 published a ground-breaking treatise on the intertidal relationships of marine invertebrates along the Pacific shores of the United States [5], planned the expedition to expand his observations to the biologically rich but then poorly studied Sea of Cortez. His friend, author John Steinbeck, called the Hidden Harbor a place of magic and wrote: "If one wished to design a secret personal bay, one would probably build something very like this little harbor."

The goal of this paper is to apply geological and geomorphological insights to explain why Puerto Escondido is so extraordinary as a natural harbor. Paleontological data, as well as the location of critical fault lines, are used to show how the coast was more open to marine circulation during the last interglacial epoch in the Late Pleistocene. The emplacement of two major barriers fixed among outer hills is the primary focus of analysis looking at boulder shapes and their variation in size and calculated weight. Estimation of wave heights necessary to transport large boulders serves as a proxy to gauge recurrent storm intensity. Lastly, geomorphologic modeling provides a means to consider the degree to which rocky-shore retreat has occurred over Holocene time in a consistently subtropical setting and the scale of erosion necessary to provide the raw materials for barrier construction.

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

Situated between the Mexican mainland and the Baja California peninsula, the Gulf of California is a marginal sea with a semi-enclosed area amounting to 210,000 km2 arrayed along a NW–SE axis stretching for 1100 km (Figure 1a). Central basins within the gulf are semi-oceanic in depth, exceeding 3200 m. The southern opening to the Pacific Ocean is 180 km wide and allows for a range of oceanographic phenomena [6] that in turn stimulates seasonal upwelling and nutrient fertilization linked to a high degree of biological productivity and species diversity [7]. More than a dozen tropical storms typically form off the cost of Acapulco at approximately 15◦ N latitude during the annual hurricane season, but most turn outward to the northwest before reaching the southern tip of the Baja California peninsula at 23◦ N Latitude [8]. Major storms are known to enter the Gulf of California—most recently, Hurricane Odile in September 2015 [9] and Hurricane Lorena in September 2019.

Puerto Escondido is located 24 km south from the town of Loreto in Baja California Sur on the Gulf of California (Figure 1b, locality 1). A small outer harbor is linked to a huge inner harbor by a narrow entrance on the south side. Viewed from hills on the inland western side (Figure 2), the harbor is notable for a pair of distinct barriers that form robust sea walls anchored to an intermediate islet. At their opposite ends, the pair of barriers are linked to the bedrock on the peninsula mainland to the north and a large island to the south that also guards the entrance to the inner harbor. In concert, the combination of natural barriers and fixed bedrock effectively seals off the lagoon from outside disturbances in the open Gulf of California.

**Figure 1.** Locality maps showing Mexico's Baja California peninsula and Gulf of California; (**a**) Mexico and border area with the Unite States, denoting key villages or cities with inset box marking the study region around the town of Loreto; (**b**) Region around Loreto in Baja California Sur, marking coastal boulder deposits (\*) at localities 1 to 4.

**Figure 2.** View east over the inner harbor at Puerto Escondido with Isla del Carmen on the horizon.

From a geological perspective, the history of faulting in western Mexico is intimately related to the origins of the Gulf of California. Tectonic separation of the Baja California peninsula from the mainland occurred due to crustal extension between 13 and 3.5 million years ago, with N–S trending faults related to Basin and Range development in western North America. Thereafter, a change in tectonic regime led to transtensional faulting with the transfer of the peninsula to the Pacific Tectonic Plate and its ongoing migration to the NW [7]. During the earlier phase, faults were oriented mainly N–S and the peninsular coast underwent major uplift west of the Loreto rift segment between 5.6 and 3.2 million years ago, amounting to 100s of meters in the Sierra de la Gigante [10]. Many of the 40 named islands in the gulf conform to fault blocks subjected to uplift as structural horsts [11]. Subsequent strike-slip faulting is oriented NW–SE perpendicular to a series of step-like spreading centers located within deep basins through the Gulf of California. A major clue as to the tectonic history of the study area around Puerto Escondido is the placement of faults registered in the landscape.

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

#### *3.1. Data Collection*

Data on topography, basin size, watershed boundaries and fault orientations are derived from a portion of the Mexican federal government map for the Juncalito quadrangle (G12C19). Drawn at a scale of 1:50,000, the greater map was issued by the Instituto Nacional de Estadistica Geografia e Informatica in 1982. Contour intervals from the government map were scanned and traced to yield a project map retaining an accuracy at 20 m intervals. The 1982 version of the map was updated to show the main details of improved harbor infrastructure since that time.

Puerto Escondido was visited on 24 and 25 April 2019, when the field data for this study were collected based foremost on a sample of 100 boulders divided equally among four transects along the upper tide line of the northern Coastal Barrier Deposit (CBD) and another 50 boulders along the southern CBD. The boundary is denoted by a prominent color due to marine algae. The definition for a boulder adapted in this exercise is that of Wentworth (1922) for an erosional clast equal or greater than 256 mm in diameter [12]. There exists no proposed upper limit in size for this category.

Collection of data on boulder size followed procedures graphically codified in Figure 3. A Brunton compass and meter tape were used to lay out transects in 50 m segments. Consistent with previous studies [1,2], the largest 25 boulders were measured manually in each transect with boulder centers spaced from 1 to 1.5 m apart. Each boulder required three measurements along principle axes (long a, intermediate b and short c). Triangular plots were employed to demonstrate variations in boulder shape, following the practice of Sneed and Folk (1958) for river pebbles [13]. 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 representative cobble of andesite was collected from the northern barrier for laboratory treatment at Williams College, where it was weighed, and its volume determined as a function of equal displacement when submerged in a beaker of water. Prior to immersion, the rock was water-proofed by spraying it with Thompson's Water Seal TM (The Thompson's Co, Cleveland, OH, USA).

**Figure 3.** Schematic portrayal of the sampling method applied at Puerto Escondido (not to scale).

#### *3.2. Hydraulic Model*

With determination of specific gravity based on laboratory testing for volume and weight, a hydraulic model may be applied to predict the energy needed to transport larger andesite blocks from a rocky shoreline to a barrier deposit as a function of wave impact. Andesite is a volcanic rock that forms from surface flows with variable thicknesses and a propensity to vertical fractures. These factors control the size and general shape of blocks loosened in the cliff face. Herein, the formula used to estimate the magnitude of storm waves applied to joint-bounded boulders is taken from equation 36 in the work of Nott [14]:

$$Hs = \frac{\left(Ps - Pw/Pw\right)a}{C\_1}$$

where *Hs* = height of the storm wave at breaking point; *Ps* = density of the boulder (tons/m3 or g/cm3) *Pw* = density of water at 1.02 g/mL; a = length of boulder on long axis in cm; and *C*<sup>1</sup> = lift coefficient (=0.178).

#### **4. Results**

#### *4.1. Topographic Base Map*

The base map adapted for use in this project treats an area of 25 km<sup>2</sup> (Figure 4). A small outer harbor open to the south occupies an area of 0.5 km2. To one side of the outer harbor, the mouth (*La Bocana*) forms a 50-m wide entrance to a much larger inner harbor covering 2.3 km2. The narrow connection between outer and inner harbors admits tidal flux but resists severe weather arriving from all directions. Hills surrounding the inner harbor inland to the west exhibit lower topography with elevations ranging between 100 and 160 m above sea level. The outer eastern edge of the harbor complex is formed by a linear front stretching 4 km from NW to SE consisting of two hills (Cerro El Chino and Cerro La Enfermería) connected to an un-named islet by barriers #1 and #2 (Figure 4). The islet rises to an elevation exceeding 80 m above sea level, whereas bedrock on the neighboring hills reaches 120 and 180 m, respectively. Formed by andesite pebbles, cobbles, and boulders, the two

natural breakwaters are the most vulnerable spots in the outer defense of the main harbor. The longer northern barrier extends for 250 m, whereas the shorter southern barrier is 140 m in length. On average, barrier width amounts to 30 m, with a mid-line 2.75 m above mean sea level. On close inspection, the inner west-facing edges of the barriers drop off abruptly into the enclosed lagoon. The outer east-facing margins are ramp-like in configuration extending at a low angle into the water.

**Figure 4.** Topographic map showing the hills surrounding Puerto Escondido and other key features including a Pleistocene fossil deposit and faults.
