*4.2. Paleontological Data from the Western Hills*

Described here for the first time, a key fossil deposit occurs in the western hills above the inner harbor at Puerto Escondido. Approximately one hectare in area (Figure 5a), the deposit is comparable to Pleistocene shell drapes found on 12-m marine terraces along of the peninsular gulf coast [15]. Here, the shell drape sits 45 m above present sea level covering the topographic saddle between hills separating the harbor lagoon on one side and Bahía Juncalito to the north (Figure 4). Loose shells in the deposit correlate with the last interglacial epoch 125,000 years ago, when sea level worldwide was 6 m higher than today based on comparisons with marine deposits from islands regarded as tectonically stable [16,17]. A precise radiometric date is not possible, because datable *Porites* corals are not found at this locality. For the most part, the Puerto Escondido drape consists of white-bleached and abundant shells from the clam *Chione californiensis*—all of which are disarticulated as separate valves. Rare, but easy to spot within the mix is a small oyster (*Ostrea fischeri*), also known to encrust rocks in an intertidal

environment. In a single sample covering 16 dm2, approximately 75 valves of the dominant *Chione* clam litter the surface (Figure 5b). Tested laterally, the deposit is rarely more than 15 cm in thickness.

**Figure 5.** Upper Pleistocene shell drape in the western hills around Puerto Escondido: (**a**) View from an elevation of 45 m above sea level looking west toward the Sierra de la Gigante in the background; (**b**) Close-up view of the shell drape dominated by disarticulated vales of the Mollusk bivalve, *Chione californiensis*. Pocket knife for scale is 9 cm in length.

The tally features whole (unbroken) valves, although fragmented shells also are present. From the sample, there is scarcely any evidence for other species of marine mollusks. The operculum of a marine gastropod (*Turbo fluctuosus*) is small and easy to overlook. That particular species is diagnostic for a herbiverous gastropod typically found living in an intertidal setting, today. Another shell hidden in the array is a predatory gastropod (*Murex elenensis*), also found today living in the intertidal zone where it feeds on other mollusks and barnacles. The *Turbo* utilizes a hard mouthpiece called a radula like a file to scrape off marine algae critical to its diet, whereas the *Murex* uses a similar mouth device to bore a hole through the shells of its prey to gain access for feeding. Larger shells are widely scattered around the deposit but few in number. They include the turkey shell (*Cardita megastrophica*), bittersweet shell (*Glycymeris maculate*), cholate shell (*Megapitaria squalida*), cockle shell (*Trachycardium panamense*), and rock oyster (*Spondylus calcifer*). The overall species list requires some effort to assemble, because these larger shells represent a clear minority within the shell assemblage. All are represented by extant species living in the Gulf of California today [18]. The fossil shell drape is observed to occur off to one side of a fault that extends southward through a narrow valley from Bahía Juncalito. Considering that shell drapes of this reputed age from marine terraces elsewhere typically occur 12 m above sea level [15], tectonic uplift of the hills around Puerto Escondido represents a local anomaly some 32 m above normal. Proximity to the master Loreto fault and massive uplift of the nearby Sierra de la Gigante [10] account for this discrepancy.

The original work by Steinbeck and Ricketts [4] includes a detailed accounting of marine life both within the inner harbor and the outer harbor. These local biological data make a useful contrast with the paleontological data. The Brown Cucumber (*Isostichopus fusca*), a holothurian typically 15 cm in length, was found to occupy the inner harbor in a population numbering in the hundreds. Sand flats on the west side of the inner harbor appeared to be sterile. In contrast, one of the richest collecting stations of the entire expedition was reported from the outer harbor, where tidal currents were strongest at the entrance. A diverse biota was recorded to include sponges, tunicates, chitons, limpets, bivalves, snails, hermit crabs, as well as numerous species of sea cucumbers and starfish.

#### *4.3. Sample Density Calculation*

Specific gravity is a precise physical property that compares the density of an object to that of one cubic centimeter of water. One of the diagnostic characteristics of all naturally occurring minerals is defined by a known specific gravity. Because the composition of igneous rocks is variable, depending on the ratio of component minerals, the specific gravity of a particular class of igneous rocks like granite, rhyolite, or andesite will vary from region to region. The andesite sample from Puerto Escondido collected for laboratory analysis was a single cobble determined to weigh 575 g. After treatment to make the sample water-tight, the cobble was submerged in a wide-mouth, graduated beaker partially filled with distilled water. The volume of water displaced through this operation amounted to 225 mL. Dividing mass by volume yielded a density of 2.55 for the andesite sample, which means it was found to be 2.55 times as dense as water. The laboratory result was subsequently applied uniformly to all further calculations using the formula cited in the methods section, above. For comparison, the specific gravity of limestone from our earlier study at nearby Isla del Carmen (Figure 1b, locality 2) was determined to be 1.86 [2] and the banded rhyolite from our study at Ensenada Almeja (Figure 1b, locality 3) was determined to be 2.16 [3].

#### *4.4. Placement of Transects and Analysis of Boulder Shapes*

The modern inter-tidal zone is visible on the exposed outer margin of the two barriers by boulders tinted light green in color with the growth of filamentous algae. Contrast with the clean supratidal zone is clearly observed looking to the NW on barrier #1 with the sea cliffs of Cerro El Chino in the background (Figure 6a). All transects in this study were set along this boundary at the top of the intertidal zone. In the opposite direction, the view shows placement of the meter tape with the un-named islet in the distance to the SE (Figure 6b).

It is worthy of note that both Cerro El Chino and the un-named islet exhibit steep sea cliffs where erosion resulted in significant coastal retreat. The effect is apparent in the asymmetry of the outer hills and related islet as registered on the topographic map (Figure 4). A view from the middle of barrier #1 looking SW shows a characteristic mixture of pebbles, cobbles, and boulders entrained in the natural breakwater (Figure 6b). Looking eastward (Figure 6c), the meter tape is extended in front of the figure and the larger boulders in the field of view fall along the boundary between the darker upper inter-tidal zone and the normal supratidal zone. Much the same mixture of eroded cobbles and boulders is visible in the shallow water behind the figure. The larger boulders are typically 1 m in maximum diameter.

Raw data on boulder size in three dimensions collected from four consecutive transects each—50 m in length—are available in Tables 1–4. Data points representing individual boulders grouped by transect are plotted on a set of Sneed-Folk triangular diagrams (Figure 7a–d), showing the actual variation in shapes. Those points clustered nearest to the core of the diagrams are most faithful to an average value with somewhat equidimensional axes in three directions. Only very seldom do points for these boulders appear in the upper-most triangle, which signifies a cube-shaped endpoint. The vast majority falls within the central part of the two tiers beneath the top triangle. However, the overall trend among those points grouped from different transects trace a similar pattern angled toward the lower right corner of the diagrams. No points are plotted in the lower left tier in any of the diagrams, but a few occur in the lower right tier most notably in Figure 7b,c. The repetitive pattern in these plots indicates a tendency toward boulders that are oblong in shape. Although the trends in shape are similar among the four samples from barrier #1, the plots have no bearing on variations in boulder size.

**Figure 6.** Boulder deposits from barrier #1; (**a**) View from the north end of the boulder deposit with the eroded cliff face of Cerro El Chino in the background; (**b**) View looking toward the south end with the eroded cliff face of an un-named islet in the distance; (**c**) View east from the center of the barrier. In each view, the anchor position of the meter tape is at the upper tide line marked by black arrows.


**Table 1.** Quantification of boulder size, volume and estimated weight from coastal bar samples through Transect 1a at the east end of Cerro El Chino (Puerto Escondido). The laboratory result for density of andesite at 2.55 gm/cm3 is applied uniformly in order to calculate wave height for each boulder.

**Table 2.** Quantification of boulder size, volume and estimated weight from coastal bar samples through Transect 1b (continuation from 1a east of El China). The laboratory result for density at 2.55 gm/cm<sup>3</sup> is applied uniformly to all samples in order to calculate wave height for each boulder.



**Table 3.** Quantification of boulder size, volume and estimated weight from coastal bar samples through Transect 1c (continuation from 1b east of El China). The laboratory result for density at 2.55 gm/cm<sup>3</sup> is applied uniformly in order to calculate wave height for each boulder.

**Table 4.** Quantification of boulder size, volume and estimated weight from coastal bar samples through Transect 1d (continuation from 1c east of El China). The laboratory result for density at 2.55 gm/cm<sup>3</sup> is applied uniformly in order to calculate wave height for each boulder.


**Figure 7.** Set of four triangular Sneed-Folk diagrams used to appraise variations in boulder shape on barrier #1; (**a**) Trend for boulders from Transect 1a; (**b**) Trend for boulders from Transect 1b; (**c**) Trend for boulders from Transect 1c; (**d**) Trend for boulders from Transect 1d. Note the similarity in slopes from sample to sample.

Comparable data from barrier #2 collected along two consecutive transects of 50 m each are registered in Tables 5 and 6. Individual boulders from the upper intertidal zone of barrier #2 are shown by data points plotted in a pair of Sneed-Folk triangular diagrams. The trends expressed in Figure 8a,b are similar both to one another, as well as to those found in Figure 7a–d. That is, the overprint of a common pattern immerges in which the constituent boulders entrained in both barriers trend toward shapes that are more elongated and not at all plate-shaped.


**Table 5.** Quantification of boulder size, volume and estimated weight from coastal bar samples collected from transect 2a. (north of Cerro Enfermería). The laboratory result for density at 2.55 gm/cm<sup>3</sup> is applied uniformly in order to calculate wave height for each boulder.

**Table 6.** Quantification of boulder size, volume and estimated weight from coastal bar samples from transect 2b. (north of Cerro Enfernmera). The laboratory result for density at 2.55 gm/cm3 is applied uniformly in order to calculate wave height for each boulder.


**Figure 8.** Pair of triangular Sneed-Folk diagrams used to appraise variations in boulder shape on barrier #2; (**a**) Trend for boulders from Transect 2a; (**b**) Trend for boulders from Transect 2b. Note similarities in slopes with those from barrier #1 in Figure 7.

#### *4.5. Analysis of Boulder Sizes*

Variations in boulder size as a function of maximum and intermediate length drawn from the data sets for barrier #1 (Tables 1–4) are plotted separately for each of four transects using bar graphs. In this case, the stacked succession of graphs in Figure 9a–d show that the extreme outlier in maximum boulder size occurs in the first transect nearest the sea cliffs on Cerro El Chino. Boulders in the size class between 41 and 55 cm in diameter become more abundant and those in the size class between 56 and 70 cm in diameter are fewer in number. Otherwise, the size distributions remain fairly consistent from transect 1b through transect 1d. However, a deviation signaling a minor reversal in the size class between 71 and 85 cm appears in a comparison of Figure 9c,d. Such a reversal could imply a change in the direction of boulder source coming from the intermediate islet to the south. The numbers involved are small.

Boulder sizes along the intermediate axis from transects 1a through 1d are plotted in the stacked bar graphs from Figure 9e–h. Not surprisingly, the outlier in extreme length is found in Figure 8e representing the transect nearest the sea cliffs on Cerro El Chino. Otherwise, boulders in the size class between 26 and 40 cm are fairly consistent in number through the four transects. Data comparing the relative frequency of boulders in different size classes from barrier #2 are plotted as bar graphs in Figure 10a,b for the long axis and Figure 10c,d for the intermediate axis. Differences between the two samples are few. Most noticeable is the diminishment of boulders in the size class between 71 and 85 cm from transect 2a to 2b in regard to length across the long axis. Otherwise, apparent differences in size variation tend to be minimal. Again, a minor deviation appears in close comparison among Figure 9f–h, with particular reference to the size class between 41 and 55 cm. Moreover, a single large boulder in the size class between 56 and 70 cm is re-established.

**Figure 9.** Parallel sets of bar graphs used to appraise variations in the long and intermediate axes on boulders from barrier #1; (**a**) Long axis from boulders in Transect 1a; (**b**) Long axis from boulders in Transect 1b; (**c**) Long axis from boulders in Transect 1c; (**d**) Long axis from boulders in Transect 1d; (**e**) intermediate axes from boulders in Transect 1a; (**f**); Intermediate axis from boulders in Transect 1b; (**g**); Intermediate axis from boulders in Transect 1c; (**h**) Intermediate axis from boulders in Transect 1d.

**Figure 10.** Parallel sets of bar graphs used to appraise variations in the log and intermediate axes on boulders from barrier #2; (**a**) Long axis from boulders in Transect 2a; (**b**) Long axis from boulders in Transect 2b; (**c**) Intermediate axis from boulders in Transect 2a; (**d**) Intermediate axis from boulders in Transect 2b.
