*4.4. Mapping and Installation of Sampling Transcects*

With the addition of ground measurements, the aerial photo (Figure 3) was consulted to draw a map of the Almeja CBB (Figure 5) from which different parts of the deposit could be quantified. The structure's subaerial exposure amounts to a total area of 13,000 m2, of which the boulder field around the rim occupies close to half at 6500 m2. As of 28 April 2019, swampy ground and open water occupied approximately 200 m<sup>2</sup> adjacent to the inner north wall of the ring. Bare ground covers a larger area and it is likely that over-wash of sea water enlarges the area of wet ground from time to time. The location and orientation of four transects across the CBB are marked on the map, the longest of which extends for 50 m sub-parallel to the bedrock escarpment in the northern part of the structure. The shortest is the N-S oriented transect crossing the south rim of the structure. In the central part of the Gulf of California, the maximum tidal range varies by as much as 2.75 m [17]. The subtidal portion of the Almeja CBB likely adds another 2500 m<sup>2</sup> to the boulder field for a total of 9000 m2.

**Figure 5.** Map of the Ensenada Almeja CBB with location of four transects used to collect data on boulder size and shape. Each square on the superimposed grid represents 100 square meters.

The tape measure laid out along transect 1 is shown in place (Figure 6a), with a mega-boulder adjacent to the first author. It turned out to be the second largest individual boulder measured in the Almeja CBB, with a long axis of 268 cm and an estimated volume of 2000 cubic decimeters. The weight of the boulder is estimated conservatively to be on the order of 3450 kg. The height of the boulder ring above the swampy ground on transect 2 stands at 1.85 m (Figure 6b). At its farthest extent seaward to the north, the base of large boulders sits in water up to 2 m below mean sea level. The largest rhyolite boulder encountered during offshore exploration from the northeast corner of the Almeja CBB is estimated to measure 6 m in length by 3 m wide and 2 m high (Figure 5).

**Figure 6.** Northern part of the Almeja CBB; (**a**) View to the northwest across Ensenada Almeja crossing the path of Transect 1 (see Figure 5)—the largest boulders in the deposit belong to this sector (figure for scale); (**b**) View due north, showing the deposit's inner wall diagonal to Transect 2 (figure for scale).
