*4.3. Source Rock and Natural Weathering*

Banded rhyolite is the dominant lithology on the headland forming the east flank of Ensenada Almeja. It is the exclusive source of eroded boulders in the Almeja CBB. The natural state of weathered bedrock is exposed in sea cliffs near the tip of the headland (Figure 3, white rectangle). From a ground view near sea level (Figure 4), the bedrock is found to be dissected by bedding planes and vertical fissures intersecting at right angles that outline oblong shapes similar to common shoe boxes. On average, such blocks are roughly three quarters of a meter in length, a half meter wide and a third of a meter in height. The apparent source of mechanical retreat worked on the rhyolite sea cliffs is hydraulic pressure exerted by wave surge against joints in the bedrock. Loosened blocks on bedding planes are poised to slide down slope into the sea aided by the force of gravity.

**Figure 4.** Bedrock exposure of banded rhyolite on the outer headland of Ensenada Almeja (see Figure 3 for location), showing pattern of intersecting vertical joints and bedding planes that facilitate boulder production under wave attack.
