Boulders and cobbles derived from a conglomerate in the underlying geologic section are also present. Small coastal valleys occur throughout the length of the project along the major drainages within the Montara Mountain watershed. The soils in these valleys are deep and moderately well drained and have developed on low terraces and alluvial fans along the stream channels. The climate of the project area is Mediterranean with a strong maritime influence. The winters are relatively warm with a short rainy season.
Summers tend to be relatively cool and dry but subject to extended periods of coastal fog.
Temperature ranges, both seasonally and diurnally, are narrow while air moisture remains relatively high. Wind is an important environmental factor on the exposed slopes of San Pedro and Montara Mountains. Extremely dense northern coastal scrub covers most of the project vicinity, especially those portions over San Pedro Mountain and along the steeper foot slopes of Montara Mountain. Small grassy openings and barren rocky areas are scattered throughout the scrub areas. The scrub extends westerly from the tunnel alignment to the cliffs above Devil's Slide, although coyote brush becomes less dominant nearer the coast.