Sandstone and sandstone-dominated melange. This is the most common upland rock type. Slopes are typically steep, and soils are well drained.
Montara Mountain granitics, which have been classified as tonalite. Characteristic soil development is thin, well-drained soils with outcrops on slope convexities. Slopes are typically quite steep, since this is all on Montara Mountain, extending to the west to San Pedro Mountain.
Greenstone. Quite in contrast to the above, greenstone doesn’t tend to produce significant outcrops. This rock is common in the northeast part of the watershed, and is drained by the North Fork.
Alluvium. Flat valley bottoms are predominantly underlain by gravelly alluvial deposits. Most of these areas are covered by residential development.
"Conglomerate". Most of the areas mapped as "conglomerate" by the USGS appear to be unconsolidated colluvial deposits from debris flows and other slope movements.
In addition, there are scattered outcrops of serpentinite and limestone. Lower parts of San Pedro valley are mapped as fill.