Bedrock 

Rocks of the Franciscan Complex underlie the entire watershed. Within the watershed, the Franciscan occurs as two distinct bedrock units: the relatively coherent (stable) Tertiary to Cretaceous age Coastal Belt terrain and the relatively incoherent (easily eroded) Tertiary to Jurassic age Central Belt terrain.

Coastal Belt rocks underlie the entire watershed except for the northeastern area of the headwaters of the North Fork. Though they have not been recognized, minor occurrences of Mesozoic volcanic rocks could be present. Franciscan Coastal Belt terrain is characterized by sandstone and interbedded siltstone and shale, with locally minor amounts of conglomerate present. Elsewhere chert, limestone, and greenstone are found. Coastal Belt rocks have been deformed by past tectonic activity. This has created a body of rock that has been broken up into coherent bedrock blocks of varying size (up to city blocks or larger) separated by shear zones and faulting; locally the bedrock is tightly folded.

Central Belt rocks crop out in the northeastern area in the headwaters of the North Fork. They underlie the subdued topography of that area. The Central Belt is a mélange characterized by blocks of bedrock, varying in size from fist size pieces to blocks up to city blocks or larger in size, in a highly sheared, mashed, and mangled clayey matrix. The blocks of bedrock can include sandstone, conglomerate, chert, greenstone, blue schist, limestone, eclogite, serpentine, amphibole, and ultramafic rocks. The subdued nature of the hillside topography overlying the central belt is a result of the weak nature of the sheared mélange matrix.

Surficial Deposits

Locally overlying the bedrock are a variety of surficial earth materials deposits that include beach sand, marine terrace deposits, dune sands, estuary deposits, landslide debris, alluvium, and soil and colluvium.

These deposits occur in a very small area at the mouth of Ten Mile River. It is likely that the beach sand and sand dune deposits interfinger along the back of the beach. The beach sands are composed of fine to coarse sand with local pebble and cobble gravel lenses. The terrace deposits are represented by poorly consolidated sand and minor amounts of gravel. The overlying dune deposits are composed of fine- to medium-grained sand.

Estuary deposits occur in the very lower reaches of Ten Mile River. They are found on both sides of the river from near the mouth to about two to three miles up river. They likely interfinger with alluvial deposits in the lower reaches. Estuary deposits are composed of unconsolidated muds, silts, and fine-grained sands. They are locally rich in organic debris. 

Landslide deposits occur everywhere throughout the watershed. They vary from small creek- side failures to large slides involving hundreds of acres. The slides are composed of a mixture of soil, colluvium, and bedrock debris carried down-slope as either intact masses or heterogeneous flows or avalanches. Locally overlying the bedrock is a variety of surficial deposits that include marine terrace deposits, dune sands, landslide debris, and alluvium.

Except for very steep to precipitous slopes, soil and colluvial deposits mantle the hillsides. They also occur on ridge tops and valley bottoms. Soils are derived from the mechanical and chemical weathering of the underlying rock or surficial deposits. The materials that make up colluvial deposits are derived by the same weathering processes that make up soils. However, colluvial deposits are characterized by being accumulations of these weathering products that have moved down slope by raindrop impact, sheetwash, and other gravity-driven processes, other than mass wasting (landsliding), to collect at the lower reaches and bottoms of hillsides. Generally deposits of soil materials thicker than about 3 to 4 feet thick are judged to be colluvial deposits. Soil and colluvial deposits are composed of a heterogeneous and poorly consolidated mixture of rock debris, sand, silt, and clay. These materials can be present in varying amounts along with organic debris.