Rock Types of Redwood National & State Parks: A Geological Tour

Redwood National and State Parks (RNSP) on California's North Coast sit at a tectonic collision zone. The rocks tell a story of subduction, accretion, and uplift.

1. Franciscan Complex (Core Bedrock)

Subduction zone mélange – scraped off Farallon Plate as it dove under North America (Jurassic-Cretaceous). Highly deformed, sheared, faulted.

• Graywacke sandstone – turbidite deposits, now metamorphosed.
• Shale / argillite – deep marine mud.
• Greenstone – altered basalt (pillow lavas) – green from chlorite/epidote.
• Chert – radiolarian ribbon chert (deep sea silica).
• Serpentinite – altered mantle peridotite (CA state rock).
• Blueschist / eclogite – high-pressure metamorphic blocks (exotic).

2. Coastal Sedimentary Rocks (Younger Cover)

• Wildcat Group (Miocene-Pliocene): Marine sandstone, siltstone, mudstone – fossiliferous (mollusks, forams).
• Falor Formation: Non-marine conglomerate, sandstone – river/delta deposits.
• Hookton Formation: Marine terrace deposits (Pleistocene).

Exposed in sea cliffs, wave-cut platforms. Fossils: whales, sharks, invertebrates.

3. Granitic Rocks (Inland / Eastern Park)

• Central Belt / Klamath Mountains plutons: Granite, granodiorite, diorite (Jurassic-Cretaceous).
• Resistant → forms peaks, domes (e.g., Rodger Peak).
• Source of coarse sediment in rivers.

4. Metamorphic Rocks

• Schist, gneiss, amphibolite – in eastern park (Klamath terranes).
• Protoliths: sedimentary/volcanic rocks metamorphosed during accretion.

5. Quaternary Deposits

• Alluvium: River gravel, sand, silt (Redwood Creek, Klamath, Smith).
• Marine terraces: Uplifted beach deposits (staircase of terraces = uplift history).
• Landslide deposits: Abundant in Franciscan terrain.

Geologic Story in Brief

1. Farallon Plate subducts → scrapes off Franciscan mélange (accretionary wedge).
2. Plutons intrude (granites).
3. Uplift & erosion → marine basins fill (Wildcat, etc.).
4. Pleistocene uplift → marine terraces.
5. Modern rivers incise → Redwood Creek canyon.

The rocks are why the redwoods grow here: Franciscan soils, fog capture on coastal ridges, seismic uplift creating drainage. Geology wrote the forest.