Drive approximately 2.5 miles north to the large turnout on the right side of the road. Descend to the river near the guardrail. Sheeted dikes that grade downstream into gabbro are exposed along the river. Mafic sheeted dikes intrude into the lighter-colored gabbro. Subparallel dikes with chilled margins can be found.

The sheeted dikes are interpreted as “feeder tubes” that carry the gabbroic magma (a dark, iron-magnesium-rich, silicate mixture) upward from the magma chamber. These dikes cut through the overlying gabbroic rock. The sheeted dikes bring magma upward to the surface where it erupts on the ocean floor to form pillow basalts. Ideally, a mass of vertical dikes that represent the axis of spreading should exist in this portion of the ophiolite. However, this axis region may be more diffuse in nature.

The photograph above is taken from Harper, 1989. The sheeted dikes can be seen across the river (dipping down to the right). The photograph on the left (also from Harper, 1989) shows a detailed picture of the darker mafic dike and the gabbro "screen". The lighter gabbro (gb) rock was intruded by the darker (mafic) dikes. The center dike is ~0.4m wide. Click on this picture for more detail.

Reference: Harper, G.D., Field Guide to the Josephine Ophiolite and Coeval Island Arc Complex, Oregon-California, in: Geologic Evolution of the Northernmost Coast Ranges and Western Klamath Mountains, California, eds. Aalto, K.R., and G.D. Harper, American Geophysical Union Field Trip Guidebook T308.