Geology
The Coquille watershed lies within two major geologic provinces and is dominated by marine sedimentary rocks. The headwaters of the South Fork of the Coquille lie in the northwestern cormer of the Klamath Mountain Province. This is a hard rock system composed of volcanic, diorite, and serpentine rocks. Faults contacts exist between the volcanic rocks leading to instability in the area and resulting in earthflows, debris slides, and slumps. The remainder of the basin lies in the southern part of the Coast Range Province. This province is primarily composed of thick sequences of bedded sandstone, susceptible to mass movement, rapid erosion, flash flooding, and landslides. Fluctuating sea levels and continued uplifting and infilling of the river channel have resulted in marine and alluvial sediment deposition and terrace formation through the lower river drainage. The towns of Myrtle Point and Coquille are situated on these alluvial deposits while Bandon is perched on a marine terrace.